Merge remote-tracking branch 'upstream/master' into overlayfs-store

src/build-remote/build-remote.cc

@@ -311,8 +311,9 @@ connected:
                 auto thisOutputId = DrvOutput{ thisOutputHash, outputName };
                 if (!store->queryRealisation(thisOutputId)) {
                     debug("missing output %s", outputName);
-                    assert(result.builtOutputs.count(thisOutputId));
-                    auto newRealisation = result.builtOutputs.at(thisOutputId);
+                    auto i = result.builtOutputs.find(outputName);
+                    assert(i != result.builtOutputs.end());
+                    auto & newRealisation = i->second;
                     missingRealisations.insert(newRealisation);
                     missingPaths.insert(newRealisation.outPath);
                 }

src/libcmd/command.cc

@@ -121,6 +121,8 @@ ref<EvalState> EvalCommand::getEvalState()
             #endif
             ;
 
+        evalState->repair = repair;
+
         if (startReplOnEvalErrors) {
             evalState->debugRepl = &AbstractNixRepl::runSimple;
         };

src/libcmd/common-eval-args.cc

@@ -153,7 +153,7 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
     for (auto & i : autoArgs) {
         auto v = state.allocValue();
         if (i.second[0] == 'E')
-            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), absPath(".")));
+            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), state.rootPath(CanonPath::fromCwd())));
         else
             v->mkString(((std::string_view) i.second).substr(1));
         res.insert(state.symbols.create(i.first), v);
@@ -161,19 +161,19 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
     return res.finish();
 }
 
-Path lookupFileArg(EvalState & state, std::string_view s)
+SourcePath lookupFileArg(EvalState & state, std::string_view s)
 {
     if (EvalSettings::isPseudoUrl(s)) {
         auto storePath = fetchers::downloadTarball(
             state.store, EvalSettings::resolvePseudoUrl(s), "source", false).first.storePath;
-        return state.store->toRealPath(storePath);
+        return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
     }
 
     else if (hasPrefix(s, "flake:")) {
         experimentalFeatureSettings.require(Xp::Flakes);
         auto flakeRef = parseFlakeRef(std::string(s.substr(6)), {}, true, false);
         auto storePath = flakeRef.resolve(state.store).fetchTree(state.store).first.storePath;
-        return state.store->toRealPath(storePath);
+        return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
     }
 
     else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
@@ -182,7 +182,7 @@ Path lookupFileArg(EvalState & state, std::string_view s)
     }
 
     else
-        return absPath(std::string(s));
+        return state.rootPath(CanonPath::fromCwd(s));
 }
 
 }

src/libcmd/common-eval-args.hh

@@ -2,14 +2,16 @@
 ///@file
 
 #include "args.hh"
+#include "common-args.hh"
 
 namespace nix {
 
 class Store;
 class EvalState;
 class Bindings;
+struct SourcePath;
 
-struct MixEvalArgs : virtual Args
+struct MixEvalArgs : virtual Args, virtual MixRepair
 {
     static constexpr auto category = "Common evaluation options";
 
@@ -25,6 +27,6 @@ private:
     std::map<std::string, std::string> autoArgs;
 };
 
-Path lookupFileArg(EvalState & state, std::string_view s);
+SourcePath lookupFileArg(EvalState & state, std::string_view s);
 
 }

src/libcmd/editor-for.cc

@@ -3,8 +3,11 @@
 
 namespace nix {
 
-Strings editorFor(const Path & file, uint32_t line)
+Strings editorFor(const SourcePath & file, uint32_t line)
 {
+    auto path = file.getPhysicalPath();
+    if (!path)
+        throw Error("cannot open '%s' in an editor because it has no physical path", file);
     auto editor = getEnv("EDITOR").value_or("cat");
     auto args = tokenizeString<Strings>(editor);
     if (line > 0 && (
@@ -13,7 +16,7 @@ Strings editorFor(const Path & file, uint32_t line)
         editor.find("vim") != std::string::npos ||
         editor.find("kak") != std::string::npos))
         args.push_back(fmt("+%d", line));
-    args.push_back(file);
+    args.push_back(path->abs());
     return args;
 }
 

src/libcmd/editor-for.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include "types.hh"
+#include "input-accessor.hh"
 
 namespace nix {
 
@@ -9,6 +10,6 @@ namespace nix {
  * Helper function to generate args that invoke $EDITOR on
  * filename:lineno.
  */
-Strings editorFor(const Path & file, uint32_t line);
+Strings editorFor(const SourcePath & file, uint32_t line);
 
 }

src/libcmd/installable-flake.cc

@@ -96,8 +96,7 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
         auto v = attr->forceValue();
 
         if (v.type() == nPath) {
-            PathSet context;
-            auto storePath = state->copyPathToStore(context, Path(v.path));
+            auto storePath = v.path().fetchToStore(state->store);
             return {{
                 .path = DerivedPath::Opaque {
                     .path = std::move(storePath),
@@ -107,10 +106,10 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
         }
 
         else if (v.type() == nString) {
-            PathSet context;
+            NixStringContext context;
             auto s = state->forceString(v, context, noPos, fmt("while evaluating the flake output attribute '%s'", attrPath));
             auto storePath = state->store->maybeParseStorePath(s);
-            if (storePath && context.count(std::string(s))) {
+            if (storePath && context.count(NixStringContextElem::Opaque { .path = *storePath })) {
                 return {{
                     .path = DerivedPath::Opaque {
                         .path = std::move(*storePath),

src/libcmd/installables.cc

@@ -449,7 +449,7 @@ Installables SourceExprCommand::parseInstallables(
         else if (file)
             state->evalFile(lookupFileArg(*state, *file), *vFile);
         else {
-            auto e = state->parseExprFromString(*expr, absPath("."));
+            auto e = state->parseExprFromString(*expr, state->rootPath(CanonPath::fromCwd()));
             state->eval(e, *vFile);
         }
 
@@ -593,8 +593,8 @@ std::vector<std::pair<ref<Installable>, BuiltPathWithResult>> Installable::build
                 std::visit(overloaded {
                     [&](const DerivedPath::Built & bfd) {
                         std::map<std::string, StorePath> outputs;
-                        for (auto & path : buildResult.builtOutputs)
-                            outputs.emplace(path.first.outputName, path.second.outPath);
+                        for (auto & [outputName, realisation] : buildResult.builtOutputs)
+                            outputs.emplace(outputName, realisation.outPath);
                         res.push_back({aux.installable, {
                             .path = BuiltPath::Built { bfd.drvPath, outputs },
                             .info = aux.info,

src/libcmd/repl.cc

@@ -40,6 +40,7 @@ extern "C" {
 #include "markdown.hh"
 #include "local-fs-store.hh"
 #include "progress-bar.hh"
+#include "print.hh"
 
 #if HAVE_BOEHMGC
 #define GC_INCLUDE_NEW
@@ -54,8 +55,6 @@ struct NixRepl
     , gc
     #endif
 {
-    std::string curDir;
-
     size_t debugTraceIndex;
 
     Strings loadedFiles;
@@ -113,7 +112,6 @@ NixRepl::NixRepl(const Strings & searchPath, nix::ref<Store> store, ref<EvalStat
     , staticEnv(new StaticEnv(false, state->staticBaseEnv.get()))
     , historyFile(getDataDir() + "/nix/repl-history")
 {
-    curDir = absPath(".");
 }
 
 
@@ -425,6 +423,7 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
 }
 
 
+// FIXME: DRY and match or use the parser
 static bool isVarName(std::string_view s)
 {
     if (s.size() == 0) return false;
@@ -592,14 +591,14 @@ bool NixRepl::processLine(std::string line)
         Value v;
         evalString(arg, v);
 
-        const auto [path, line] = [&] () -> std::pair<Path, uint32_t> {
+        const auto [path, line] = [&] () -> std::pair<SourcePath, uint32_t> {
             if (v.type() == nPath || v.type() == nString) {
-                PathSet context;
+                NixStringContext context;
                 auto path = state->coerceToPath(noPos, v, context, "while evaluating the filename to edit");
                 return {path, 0};
             } else if (v.isLambda()) {
                 auto pos = state->positions[v.lambda.fun->pos];
-                if (auto path = std::get_if<Path>(&pos.origin))
+                if (auto path = std::get_if<SourcePath>(&pos.origin))
                     return {*path, pos.line};
                 else
                     throw Error("'%s' cannot be shown in an editor", pos);
@@ -874,8 +873,7 @@ void NixRepl::addVarToScope(const Symbol name, Value & v)
 
 Expr * NixRepl::parseString(std::string s)
 {
-    Expr * e = state->parseExprFromString(std::move(s), curDir, staticEnv);
-    return e;
+    return state->parseExprFromString(std::move(s), state->rootPath(CanonPath::fromCwd()), staticEnv);
 }
 
 
@@ -894,17 +892,6 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
 }
 
 
-std::ostream & printStringValue(std::ostream & str, const char * string) {
-    str << "\"";
-    for (const char * i = string; *i; i++)
-        if (*i == '\"' || *i == '\\') str << "\\" << *i;
-        else if (*i == '\n') str << "\\n";
-        else if (*i == '\r') str << "\\r";
-        else if (*i == '\t') str << "\\t";
-        else str << *i;
-    str << "\"";
-    return str;
-}
 
 
 // FIXME: lot of cut&paste from Nix's eval.cc.
@@ -922,17 +909,19 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
         break;
 
     case nBool:
-        str << ANSI_CYAN << (v.boolean ? "true" : "false") << ANSI_NORMAL;
+        str << ANSI_CYAN;
+        printLiteralBool(str, v.boolean);
+        str << ANSI_NORMAL;
         break;
 
     case nString:
         str << ANSI_WARNING;
-        printStringValue(str, v.string.s);
+        printLiteralString(str, v.string.s);
         str << ANSI_NORMAL;
         break;
 
     case nPath:
-        str << ANSI_GREEN << v.path << ANSI_NORMAL; // !!! escaping?
+        str << ANSI_GREEN << v.path().to_string() << ANSI_NORMAL; // !!! escaping?
         break;
 
     case nNull:
@@ -947,7 +936,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
         if (isDrv) {
             str << "«derivation ";
             Bindings::iterator i = v.attrs->find(state->sDrvPath);
-            PathSet context;
+            NixStringContext context;
             if (i != v.attrs->end())
                 str << state->store->printStorePath(state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation"));
             else
@@ -964,10 +953,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
                 sorted.emplace(state->symbols[i.name], i.value);
 
             for (auto & i : sorted) {
-                if (isVarName(i.first))
-                    str << i.first;
-                else
-                    printStringValue(str, i.first.c_str());
+                printAttributeName(str, i.first);
                 str << " = ";
                 if (seen.count(i.second))
                     str << "«repeated»";

src/libexpr/attr-path.cc

@@ -106,7 +106,7 @@ std::pair<Value *, PosIdx> findAlongAttrPath(EvalState & state, const std::strin
 }
 
 
-std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what)
+std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what)
 {
     Value * v2;
     try {
@@ -118,21 +118,25 @@ std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value &
 
     // FIXME: is it possible to extract the Pos object instead of doing this
     //        toString + parsing?
-    auto pos = state.forceString(*v2, noPos, "while evaluating the 'meta.position' attribute of a derivation");
+    NixStringContext context;
+    auto path = state.coerceToPath(noPos, *v2, context, "while evaluating the 'meta.position' attribute of a derivation");
 
-    auto colon = pos.rfind(':');
-    if (colon == std::string::npos)
-        throw ParseError("cannot parse meta.position attribute '%s'", pos);
+    auto fn = path.path.abs();
+
+    auto fail = [fn]() {
+        throw ParseError("cannot parse 'meta.position' attribute '%s'", fn);
+    };
 
-    std::string filename(pos, 0, colon);
-    unsigned int lineno;
     try {
-        lineno = std::stoi(std::string(pos, colon + 1, std::string::npos));
+        auto colon = fn.rfind(':');
+        if (colon == std::string::npos) fail();
+        std::string filename(fn, 0, colon);
+        auto lineno = std::stoi(std::string(fn, colon + 1, std::string::npos));
+        return {CanonPath(fn.substr(0, colon)), lineno};
     } catch (std::invalid_argument & e) {
-        throw ParseError("cannot parse line number '%s'", pos);
+        fail();
+        abort();
     }
-
-    return { std::move(filename), lineno };
 }
 
 

src/libexpr/attr-path.hh

@@ -20,7 +20,7 @@ std::pair<Value *, PosIdx> findAlongAttrPath(
 /**
  * Heuristic to find the filename and lineno or a nix value.
  */
-std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what);
+std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what);
 
 std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s);
 

src/libexpr/eval-cache.cc

@@ -47,7 +47,7 @@ struct AttrDb
     {
         auto state(_state->lock());
 
-        Path cacheDir = getCacheDir() + "/nix/eval-cache-v4";
+        Path cacheDir = getCacheDir() + "/nix/eval-cache-v5";
         createDirs(cacheDir);
 
         Path dbPath = cacheDir + "/" + fingerprint.to_string(Base16, false) + ".sqlite";
@@ -300,7 +300,7 @@ struct AttrDb
                 NixStringContext context;
                 if (!queryAttribute.isNull(3))
                     for (auto & s : tokenizeString<std::vector<std::string>>(queryAttribute.getStr(3), ";"))
-                        context.push_back(NixStringContextElem::parse(cfg, s));
+                        context.insert(NixStringContextElem::parse(s));
                 return {{rowId, string_t{queryAttribute.getStr(2), context}}};
             }
             case AttrType::Bool:
@@ -442,8 +442,10 @@ Value & AttrCursor::forceValue()
         if (v.type() == nString)
             cachedValue = {root->db->setString(getKey(), v.string.s, v.string.context),
                            string_t{v.string.s, {}}};
-        else if (v.type() == nPath)
-            cachedValue = {root->db->setString(getKey(), v.path), string_t{v.path, {}}};
+        else if (v.type() == nPath) {
+            auto path = v.path().path;
+            cachedValue = {root->db->setString(getKey(), path.abs()), string_t{path.abs(), {}}};
+        }
         else if (v.type() == nBool)
             cachedValue = {root->db->setBool(getKey(), v.boolean), v.boolean};
         else if (v.type() == nInt)
@@ -580,7 +582,7 @@ std::string AttrCursor::getString()
     if (v.type() != nString && v.type() != nPath)
         root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
 
-    return v.type() == nString ? v.string.s : v.path;
+    return v.type() == nString ? v.string.s : v.path().to_string();
 }
 
 string_t AttrCursor::getStringWithContext()
@@ -619,10 +621,13 @@ string_t AttrCursor::getStringWithContext()
 
     auto & v = forceValue();
 
-    if (v.type() == nString)
-        return {v.string.s, v.getContext(*root->state.store)};
+    if (v.type() == nString) {
+        NixStringContext context;
+        copyContext(v, context);
+        return {v.string.s, std::move(context)};
+    }
     else if (v.type() == nPath)
-        return {v.path, {}};
+        return {v.path().to_string(), {}};
     else
         root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
 }

src/libexpr/eval.cc

@@ -9,6 +9,7 @@
 #include "filetransfer.hh"
 #include "function-trace.hh"
 #include "profiles.hh"
+#include "print.hh"
 
 #include <algorithm>
 #include <chrono>
@@ -104,21 +105,13 @@ void Value::print(const SymbolTable & symbols, std::ostream & str,
         str << integer;
         break;
     case tBool:
-        str << (boolean ? "true" : "false");
+        printLiteralBool(str, boolean);
         break;
     case tString:
-        str << "\"";
-        for (const char * i = string.s; *i; i++)
-            if (*i == '\"' || *i == '\\') str << "\\" << *i;
-            else if (*i == '\n') str << "\\n";
-            else if (*i == '\r') str << "\\r";
-            else if (*i == '\t') str << "\\t";
-            else if (*i == '$' && *(i+1) == '{') str << "\\" << *i;
-            else str << *i;
-        str << "\"";
+        printLiteralString(str, string.s);
         break;
     case tPath:
-        str << path; // !!! escaping?
+        str << path().to_string(); // !!! escaping?
         break;
     case tNull:
         str << "null";
@@ -542,6 +535,7 @@ EvalState::EvalState(
     , sOutputSpecified(symbols.create("outputSpecified"))
     , repair(NoRepair)
     , emptyBindings(0)
+    , derivationInternal(rootPath(CanonPath("/builtin/derivation.nix")))
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , debugRepl(nullptr)
@@ -616,15 +610,14 @@ void EvalState::allowAndSetStorePathString(const StorePath & storePath, Value &
 {
     allowPath(storePath);
 
-    auto path = store->printStorePath(storePath);
-    v.mkString(path, PathSet({path}));
+    mkStorePathString(storePath, v);
 }
 
-Path EvalState::checkSourcePath(const Path & path_)
+SourcePath EvalState::checkSourcePath(const SourcePath & path_)
 {
     if (!allowedPaths) return path_;
 
-    auto i = resolvedPaths.find(path_);
+    auto i = resolvedPaths.find(path_.path.abs());
     if (i != resolvedPaths.end())
         return i->second;
 
@@ -634,9 +627,9 @@ Path EvalState::checkSourcePath(const Path & path_)
      * attacker can't append ../../... to a path that would be in allowedPaths
      * and thus leak symlink targets.
      */
-    Path abspath = canonPath(path_);
+    Path abspath = canonPath(path_.path.abs());
 
-    if (hasPrefix(abspath, corepkgsPrefix)) return abspath;
+    if (hasPrefix(abspath, corepkgsPrefix)) return CanonPath(abspath);
 
     for (auto & i : *allowedPaths) {
         if (isDirOrInDir(abspath, i)) {
@@ -654,11 +647,11 @@ Path EvalState::checkSourcePath(const Path & path_)
 
     /* Resolve symlinks. */
     debug("checking access to '%s'", abspath);
-    Path path = canonPath(abspath, true);
+    SourcePath path = CanonPath(canonPath(abspath, true));
 
     for (auto & i : *allowedPaths) {
-        if (isDirOrInDir(path, i)) {
-            resolvedPaths[path_] = path;
+        if (isDirOrInDir(path.path.abs(), i)) {
+            resolvedPaths.insert_or_assign(path_.path.abs(), path);
             return path;
         }
     }
@@ -686,12 +679,12 @@ void EvalState::checkURI(const std::string & uri)
     /* If the URI is a path, then check it against allowedPaths as
        well. */
     if (hasPrefix(uri, "/")) {
-        checkSourcePath(uri);
+        checkSourcePath(CanonPath(uri));
         return;
     }
 
     if (hasPrefix(uri, "file://")) {
-        checkSourcePath(std::string(uri, 7));
+        checkSourcePath(CanonPath(std::string(uri, 7)));
         return;
     }
 
@@ -699,7 +692,7 @@ void EvalState::checkURI(const std::string & uri)
 }
 
 
-Path EvalState::toRealPath(const Path & path, const PathSet & context)
+Path EvalState::toRealPath(const Path & path, const NixStringContext & context)
 {
     // FIXME: check whether 'path' is in 'context'.
     return
@@ -951,34 +944,34 @@ void Value::mkString(std::string_view s)
 }
 
 
-static void copyContextToValue(Value & v, const PathSet & context)
+static void copyContextToValue(Value & v, const NixStringContext & context)
 {
     if (!context.empty()) {
         size_t n = 0;
         v.string.context = (const char * *)
             allocBytes((context.size() + 1) * sizeof(char *));
         for (auto & i : context)
-            v.string.context[n++] = dupString(i.c_str());
+            v.string.context[n++] = dupString(i.to_string().c_str());
         v.string.context[n] = 0;
     }
 }
 
-void Value::mkString(std::string_view s, const PathSet & context)
+void Value::mkString(std::string_view s, const NixStringContext & context)
 {
     mkString(s);
     copyContextToValue(*this, context);
 }
 
-void Value::mkStringMove(const char * s, const PathSet & context)
+void Value::mkStringMove(const char * s, const NixStringContext & context)
 {
     mkString(s);
     copyContextToValue(*this, context);
 }
 
 
-void Value::mkPath(std::string_view s)
+void Value::mkPath(const SourcePath & path)
 {
-    mkPath(makeImmutableString(s));
+    mkPath(makeImmutableString(path.path.abs()));
 }
 
 
@@ -1034,9 +1027,9 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 void EvalState::mkPos(Value & v, PosIdx p)
 {
     auto pos = positions[p];
-    if (auto path = std::get_if<Path>(&pos.origin)) {
+    if (auto path = std::get_if<SourcePath>(&pos.origin)) {
         auto attrs = buildBindings(3);
-        attrs.alloc(sFile).mkString(*path);
+        attrs.alloc(sFile).mkString(path->path.abs());
         attrs.alloc(sLine).mkInt(pos.line);
         attrs.alloc(sColumn).mkInt(pos.column);
         v.mkAttrs(attrs);
@@ -1045,6 +1038,16 @@ void EvalState::mkPos(Value & v, PosIdx p)
 }
 
 
+void EvalState::mkStorePathString(const StorePath & p, Value & v)
+{
+    v.mkString(
+        store->printStorePath(p),
+        NixStringContext {
+            NixStringContextElem::Opaque { .path = p },
+        });
+}
+
+
 /* Create a thunk for the delayed computation of the given expression
    in the given environment.  But if the expression is a variable,
    then look it up right away.  This significantly reduces the number
@@ -1092,7 +1095,7 @@ Value * ExprPath::maybeThunk(EvalState & state, Env & env)
 }
 
 
-void EvalState::evalFile(const Path & path_, Value & v, bool mustBeTrivial)
+void EvalState::evalFile(const SourcePath & path_, Value & v, bool mustBeTrivial)
 {
     auto path = checkSourcePath(path_);
 
@@ -1102,7 +1105,7 @@ void EvalState::evalFile(const Path & path_, Value & v, bool mustBeTrivial)
         return;
     }
 
-    Path resolvedPath = resolveExprPath(path);
+    auto resolvedPath = resolveExprPath(path);
     if ((i = fileEvalCache.find(resolvedPath)) != fileEvalCache.end()) {
         v = i->second;
         return;
@@ -1130,8 +1133,8 @@ void EvalState::resetFileCache()
 
 
 void EvalState::cacheFile(
-    const Path & path,
-    const Path & resolvedPath,
+    const SourcePath & path,
+    const SourcePath & resolvedPath,
     Expr * e,
     Value & v,
     bool mustBeTrivial)
@@ -1145,7 +1148,7 @@ void EvalState::cacheFile(
                 *e,
                 this->baseEnv,
                 e->getPos() ? static_cast<std::shared_ptr<AbstractPos>>(positions[e->getPos()]) : nullptr,
-                "while evaluating the file '%1%':", resolvedPath)
+                "while evaluating the file '%1%':", resolvedPath.to_string())
             : nullptr;
 
         // Enforce that 'flake.nix' is a direct attrset, not a
@@ -1155,7 +1158,7 @@ void EvalState::cacheFile(
             error("file '%s' must be an attribute set", path).debugThrow<EvalError>();
         eval(e, v);
     } catch (Error & e) {
-        addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath);
+        addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath.to_string());
         throw;
     }
 
@@ -1416,8 +1419,8 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     } catch (Error & e) {
         if (pos2) {
             auto pos2r = state.positions[pos2];
-            auto origin = std::get_if<Path>(&pos2r.origin);
-            if (!(origin && *origin == state.derivationNixPath))
+            auto origin = std::get_if<SourcePath>(&pos2r.origin);
+            if (!(origin && *origin == state.derivationInternal))
                 state.addErrorTrace(e, pos2, "while evaluating the attribute '%1%'",
                     showAttrPath(state, env, attrPath));
         }
@@ -1907,7 +1910,7 @@ void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const Po
 
 void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     std::vector<BackedStringView> s;
     size_t sSize = 0;
     NixInt n = 0;
@@ -1990,7 +1993,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     else if (firstType == nPath) {
         if (!context.empty())
             state.error("a string that refers to a store path cannot be appended to a path").atPos(pos).withFrame(env, *this).debugThrow<EvalError>();
-        v.mkPath(canonPath(str()));
+        v.mkPath(CanonPath(canonPath(str())));
     } else
         v.mkStringMove(c_str(), context);
 }
@@ -2116,26 +2119,15 @@ std::string_view EvalState::forceString(Value & v, const PosIdx pos, std::string
 }
 
 
-void copyContext(const Value & v, PathSet & context)
+void copyContext(const Value & v, NixStringContext & context)
 {
     if (v.string.context)
         for (const char * * p = v.string.context; *p; ++p)
-            context.insert(*p);
+            context.insert(NixStringContextElem::parse(*p));
 }
 
 
-NixStringContext Value::getContext(const Store & store)
-{
-    NixStringContext res;
-    assert(internalType == tString);
-    if (string.context)
-        for (const char * * p = string.context; *p; ++p)
-            res.push_back(NixStringContextElem::parse(store, *p));
-    return res;
-}
-
-
-std::string_view EvalState::forceString(Value & v, PathSet & context, const PosIdx pos, std::string_view errorCtx)
+std::string_view EvalState::forceString(Value & v, NixStringContext & context, const PosIdx pos, std::string_view errorCtx)
 {
     auto s = forceString(v, pos, errorCtx);
     copyContext(v, context);
@@ -2165,7 +2157,7 @@ bool EvalState::isDerivation(Value & v)
 
 
 std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value & v,
-    PathSet & context, bool coerceMore, bool copyToStore)
+    NixStringContext & context, bool coerceMore, bool copyToStore)
 {
     auto i = v.attrs->find(sToString);
     if (i != v.attrs->end()) {
@@ -2179,8 +2171,14 @@ std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value &
     return {};
 }
 
-BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &context,
-    std::string_view errorCtx, bool coerceMore, bool copyToStore, bool canonicalizePath)
+BackedStringView EvalState::coerceToString(
+    const PosIdx pos,
+    Value & v,
+    NixStringContext & context,
+    std::string_view errorCtx,
+    bool coerceMore,
+    bool copyToStore,
+    bool canonicalizePath)
 {
     forceValue(v, pos);
 
@@ -2190,12 +2188,14 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &
     }
 
     if (v.type() == nPath) {
-        BackedStringView path(PathView(v.path));
-        if (canonicalizePath)
-            path = canonPath(*path);
-        if (copyToStore)
-            path = store->printStorePath(copyPathToStore(context, std::move(path).toOwned()));
-        return path;
+        return
+            !canonicalizePath && !copyToStore
+            ? // FIXME: hack to preserve path literals that end in a
+              // slash, as in /foo/${x}.
+              v._path
+            : copyToStore
+            ? store->printStorePath(copyPathToStore(context, v.path()))
+            : std::string(v.path().path.abs());
     }
 
     if (v.type() == nAttrs) {
@@ -2256,40 +2256,40 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &
 }
 
 
-StorePath EvalState::copyPathToStore(PathSet & context, const Path & path)
+StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePath & path)
 {
-    if (nix::isDerivation(path))
+    if (nix::isDerivation(path.path.abs()))
         error("file names are not allowed to end in '%1%'", drvExtension).debugThrow<EvalError>();
 
-    auto dstPath = [&]() -> StorePath
-    {
-        auto i = srcToStore.find(path);
-        if (i != srcToStore.end()) return i->second;
+    auto i = srcToStore.find(path);
 
-        auto dstPath = settings.readOnlyMode
-            ? store->computeStorePathForPath(std::string(baseNameOf(path)), checkSourcePath(path)).first
-            : store->addToStore(std::string(baseNameOf(path)), checkSourcePath(path), FileIngestionMethod::Recursive, htSHA256, defaultPathFilter, repair);
-        allowPath(dstPath);
-        srcToStore.insert_or_assign(path, dstPath);
-        printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
-        return dstPath;
-    }();
+    auto dstPath = i != srcToStore.end()
+        ? i->second
+        : [&]() {
+            auto dstPath = path.fetchToStore(store, path.baseName(), nullptr, repair);
+            allowPath(dstPath);
+            srcToStore.insert_or_assign(path, dstPath);
+            printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
+            return dstPath;
+        }();
 
-    context.insert(store->printStorePath(dstPath));
+    context.insert(NixStringContextElem::Opaque {
+        .path = dstPath
+    });
     return dstPath;
 }
 
 
-Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
+SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx)
 {
     auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (path == "" || path[0] != '/')
         error("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
-    return path;
+    return CanonPath(path);
 }
 
 
-StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
+StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx)
 {
     auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (auto storePath = store->maybeParseStorePath(path))
@@ -2328,7 +2328,7 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v
             return strcmp(v1.string.s, v2.string.s) == 0;
 
         case nPath:
-            return strcmp(v1.path, v2.path) == 0;
+            return strcmp(v1._path, v2._path) == 0;
 
         case nNull:
             return true;
@@ -2455,8 +2455,8 @@ void EvalState::printStats()
                     else
                         obj["name"] = nullptr;
                     if (auto pos = positions[fun->pos]) {
-                        if (auto path = std::get_if<Path>(&pos.origin))
-                            obj["file"] = *path;
+                        if (auto path = std::get_if<SourcePath>(&pos.origin))
+                            obj["file"] = path->to_string();
                         obj["line"] = pos.line;
                         obj["column"] = pos.column;
                     }
@@ -2470,8 +2470,8 @@ void EvalState::printStats()
                 for (auto & i : attrSelects) {
                     json obj = json::object();
                     if (auto pos = positions[i.first]) {
-                        if (auto path = std::get_if<Path>(&pos.origin))
-                            obj["file"] = *path;
+                        if (auto path = std::get_if<SourcePath>(&pos.origin))
+                            obj["file"] = path->to_string();
                         obj["line"] = pos.line;
                         obj["column"] = pos.column;
                     }
@@ -2496,7 +2496,7 @@ void EvalState::printStats()
 }
 
 
-std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
+std::string ExternalValueBase::coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
 {
     throw TypeError({
         .msg = hintfmt("cannot coerce %1% to a string", showType())

src/libexpr/eval.hh

@@ -8,6 +8,7 @@
 #include "symbol-table.hh"
 #include "config.hh"
 #include "experimental-features.hh"
+#include "input-accessor.hh"
 
 #include <map>
 #include <optional>
@@ -56,20 +57,14 @@ void printEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env &
 
 std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env);
 
-void copyContext(const Value & v, PathSet & context);
-
-
-/**
- * Cache for calls to addToStore(); maps source paths to the store
- * paths.
- */
-typedef std::map<Path, StorePath> SrcToStore;
+void copyContext(const Value & v, NixStringContext & context);
 
 
 std::string printValue(const EvalState & state, const Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
 
+// FIXME: maybe change this to an std::variant<SourcePath, URL>.
 typedef std::pair<std::string, std::string> SearchPathElem;
 typedef std::list<SearchPathElem> SearchPath;
 
@@ -137,8 +132,6 @@ public:
     SymbolTable symbols;
     PosTable positions;
 
-    static inline std::string derivationNixPath = "//builtin/derivation.nix";
-
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,
@@ -149,7 +142,6 @@ public:
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
         sPrefix,
         sOutputSpecified;
-    Symbol sDerivationNix;
 
     /**
      * If set, force copying files to the Nix store even if they
@@ -165,6 +157,8 @@ public:
 
     Bindings emptyBindings;
 
+    const SourcePath derivationInternal;
+
     /**
      * Store used to materialise .drv files.
      */
@@ -234,15 +228,18 @@ public:
     }
 
 private:
-    SrcToStore srcToStore;
+
+    /* Cache for calls to addToStore(); maps source paths to the store
+       paths. */
+    std::map<SourcePath, StorePath> srcToStore;
 
     /**
      * A cache from path names to parse trees.
      */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Expr *, std::less<Path>, traceable_allocator<std::pair<const Path, Expr *>>> FileParseCache;
+    typedef std::map<SourcePath, Expr *, std::less<SourcePath>, traceable_allocator<std::pair<const SourcePath, Expr *>>> FileParseCache;
 #else
-    typedef std::map<Path, Expr *> FileParseCache;
+    typedef std::map<SourcePath, Expr *> FileParseCache;
 #endif
     FileParseCache fileParseCache;
 
@@ -250,9 +247,9 @@ private:
      * A cache from path names to values.
      */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Value, std::less<Path>, traceable_allocator<std::pair<const Path, Value>>> FileEvalCache;
+    typedef std::map<SourcePath, Value, std::less<SourcePath>, traceable_allocator<std::pair<const SourcePath, Value>>> FileEvalCache;
 #else
-    typedef std::map<Path, Value> FileEvalCache;
+    typedef std::map<SourcePath, Value> FileEvalCache;
 #endif
     FileEvalCache fileEvalCache;
 
@@ -263,7 +260,7 @@ private:
     /**
      * Cache used by checkSourcePath().
      */
-    std::unordered_map<Path, Path> resolvedPaths;
+    std::unordered_map<Path, SourcePath> resolvedPaths;
 
     /**
      * Cache used by prim_match().
@@ -294,6 +291,12 @@ public:
 
     SearchPath getSearchPath() { return searchPath; }
 
+    /**
+     * Return a `SourcePath` that refers to `path` in the root
+     * filesystem.
+     */
+    SourcePath rootPath(CanonPath path);
+
     /**
      * Allow access to a path.
      */
@@ -314,7 +317,7 @@ public:
      * Check whether access to a path is allowed and throw an error if
      * not. Otherwise return the canonicalised path.
      */
-    Path checkSourcePath(const Path & path);
+    SourcePath checkSourcePath(const SourcePath & path);
 
     void checkURI(const std::string & uri);
 
@@ -327,19 +330,19 @@ public:
      * intended to distinguish between import-from-derivation and
      * sources stored in the actual /nix/store.
      */
-    Path toRealPath(const Path & path, const PathSet & context);
+    Path toRealPath(const Path & path, const NixStringContext & context);
 
     /**
      * Parse a Nix expression from the specified file.
      */
-    Expr * parseExprFromFile(const Path & path);
-    Expr * parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parseExprFromFile(const SourcePath & path);
+    Expr * parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv);
 
     /**
      * Parse a Nix expression from the specified string.
      */
-    Expr * parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv);
-    Expr * parseExprFromString(std::string s, const Path & basePath);
+    Expr * parseExprFromString(std::string s, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parseExprFromString(std::string s, const SourcePath & basePath);
 
     Expr * parseStdin();
 
@@ -348,14 +351,14 @@ public:
      * form. Optionally enforce that the top-level expression is
      * trivial (i.e. doesn't require arbitrary computation).
      */
-    void evalFile(const Path & path, Value & v, bool mustBeTrivial = false);
+    void evalFile(const SourcePath & path, Value & v, bool mustBeTrivial = false);
 
     /**
      * Like `evalFile`, but with an already parsed expression.
      */
     void cacheFile(
-        const Path & path,
-        const Path & resolvedPath,
+        const SourcePath & path,
+        const SourcePath & resolvedPath,
         Expr * e,
         Value & v,
         bool mustBeTrivial = false);
@@ -365,8 +368,8 @@ public:
     /**
      * Look up a file in the search path.
      */
-    Path findFile(const std::string_view path);
-    Path findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos = noPos);
+    SourcePath findFile(const std::string_view path);
+    SourcePath findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos = noPos);
 
     /**
      * If the specified search path element is a URI, download it.
@@ -423,7 +426,7 @@ public:
      */
     void forceFunction(Value & v, const PosIdx pos, std::string_view errorCtx);
     std::string_view forceString(Value & v, const PosIdx pos, std::string_view errorCtx);
-    std::string_view forceString(Value & v, PathSet & context, const PosIdx pos, std::string_view errorCtx);
+    std::string_view forceString(Value & v, NixStringContext & context, const PosIdx pos, std::string_view errorCtx);
     std::string_view forceStringNoCtx(Value & v, const PosIdx pos, std::string_view errorCtx);
 
     [[gnu::noinline]]
@@ -439,7 +442,7 @@ public:
     bool isDerivation(Value & v);
 
     std::optional<std::string> tryAttrsToString(const PosIdx pos, Value & v,
-        PathSet & context, bool coerceMore = false, bool copyToStore = true);
+        NixStringContext & context, bool coerceMore = false, bool copyToStore = true);
 
     /**
      * String coercion.
@@ -449,12 +452,12 @@ public:
      * booleans and lists to a string.  If `copyToStore` is set,
      * referenced paths are copied to the Nix store as a side effect.
      */
-    BackedStringView coerceToString(const PosIdx pos, Value & v, PathSet & context,
+    BackedStringView coerceToString(const PosIdx pos, Value & v, NixStringContext & context,
         std::string_view errorCtx,
         bool coerceMore = false, bool copyToStore = true,
         bool canonicalizePath = true);
 
-    StorePath copyPathToStore(PathSet & context, const Path & path);
+    StorePath copyPathToStore(NixStringContext & context, const SourcePath & path);
 
     /**
      * Path coercion.
@@ -463,12 +466,12 @@ public:
      * path.  The result is guaranteed to be a canonicalised, absolute
      * path.  Nothing is copied to the store.
      */
-    Path coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
+    SourcePath coerceToPath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx);
 
     /**
      * Like coerceToPath, but the result must be a store path.
      */
-    StorePath coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
+    StorePath coerceToStorePath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx);
 
 public:
 
@@ -525,7 +528,7 @@ private:
         char * text,
         size_t length,
         Pos::Origin origin,
-        Path basePath,
+        const SourcePath & basePath,
         std::shared_ptr<StaticEnv> & staticEnv);
 
 public:
@@ -573,6 +576,12 @@ public:
     void mkThunk_(Value & v, Expr * expr);
     void mkPos(Value & v, PosIdx pos);
 
+    /* Create a string representing a store path.
+
+       The string is the printed store path with a context containing a single
+       `Opaque` element of that store path. */
+    void mkStorePathString(const StorePath & storePath, Value & v);
+
     void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
 
     /**
@@ -584,7 +593,7 @@ public:
      * Realise the given context, and return a mapping from the placeholders
      * used to construct the associated value to their final store path
      */
-    [[nodiscard]] StringMap realiseContext(const PathSet & context);
+    [[nodiscard]] StringMap realiseContext(const NixStringContext & context);
 
 private:
 
@@ -650,7 +659,7 @@ std::string showType(const Value & v);
 /**
  * If `path` refers to a directory, then append "/default.nix".
  */
-Path resolveExprPath(Path path);
+SourcePath resolveExprPath(const SourcePath & path);
 
 struct InvalidPathError : EvalError
 {

src/libexpr/flake/config.cc

@@ -31,7 +31,7 @@ static void writeTrustedList(const TrustedList & trustedList)
 
 void ConfigFile::apply()
 {
-    std::set<std::string> whitelist{"bash-prompt", "bash-prompt-prefix", "bash-prompt-suffix", "flake-registry"};
+    std::set<std::string> whitelist{"bash-prompt", "bash-prompt-prefix", "bash-prompt-suffix", "flake-registry", "commit-lockfile-summary"};
 
     for (auto & [name, value] : settings) {
 

src/libexpr/flake/flake.cc

@@ -222,9 +222,9 @@ static Flake getFlake(
         throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", lockedRef, lockedRef.subdir);
 
     Value vInfo;
-    state.evalFile(flakeFile, vInfo, true); // FIXME: symlink attack
+    state.evalFile(CanonPath(flakeFile), vInfo, true); // FIXME: symlink attack
 
-    expectType(state, nAttrs, vInfo, state.positions.add({flakeFile}, 1, 1));
+    expectType(state, nAttrs, vInfo, state.positions.add({CanonPath(flakeFile)}, 1, 1));
 
     if (auto description = vInfo.attrs->get(state.sDescription)) {
         expectType(state, nString, *description->value, description->pos);
@@ -265,7 +265,7 @@ static Flake getFlake(
                     state.symbols[setting.name],
                     std::string(state.forceStringNoCtx(*setting.value, setting.pos, "")));
             else if (setting.value->type() == nPath) {
-                PathSet emptyContext = {};
+                NixStringContext emptyContext = {};
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
                     state.coerceToString(setting.pos, *setting.value, emptyContext, "", false, true, true) .toOwned());
@@ -745,7 +745,7 @@ void callFlake(EvalState & state,
         state.vCallFlake = allocRootValue(state.allocValue());
         state.eval(state.parseExprFromString(
             #include "call-flake.nix.gen.hh"
-            , "/"), **state.vCallFlake);
+            , CanonPath::root), **state.vCallFlake);
     }
 
     state.callFunction(**state.vCallFlake, *vLocks, *vTmp1, noPos);

src/libexpr/flake/lockfile.cc

@@ -234,6 +234,11 @@ bool LockFile::operator ==(const LockFile & other) const
     return toJSON() == other.toJSON();
 }
 
+bool LockFile::operator !=(const LockFile & other) const
+{
+    return !(*this == other);
+}
+
 InputPath parseInputPath(std::string_view s)
 {
     InputPath path;

src/libexpr/flake/lockfile.hh

@@ -73,6 +73,9 @@ struct LockFile
     std::optional<FlakeRef> isUnlocked() const;
 
     bool operator ==(const LockFile & other) const;
+    // Needed for old gcc versions that don't synthesize it (like gcc 8.2.2
+    // that is still the default on aarch64-linux)
+    bool operator !=(const LockFile & other) const;
 
     std::shared_ptr<Node> findInput(const InputPath & path);
 

src/libexpr/get-drvs.cc

@@ -71,7 +71,7 @@ std::optional<StorePath> DrvInfo::queryDrvPath() const
 {
     if (!drvPath && attrs) {
         Bindings::iterator i = attrs->find(state->sDrvPath);
-        PathSet context;
+        NixStringContext context;
         if (i == attrs->end())
             drvPath = {std::nullopt};
         else
@@ -93,7 +93,7 @@ StorePath DrvInfo::queryOutPath() const
 {
     if (!outPath && attrs) {
         Bindings::iterator i = attrs->find(state->sOutPath);
-        PathSet context;
+        NixStringContext context;
         if (i != attrs->end())
             outPath = state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the output path of a derivation");
     }
@@ -124,7 +124,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
                     /* And evaluate its ‘outPath’ attribute. */
                     Bindings::iterator outPath = out->value->attrs->find(state->sOutPath);
                     if (outPath == out->value->attrs->end()) continue; // FIXME: throw error?
-                    PathSet context;
+                    NixStringContext context;
                     outputs.emplace(output, state->coerceToStorePath(outPath->pos, *outPath->value, context, "while evaluating an output path of a derivation"));
                 } else
                     outputs.emplace(output, std::nullopt);

src/libexpr/nixexpr.cc

@@ -3,6 +3,7 @@
 #include "eval.hh"
 #include "symbol-table.hh"
 #include "util.hh"
+#include "print.hh"
 
 #include <cstdlib>
 
@@ -31,9 +32,9 @@ struct PosAdapter : AbstractPos
                 // Get rid of the null terminators added by the parser.
                 return std::string(s.source->c_str());
             },
-            [](const Path & path) -> std::optional<std::string> {
+            [](const SourcePath & path) -> std::optional<std::string> {
                 try {
-                    return readFile(path);
+                    return path.readFile();
                 } catch (Error &) {
                     return std::nullopt;
                 }
@@ -47,7 +48,7 @@ struct PosAdapter : AbstractPos
             [&](const Pos::none_tag &) { out << "«none»"; },
             [&](const Pos::Stdin &) { out << "«stdin»"; },
             [&](const Pos::String & s) { out << "«string»"; },
-            [&](const Path & path) { out << path; }
+            [&](const SourcePath & path) { out << path; }
         }, origin);
     }
 };
@@ -60,45 +61,12 @@ Pos::operator std::shared_ptr<AbstractPos>() const
     return pos;
 }
 
-/* Displaying abstract syntax trees. */
-
-static void showString(std::ostream & str, std::string_view s)
-{
-    str << '"';
-    for (auto c : s)
-        if (c == '"' || c == '\\' || c == '$') str << "\\" << c;
-        else if (c == '\n') str << "\\n";
-        else if (c == '\r') str << "\\r";
-        else if (c == '\t') str << "\\t";
-        else str << c;
-    str << '"';
-}
-
+// FIXME: remove, because *symbols* are abstract and do not have a single
+//        textual representation; see printIdentifier()
 std::ostream & operator <<(std::ostream & str, const SymbolStr & symbol)
 {
     std::string_view s = symbol;
-
-    if (s.empty())
-        str << "\"\"";
-    else if (s == "if") // FIXME: handle other keywords
-        str << '"' << s << '"';
-    else {
-        char c = s[0];
-        if (!((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_')) {
-            showString(str, s);
-            return str;
-        }
-        for (auto c : s)
-            if (!((c >= 'a' && c <= 'z') ||
-                  (c >= 'A' && c <= 'Z') ||
-                  (c >= '0' && c <= '9') ||
-                  c == '_' || c == '\'' || c == '-')) {
-                showString(str, s);
-                return str;
-            }
-        str << s;
-    }
-    return str;
+    return printIdentifier(str, s);
 }
 
 void Expr::show(const SymbolTable & symbols, std::ostream & str) const
@@ -118,7 +86,7 @@ void ExprFloat::show(const SymbolTable & symbols, std::ostream & str) const
 
 void ExprString::show(const SymbolTable & symbols, std::ostream & str) const
 {
-    showString(str, s);
+    printLiteralString(str, s);
 }
 
 void ExprPath::show(const SymbolTable & symbols, std::ostream & str) const

src/libexpr/nixexpr.hh

@@ -34,7 +34,7 @@ struct Pos
     struct Stdin { ref<std::string> source; };
     struct String { ref<std::string> source; };
 
-    typedef std::variant<none_tag, Stdin, String, Path> Origin;
+    typedef std::variant<none_tag, Stdin, String, SourcePath> Origin;
 
     Origin origin;
 

src/libexpr/parser.y

@@ -31,7 +31,7 @@ namespace nix {
         EvalState & state;
         SymbolTable & symbols;
         Expr * result;
-        Path basePath;
+        SourcePath basePath;
         PosTable::Origin origin;
         std::optional<ErrorInfo> error;
     };
@@ -509,7 +509,7 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath({$1.p, $1.l}, data->basePath));
+    Path path(absPath({$1.p, $1.l}, data->basePath.path.abs()));
     /* add back in the trailing '/' to the first segment */
     if ($1.p[$1.l-1] == '/' && $1.l > 1)
       path += "/";
@@ -651,7 +651,7 @@ Expr * EvalState::parse(
     char * text,
     size_t length,
     Pos::Origin origin,
-    Path basePath,
+    const SourcePath & basePath,
     std::shared_ptr<StaticEnv> & staticEnv)
 {
     yyscan_t scanner;
@@ -675,48 +675,36 @@ Expr * EvalState::parse(
 }
 
 
-Path resolveExprPath(Path path)
+SourcePath resolveExprPath(const SourcePath & path)
 {
-    assert(path[0] == '/');
-
-    unsigned int followCount = 0, maxFollow = 1024;
-
     /* If `path' is a symlink, follow it.  This is so that relative
        path references work. */
-    struct stat st;
-    while (true) {
-        // Basic cycle/depth limit to avoid infinite loops.
-        if (++followCount >= maxFollow)
-            throw Error("too many symbolic links encountered while traversing the path '%s'", path);
-        st = lstat(path);
-        if (!S_ISLNK(st.st_mode)) break;
-        path = absPath(readLink(path), dirOf(path));
-    }
+    auto path2 = path.resolveSymlinks();
 
     /* If `path' refers to a directory, append `/default.nix'. */
-    if (S_ISDIR(st.st_mode))
-        path = canonPath(path + "/default.nix");
+    if (path2.lstat().type == InputAccessor::tDirectory)
+        return path2 + "default.nix";
 
-    return path;
+    return path2;
 }
 
 
-Expr * EvalState::parseExprFromFile(const Path & path)
+Expr * EvalState::parseExprFromFile(const SourcePath & path)
 {
     return parseExprFromFile(path, staticBaseEnv);
 }
 
 
-Expr * EvalState::parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv)
 {
-    auto buffer = readFile(path);
-    // readFile should have left some extra space for terminators
+    auto buffer = path.readFile();
+    // readFile hopefully have left some extra space for terminators
     buffer.append("\0\0", 2);
-    return parse(buffer.data(), buffer.size(), path, dirOf(path), staticEnv);
+    return parse(buffer.data(), buffer.size(), Pos::Origin(path), path.parent(), staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s_, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromString(std::string s_, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
     auto s = make_ref<std::string>(std::move(s_));
     s->append("\0\0", 2);
@@ -724,7 +712,7 @@ Expr * EvalState::parseExprFromString(std::string s_, const Path & basePath, std
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath)
+Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath)
 {
     return parseExprFromString(std::move(s), basePath, staticBaseEnv);
 }
@@ -737,7 +725,7 @@ Expr * EvalState::parseStdin()
     // drainFD should have left some extra space for terminators
     buffer.append("\0\0", 2);
     auto s = make_ref<std::string>(std::move(buffer));
-    return parse(s->data(), s->size(), Pos::Stdin{.source = s}, absPath("."), staticBaseEnv);
+    return parse(s->data(), s->size(), Pos::Stdin{.source = s}, rootPath(CanonPath::fromCwd()), staticBaseEnv);
 }
 
 
@@ -757,13 +745,13 @@ void EvalState::addToSearchPath(const std::string & s)
 }
 
 
-Path EvalState::findFile(const std::string_view path)
+SourcePath EvalState::findFile(const std::string_view path)
 {
     return findFile(searchPath, path);
 }
 
 
-Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos)
+SourcePath EvalState::findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos)
 {
     for (auto & i : searchPath) {
         std::string suffix;
@@ -779,11 +767,11 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
         auto r = resolveSearchPathElem(i);
         if (!r.first) continue;
         Path res = r.second + suffix;
-        if (pathExists(res)) return canonPath(res);
+        if (pathExists(res)) return CanonPath(canonPath(res));
     }
 
     if (hasPrefix(path, "nix/"))
-        return concatStrings(corepkgsPrefix, path.substr(4));
+        return CanonPath(concatStrings(corepkgsPrefix, path.substr(4)));
 
     debugThrow(ThrownError({
         .msg = hintfmt(evalSettings.pureEval

src/libexpr/paths.cc

@@ -0,0 +1,10 @@
+#include "eval.hh"
+
+namespace nix {
+
+SourcePath EvalState::rootPath(CanonPath path)
+{
+    return std::move(path);
+}
+
+}

src/libexpr/primops.cc

@@ -38,17 +38,16 @@ namespace nix {
 InvalidPathError::InvalidPathError(const Path & path) :
     EvalError("path '%s' is not valid", path), path(path) {}
 
-StringMap EvalState::realiseContext(const PathSet & context)
+StringMap EvalState::realiseContext(const NixStringContext & context)
 {
     std::vector<DerivedPath::Built> drvs;
     StringMap res;
 
-    for (auto & c_ : context) {
+    for (auto & c : context) {
         auto ensureValid = [&](const StorePath & p) {
             if (!store->isValidPath(p))
                 debugThrowLastTrace(InvalidPathError(store->printStorePath(p)));
         };
-        auto c = NixStringContextElem::parse(*store, c_);
         std::visit(overloaded {
             [&](const NixStringContextElem::Built & b) {
                 drvs.push_back(DerivedPath::Built {
@@ -110,16 +109,16 @@ struct RealisePathFlags {
     bool checkForPureEval = true;
 };
 
-static Path realisePath(EvalState & state, const PosIdx pos, Value & v, const RealisePathFlags flags = {})
+static SourcePath realisePath(EvalState & state, const PosIdx pos, Value & v, const RealisePathFlags flags = {})
 {
-    PathSet context;
+    NixStringContext context;
 
     auto path = state.coerceToPath(noPos, v, context, "while realising the context of a path");
 
     try {
         StringMap rewrites = state.realiseContext(context);
 
-        auto realPath = state.toRealPath(rewriteStrings(path, rewrites), context);
+        auto realPath = state.rootPath(CanonPath(state.toRealPath(rewriteStrings(path.path.abs(), rewrites), context)));
 
         return flags.checkForPureEval
             ? state.checkSourcePath(realPath)
@@ -158,7 +157,12 @@ static void mkOutputString(
             /* FIXME: we need to depend on the basic derivation, not
                derivation */
             : downstreamPlaceholder(*state.store, drvPath, o.first),
-        {"!" + o.first + "!" + state.store->printStorePath(drvPath)});
+        NixStringContext {
+            NixStringContextElem::Built {
+                .drvPath = drvPath,
+                .output = o.first,
+            }
+        });
 }
 
 /* Load and evaluate an expression from path specified by the
@@ -166,28 +170,30 @@ static void mkOutputString(
 static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * vScope, Value & v)
 {
     auto path = realisePath(state, pos, vPath);
+    auto path2 = path.path.abs();
 
     // FIXME
     auto isValidDerivationInStore = [&]() -> std::optional<StorePath> {
-        if (!state.store->isStorePath(path))
+        if (!state.store->isStorePath(path2))
             return std::nullopt;
-        auto storePath = state.store->parseStorePath(path);
-        if (!(state.store->isValidPath(storePath) && isDerivation(path)))
+        auto storePath = state.store->parseStorePath(path2);
+        if (!(state.store->isValidPath(storePath) && isDerivation(path2)))
             return std::nullopt;
         return storePath;
     };
 
-    if (auto optStorePath = isValidDerivationInStore()) {
-        auto storePath = *optStorePath;
-        Derivation drv = state.store->readDerivation(storePath);
+    if (auto storePath = isValidDerivationInStore()) {
+        Derivation drv = state.store->readDerivation(*storePath);
         auto attrs = state.buildBindings(3 + drv.outputs.size());
-        attrs.alloc(state.sDrvPath).mkString(path, {"=" + path});
+        attrs.alloc(state.sDrvPath).mkString(path2, {
+            NixStringContextElem::DrvDeep { .drvPath = *storePath },
+        });
         attrs.alloc(state.sName).mkString(drv.env["name"]);
         auto & outputsVal = attrs.alloc(state.sOutputs);
         state.mkList(outputsVal, drv.outputs.size());
 
         for (const auto & [i, o] : enumerate(drv.outputs)) {
-            mkOutputString(state, attrs, storePath, drv, o);
+            mkOutputString(state, attrs, *storePath, drv, o);
             (outputsVal.listElems()[i] = state.allocValue())->mkString(o.first);
         }
 
@@ -198,7 +204,7 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
             state.vImportedDrvToDerivation = allocRootValue(state.allocValue());
             state.eval(state.parseExprFromString(
                 #include "imported-drv-to-derivation.nix.gen.hh"
-                , "/"), **state.vImportedDrvToDerivation);
+                , CanonPath::root), **state.vImportedDrvToDerivation);
         }
 
         state.forceFunction(**state.vImportedDrvToDerivation, pos, "while evaluating imported-drv-to-derivation.nix.gen.hh");
@@ -206,10 +212,10 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
         state.forceAttrs(v, pos, "while calling imported-drv-to-derivation.nix.gen.hh");
     }
 
-    else if (path == corepkgsPrefix + "fetchurl.nix") {
+    else if (path2 == corepkgsPrefix + "fetchurl.nix") {
         state.eval(state.parseExprFromString(
             #include "fetchurl.nix.gen.hh"
-            , "/"), v);
+            , CanonPath::root), v);
     }
 
     else {
@@ -330,7 +336,7 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
 
     std::string sym(state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.importNative"));
 
-    void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
+    void *handle = dlopen(path.path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
         state.debugThrowLastTrace(EvalError("could not open '%1%': %2%", path, dlerror()));
 
@@ -358,7 +364,7 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     auto count = args[0]->listSize();
     if (count == 0)
         state.error("at least one argument to 'exec' required").atPos(pos).debugThrow<EvalError>();
-    PathSet context;
+    NixStringContext context;
     auto program = state.coerceToString(pos, *elems[0], context,
             "while evaluating the first element of the argument passed to builtins.exec",
             false, false).toOwned();
@@ -378,7 +384,7 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     auto output = runProgram(program, true, commandArgs);
     Expr * parsed;
     try {
-        parsed = state.parseExprFromString(std::move(output), "/");
+        parsed = state.parseExprFromString(std::move(output), state.rootPath(CanonPath::root));
     } catch (Error & e) {
         e.addTrace(state.positions[pos], "while parsing the output from '%1%'", program);
         throw;
@@ -588,7 +594,7 @@ struct CompareValues
                 case nString:
                     return strcmp(v1->string.s, v2->string.s) < 0;
                 case nPath:
-                    return strcmp(v1->path, v2->path) < 0;
+                    return strcmp(v1->_path, v2->_path) < 0;
                 case nList:
                     // Lexicographic comparison
                     for (size_t i = 0;; i++) {
@@ -700,12 +706,14 @@ static RegisterPrimOp primop_genericClosure(RegisterPrimOp::Info {
     .arity = 1,
     .doc = R"(
       Take an *attrset* with values named `startSet` and `operator` in order to
-      return a *list of attrsets* by starting with the `startSet`, recursively
-      applying the `operator` function to each element. The *attrsets* in the
-      `startSet` and produced by the `operator` must each contain value named
-      `key` which are comparable to each other. The result is produced by
-      repeatedly calling the operator for each element encountered with a
-      unique key, terminating when no new elements are produced. For example,
+      return a *list of attrsets* by starting with the `startSet` and recursively
+      applying the `operator` function to each `item`. The *attrsets* in the
+      `startSet` and the *attrsets* produced by `operator` must contain a value
+      named `key` which is comparable. The result is produced by calling `operator`
+      for each `item` with a value for `key` that has not been called yet including
+      newly produced `item`s. The function terminates when no new `item`s are
+      produced. The resulting *list of attrsets* contains only *attrsets* with a
+      unique key. For example,
 
       ```
       builtins.genericClosure {
@@ -768,7 +776,7 @@ static RegisterPrimOp primop_abort({
     )",
     .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
-        PathSet context;
+        NixStringContext context;
         auto s = state.coerceToString(pos, *args[0], context,
                 "while evaluating the error message passed to builtins.abort").toOwned();
         state.debugThrowLastTrace(Abort("evaluation aborted with the following error message: '%1%'", s));
@@ -787,7 +795,7 @@ static RegisterPrimOp primop_throw({
     )",
     .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
-      PathSet context;
+      NixStringContext context;
       auto s = state.coerceToString(pos, *args[0], context,
               "while evaluating the error message passed to builtin.throw").toOwned();
       state.debugThrowLastTrace(ThrownError(s));
@@ -800,7 +808,7 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
         state.forceValue(*args[1], pos);
         v = *args[1];
     } catch (Error & e) {
-        PathSet context;
+        NixStringContext context;
         auto message = state.coerceToString(pos, *args[0], context,
                 "while evaluating the error message passed to builtins.addErrorContext",
                 false, false).toOwned();
@@ -1086,7 +1094,7 @@ drvName, Bindings * attrs, Value & v)
     Derivation drv;
     drv.name = drvName;
 
-    PathSet context;
+    NixStringContext context;
 
     bool contentAddressed = false;
     bool isImpure = false;
@@ -1232,8 +1240,7 @@ drvName, Bindings * attrs, Value & v)
     /* Everything in the context of the strings in the derivation
        attributes should be added as dependencies of the resulting
        derivation. */
-    for (auto & c_ : context) {
-        auto c = NixStringContextElem::parse(*state.store, c_);
+    for (auto & c : context) {
         std::visit(overloaded {
             /* Since this allows the builder to gain access to every
                path in the dependency graph of the derivation (including
@@ -1293,7 +1300,13 @@ drvName, Bindings * attrs, Value & v)
         auto h = newHashAllowEmpty(*outputHash, parseHashTypeOpt(outputHashAlgo));
 
         auto method = ingestionMethod.value_or(FileIngestionMethod::Flat);
-        auto outPath = state.store->makeFixedOutputPath(method, h, drvName);
+        auto outPath = state.store->makeFixedOutputPath(drvName, FixedOutputInfo {
+            .hash = {
+                .method = method,
+                .hash = h,
+            },
+            .references = {},
+        });
         drv.env["out"] = state.store->printStorePath(outPath);
         drv.outputs.insert_or_assign("out",
             DerivationOutput::CAFixed {
@@ -1386,7 +1399,9 @@ drvName, Bindings * attrs, Value & v)
     }
 
     auto result = state.buildBindings(1 + drv.outputs.size());
-    result.alloc(state.sDrvPath).mkString(drvPathS, {"=" + drvPathS});
+    result.alloc(state.sDrvPath).mkString(drvPathS, {
+        NixStringContextElem::DrvDeep { .drvPath = drvPath },
+    });
     for (auto & i : drv.outputs)
         mkOutputString(state, result, drvPath, drv, i);
 
@@ -1431,9 +1446,9 @@ static RegisterPrimOp primop_placeholder({
 /* Convert the argument to a path.  !!! obsolete? */
 static void prim_toPath(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
-    Path path = state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.toPath");
-    v.mkString(canonPath(path), context);
+    NixStringContext context;
+    auto path = state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.toPath");
+    v.mkString(path.path.abs(), context);
 }
 
 static RegisterPrimOp primop_toPath({
@@ -1462,22 +1477,23 @@ static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args,
             .errPos = state.positions[pos]
         }));
 
-    PathSet context;
-    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.storePath"));
+    NixStringContext context;
+    auto path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.storePath")).path;
     /* Resolve symlinks in ‘path’, unless ‘path’ itself is a symlink
        directly in the store.  The latter condition is necessary so
        e.g. nix-push does the right thing. */
-    if (!state.store->isStorePath(path)) path = canonPath(path, true);
-    if (!state.store->isInStore(path))
+    if (!state.store->isStorePath(path.abs()))
+        path = CanonPath(canonPath(path.abs(), true));
+    if (!state.store->isInStore(path.abs()))
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("path '%1%' is not in the Nix store", path),
             .errPos = state.positions[pos]
         }));
-    auto path2 = state.store->toStorePath(path).first;
+    auto path2 = state.store->toStorePath(path.abs()).first;
     if (!settings.readOnlyMode)
         state.store->ensurePath(path2);
-    context.insert(state.store->printStorePath(path2));
-    v.mkString(path, context);
+    context.insert(NixStringContextElem::Opaque { .path = path2 });
+    v.mkString(path.abs(), context);
 }
 
 static RegisterPrimOp primop_storePath({
@@ -1508,7 +1524,7 @@ static void prim_pathExists(EvalState & state, const PosIdx pos, Value * * args,
     auto path = realisePath(state, pos, *args[0], { .checkForPureEval = false });
 
     try {
-        v.mkBool(pathExists(state.checkSourcePath(path)));
+        v.mkBool(state.checkSourcePath(path).pathExists());
     } catch (SysError & e) {
         /* Don't give away info from errors while canonicalising
            ‘path’ in restricted mode. */
@@ -1532,7 +1548,7 @@ static RegisterPrimOp primop_pathExists({
    following the last slash. */
 static void prim_baseNameOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     v.mkString(baseNameOf(*state.coerceToString(pos, *args[0], context,
             "while evaluating the first argument passed to builtins.baseNameOf",
             false, false)), context);
@@ -1554,12 +1570,18 @@ static RegisterPrimOp primop_baseNameOf({
    of the argument. */
 static void prim_dirOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
-    auto path = state.coerceToString(pos, *args[0], context,
-            "while evaluating the first argument passed to builtins.dirOf",
+    state.forceValue(*args[0], pos);
+    if (args[0]->type() == nPath) {
+        auto path = args[0]->path();
+        v.mkPath(path.path.isRoot() ? path : path.parent());
+    } else {
+        NixStringContext context;
+        auto path = state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to 'builtins.dirOf'",
             false, false);
-    auto dir = dirOf(*path);
-    if (args[0]->type() == nPath) v.mkPath(dir); else v.mkString(dir, context);
+        auto dir = dirOf(*path);
+        v.mkString(dir, context);
+    }
 }
 
 static RegisterPrimOp primop_dirOf({
@@ -1577,13 +1599,13 @@ static RegisterPrimOp primop_dirOf({
 static void prim_readFile(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     auto path = realisePath(state, pos, *args[0]);
-    auto s = readFile(path);
+    auto s = path.readFile();
     if (s.find((char) 0) != std::string::npos)
         state.debugThrowLastTrace(Error("the contents of the file '%1%' cannot be represented as a Nix string", path));
     StorePathSet refs;
-    if (state.store->isInStore(path)) {
+    if (state.store->isInStore(path.path.abs())) {
         try {
-            refs = state.store->queryPathInfo(state.store->toStorePath(path).first)->references;
+            refs = state.store->queryPathInfo(state.store->toStorePath(path.path.abs()).first)->references;
         } catch (Error &) { // FIXME: should be InvalidPathError
         }
         // Re-scan references to filter down to just the ones that actually occur in the file.
@@ -1591,7 +1613,12 @@ static void prim_readFile(EvalState & state, const PosIdx pos, Value * * args, V
         refsSink << s;
         refs = refsSink.getResultPaths();
     }
-    auto context = state.store->printStorePathSet(refs);
+    NixStringContext context;
+    for (auto && p : std::move(refs)) {
+        context.insert(NixStringContextElem::Opaque {
+            .path = std::move((StorePath &&)p),
+        });
+    }
     v.mkString(s, context);
 }
 
@@ -1622,7 +1649,7 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
 
         i = getAttr(state, state.sPath, v2->attrs, "in an element of the __nixPath");
 
-        PathSet context;
+        NixStringContext context;
         auto path = state.coerceToString(pos, *i->value, context,
                 "while evaluating the `path` attribute of an element of the list passed to builtins.findFile",
                 false, false).toOwned();
@@ -1664,7 +1691,7 @@ static void prim_hashFile(EvalState & state, const PosIdx pos, Value * * args, V
 
     auto path = realisePath(state, pos, *args[1]);
 
-    v.mkString(hashFile(*ht, path).to_string(Base16, false));
+    v.mkString(hashString(*ht, path.readFile()).to_string(Base16, false));
 }
 
 static RegisterPrimOp primop_hashFile({
@@ -1678,26 +1705,20 @@ static RegisterPrimOp primop_hashFile({
     .fun = prim_hashFile,
 });
 
-
-/* Stringize a directory entry enum. Used by `readFileType' and `readDir'. */
-static const char * dirEntTypeToString(unsigned char dtType)
+static std::string_view fileTypeToString(InputAccessor::Type type)
 {
-    /* Enum DT_(DIR|LNK|REG|UNKNOWN) */
-    switch(dtType) {
-        case DT_REG: return "regular";   break;
-        case DT_DIR: return "directory"; break;
-        case DT_LNK: return "symlink";   break;
-        default:     return "unknown";   break;
-    }
-    return "unknown";  /* Unreachable */
+    return
+        type == InputAccessor::Type::tRegular ? "regular" :
+        type == InputAccessor::Type::tDirectory ? "directory" :
+        type == InputAccessor::Type::tSymlink ? "symlink" :
+        "unknown";
 }
 
-
 static void prim_readFileType(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     auto path = realisePath(state, pos, *args[0]);
     /* Retrieve the directory entry type and stringize it. */
-    v.mkString(dirEntTypeToString(getFileType(path)));
+    v.mkString(fileTypeToString(path.lstat().type));
 }
 
 static RegisterPrimOp primop_readFileType({
@@ -1718,8 +1739,7 @@ static void prim_readDir(EvalState & state, const PosIdx pos, Value * * args, Va
     // Retrieve directory entries for all nodes in a directory.
     // This is similar to `getFileType` but is optimized to reduce system calls
     // on many systems.
-    DirEntries entries = readDirectory(path);
-
+    auto entries = path.readDirectory();
     auto attrs = state.buildBindings(entries.size());
 
     // If we hit unknown directory entry types we may need to fallback to
@@ -1728,22 +1748,21 @@ static void prim_readDir(EvalState & state, const PosIdx pos, Value * * args, Va
     // `builtins.readFileType` application.
     Value * readFileType = nullptr;
 
-    for (auto & ent : entries) {
-        auto & attr = attrs.alloc(ent.name);
-        if (ent.type == DT_UNKNOWN) {
+    for (auto & [name, type] : entries) {
+        auto & attr = attrs.alloc(name);
+        if (!type) {
             // Some filesystems or operating systems may not be able to return
             // detailed node info quickly in this case we produce a thunk to
             // query the file type lazily.
             auto epath = state.allocValue();
-            Path path2 = path + "/" + ent.name;
-            epath->mkString(path2);
+            epath->mkPath(path + name);
             if (!readFileType)
                 readFileType = &state.getBuiltin("readFileType");
             attr.mkApp(readFileType, epath);
         } else {
             // This branch of the conditional is much more likely.
             // Here we just stringize the directory entry type.
-            attr.mkString(dirEntTypeToString(ent.type));
+            attr.mkString(fileTypeToString(*type));
         }
     }
 
@@ -1781,7 +1800,7 @@ static RegisterPrimOp primop_readDir({
 static void prim_toXML(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     std::ostringstream out;
-    PathSet context;
+    NixStringContext context;
     printValueAsXML(state, true, false, *args[0], out, context, pos);
     v.mkString(out.str(), context);
 }
@@ -1889,7 +1908,7 @@ static RegisterPrimOp primop_toXML({
 static void prim_toJSON(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     std::ostringstream out;
-    PathSet context;
+    NixStringContext context;
     printValueAsJSON(state, true, *args[0], pos, out, context);
     v.mkString(out.str(), context);
 }
@@ -1939,22 +1958,23 @@ static RegisterPrimOp primop_fromJSON({
    as an input by derivations. */
 static void prim_toFile(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     std::string name(state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.toFile"));
     std::string contents(state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.toFile"));
 
     StorePathSet refs;
 
-    for (auto path : context) {
-        if (path.at(0) != '/')
+    for (auto c : context) {
+        if (auto p = std::get_if<NixStringContextElem::Opaque>(&c))
+            refs.insert(p->path);
+        else
             state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt(
                     "in 'toFile': the file named '%1%' must not contain a reference "
                     "to a derivation but contains (%2%)",
-                    name, path),
+                    name, c.to_string()),
                 .errPos = state.positions[pos]
             }));
-        refs.insert(state.store->parseStorePath(path));
     }
 
     auto storePath = settings.readOnlyMode
@@ -2049,13 +2069,13 @@ static RegisterPrimOp primop_toFile({
 static void addPath(
     EvalState & state,
     const PosIdx pos,
-    const std::string & name,
+    std::string_view name,
     Path path,
     Value * filterFun,
     FileIngestionMethod method,
     const std::optional<Hash> expectedHash,
     Value & v,
-    const PathSet & context)
+    const NixStringContext & context)
 {
     try {
         // FIXME: handle CA derivation outputs (where path needs to
@@ -2077,7 +2097,7 @@ static void addPath(
 
         path = evalSettings.pureEval && expectedHash
             ? path
-            : state.checkSourcePath(path);
+            : state.checkSourcePath(CanonPath(path)).path.abs();
 
         PathFilter filter = filterFun ? ([&](const Path & path) {
             auto st = lstat(path);
@@ -2103,7 +2123,13 @@ static void addPath(
 
         std::optional<StorePath> expectedStorePath;
         if (expectedHash)
-            expectedStorePath = state.store->makeFixedOutputPath(method, *expectedHash, name);
+            expectedStorePath = state.store->makeFixedOutputPath(name, FixedOutputInfo {
+                .hash = {
+                    .method = method,
+                    .hash = *expectedHash,
+                },
+                .references = {},
+            });
 
         if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
             StorePath dstPath = settings.readOnlyMode
@@ -2123,10 +2149,11 @@ static void addPath(
 
 static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
-    Path path = state.coerceToPath(pos, *args[1], context, "while evaluating the second argument (the path to filter) passed to builtins.filterSource");
+    NixStringContext context;
+    auto path = state.coerceToPath(pos, *args[1], context,
+        "while evaluating the second argument (the path to filter) passed to builtins.filterSource");
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.filterSource");
-    addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
+    addPath(state, pos, path.baseName(), path.path.abs(), args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
 }
 
 static RegisterPrimOp primop_filterSource({
@@ -2186,18 +2213,19 @@ static RegisterPrimOp primop_filterSource({
 
 static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.path");
-    Path path;
+    std::optional<SourcePath> path;
     std::string name;
     Value * filterFun = nullptr;
     auto method = FileIngestionMethod::Recursive;
     std::optional<Hash> expectedHash;
-    PathSet context;
+    NixStringContext context;
+
+    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to 'builtins.path'");
 
     for (auto & attr : *args[0]->attrs) {
         auto n = state.symbols[attr.name];
         if (n == "path")
-            path = state.coerceToPath(attr.pos, *attr.value, context, "while evaluating the `path` attribute passed to builtins.path");
+            path.emplace(state.coerceToPath(attr.pos, *attr.value, context, "while evaluating the 'path' attribute passed to 'builtins.path'"));
         else if (attr.name == state.sName)
             name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `name` attribute passed to builtins.path");
         else if (n == "filter")
@@ -2212,15 +2240,15 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
                 .errPos = state.positions[attr.pos]
             }));
     }
-    if (path.empty())
+    if (!path)
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("missing required 'path' attribute in the first argument to builtins.path"),
             .errPos = state.positions[pos]
         }));
     if (name.empty())
-        name = baseNameOf(path);
+        name = path->baseName();
 
-    addPath(state, pos, name, path, filterFun, method, expectedHash, v, context);
+    addPath(state, pos, name, path->path.abs(), filterFun, method, expectedHash, v, context);
 }
 
 static RegisterPrimOp primop_path({
@@ -3526,7 +3554,7 @@ static RegisterPrimOp primop_lessThan({
    `"/nix/store/whatever..."'. */
 static void prim_toString(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     auto s = state.coerceToString(pos, *args[0], context,
             "while evaluating the first argument passed to builtins.toString",
             true, false);
@@ -3565,7 +3593,7 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value * * args,
 {
     int start = state.forceInt(*args[0], pos, "while evaluating the first argument (the start offset) passed to builtins.substring");
     int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
-    PathSet context;
+    NixStringContext context;
     auto s = state.coerceToString(pos, *args[2], context, "while evaluating the third argument (the string) passed to builtins.substring");
 
     if (start < 0)
@@ -3599,7 +3627,7 @@ static RegisterPrimOp primop_substring({
 
 static void prim_stringLength(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.stringLength");
     v.mkInt(s->size());
 }
@@ -3625,7 +3653,7 @@ static void prim_hashString(EvalState & state, const PosIdx pos, Value * * args,
             .errPos = state.positions[pos]
         }));
 
-    PathSet context; // discarded
+    NixStringContext context; // discarded
     auto s = state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.hashString");
 
     v.mkString(hashString(*ht, s).to_string(Base16, false));
@@ -3671,7 +3699,7 @@ void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 
         auto regex = state.regexCache->get(re);
 
-        PathSet context;
+        NixStringContext context;
         const auto str = state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.match");
 
         std::cmatch match;
@@ -3751,7 +3779,7 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 
         auto regex = state.regexCache->get(re);
 
-        PathSet context;
+        NixStringContext context;
         const auto str = state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.split");
 
         auto begin = std::cregex_iterator(str.begin(), str.end(), regex);
@@ -3848,7 +3876,7 @@ static RegisterPrimOp primop_split({
 
 static void prim_concatStringsSep(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
 
     auto sep = state.forceString(*args[0], context, pos, "while evaluating the first argument (the separator string) passed to builtins.concatStringsSep");
     state.forceList(*args[1], pos, "while evaluating the second argument (the list of strings to concat) passed to builtins.concatStringsSep");
@@ -3888,15 +3916,15 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
     for (auto elem : args[0]->listItems())
         from.emplace_back(state.forceString(*elem, pos, "while evaluating one of the strings to replace passed to builtins.replaceStrings"));
 
-    std::vector<std::pair<std::string, PathSet>> to;
+    std::vector<std::pair<std::string, NixStringContext>> to;
     to.reserve(args[1]->listSize());
     for (auto elem : args[1]->listItems()) {
-        PathSet ctx;
+        NixStringContext ctx;
         auto s = state.forceString(*elem, ctx, pos, "while evaluating one of the replacement strings passed to builtins.replaceStrings");
         to.emplace_back(s, std::move(ctx));
     }
 
-    PathSet context;
+    NixStringContext context;
     auto s = state.forceString(*args[2], context, pos, "while evaluating the third argument passed to builtins.replaceStrings");
 
     std::string res;
@@ -4138,7 +4166,6 @@ void EvalState::createBaseEnv()
 
     /* Add a wrapper around the derivation primop that computes the
        `drvPath' and `outPath' attributes lazily. */
-    sDerivationNix = symbols.create(derivationNixPath);
     auto vDerivation = allocValue();
     addConstant("derivation", vDerivation);
 
@@ -4155,7 +4182,7 @@ void EvalState::createBaseEnv()
         // the parser needs two NUL bytes as terminators; one of them
         // is implied by being a C string.
         "\0";
-    eval(parse(code, sizeof(code), derivationNixPath, "/", staticBaseEnv), *vDerivation);
+    eval(parse(code, sizeof(code), derivationInternal, {CanonPath::root}, staticBaseEnv), *vDerivation);
 }
 
 

src/libexpr/primops/context.cc

@@ -7,7 +7,7 @@ namespace nix {
 
 static void prim_unsafeDiscardStringContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.unsafeDiscardStringContext");
     v.mkString(*s);
 }
@@ -17,7 +17,7 @@ static RegisterPrimOp primop_unsafeDiscardStringContext("__unsafeDiscardStringCo
 
 static void prim_hasContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     state.forceString(*args[0], context, pos, "while evaluating the argument passed to builtins.hasContext");
     v.mkBool(!context.empty());
 }
@@ -33,17 +33,18 @@ static RegisterPrimOp primop_hasContext("__hasContext", 1, prim_hasContext);
    drv.inputDrvs. */
 static void prim_unsafeDiscardOutputDependency(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.unsafeDiscardOutputDependency");
 
-    PathSet context2;
-    for (auto && p : context) {
-        auto c = NixStringContextElem::parse(*state.store, p);
+    NixStringContext context2;
+    for (auto && c : context) {
         if (auto * ptr = std::get_if<NixStringContextElem::DrvDeep>(&c)) {
-            context2.emplace(state.store->printStorePath(ptr->drvPath));
+            context2.emplace(NixStringContextElem::Opaque {
+                .path = ptr->drvPath
+            });
         } else {
             /* Can reuse original item */
-            context2.emplace(std::move(p));
+            context2.emplace(std::move(c));
         }
     }
 
@@ -79,22 +80,21 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
         bool allOutputs = false;
         Strings outputs;
     };
-    PathSet context;
+    NixStringContext context;
     state.forceString(*args[0], context, pos, "while evaluating the argument passed to builtins.getContext");
     auto contextInfos = std::map<StorePath, ContextInfo>();
-    for (const auto & p : context) {
-        NixStringContextElem ctx = NixStringContextElem::parse(*state.store, p);
+    for (auto && i : context) {
         std::visit(overloaded {
-            [&](NixStringContextElem::DrvDeep & d) {
-                contextInfos[d.drvPath].allOutputs = true;
+            [&](NixStringContextElem::DrvDeep && d) {
+                contextInfos[std::move(d.drvPath)].allOutputs = true;
             },
-            [&](NixStringContextElem::Built & b) {
-                contextInfos[b.drvPath].outputs.emplace_back(std::move(b.output));
+            [&](NixStringContextElem::Built && b) {
+                contextInfos[std::move(b.drvPath)].outputs.emplace_back(std::move(b.output));
             },
-            [&](NixStringContextElem::Opaque & o) {
-                contextInfos[o.path].path = true;
+            [&](NixStringContextElem::Opaque && o) {
+                contextInfos[std::move(o.path)].path = true;
             },
-        }, ctx.raw());
+        }, ((NixStringContextElem &&) i).raw());
     }
 
     auto attrs = state.buildBindings(contextInfos.size());
@@ -129,7 +129,7 @@ static RegisterPrimOp primop_getContext("__getContext", 1, prim_getContext);
 */
 static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    PathSet context;
+    NixStringContext context;
     auto orig = state.forceString(*args[0], context, noPos, "while evaluating the first argument passed to builtins.appendContext");
 
     state.forceAttrs(*args[1], pos, "while evaluating the second argument passed to builtins.appendContext");
@@ -143,13 +143,16 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
                 .msg = hintfmt("context key '%s' is not a store path", name),
                 .errPos = state.positions[i.pos]
             });
+        auto namePath = state.store->parseStorePath(name);
         if (!settings.readOnlyMode)
-            state.store->ensurePath(state.store->parseStorePath(name));
+            state.store->ensurePath(namePath);
         state.forceAttrs(*i.value, i.pos, "while evaluating the value of a string context");
         auto iter = i.value->attrs->find(sPath);
         if (iter != i.value->attrs->end()) {
             if (state.forceBool(*iter->value, iter->pos, "while evaluating the `path` attribute of a string context"))
-                context.emplace(name);
+                context.emplace(NixStringContextElem::Opaque {
+                    .path = namePath,
+                });
         }
 
         iter = i.value->attrs->find(sAllOutputs);
@@ -161,7 +164,9 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
                         .errPos = state.positions[i.pos]
                     });
                 }
-                context.insert(concatStrings("=", name));
+                context.emplace(NixStringContextElem::DrvDeep {
+                    .drvPath = namePath,
+                });
             }
         }
 
@@ -176,7 +181,10 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
             }
             for (auto elem : iter->value->listItems()) {
                 auto outputName = state.forceStringNoCtx(*elem, iter->pos, "while evaluating an output name within a string context");
-                context.insert(concatStrings("!", outputName, "!", name));
+                context.emplace(NixStringContextElem::Built {
+                    .drvPath = namePath,
+                    .output = std::string { outputName },
+                });
             }
         }
     }

src/libexpr/primops/fetchClosure.cc

@@ -18,7 +18,7 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
         const auto & attrName = state.symbols[attr.name];
 
         if (attrName == "fromPath") {
-            PathSet context;
+            NixStringContext context;
             fromPath = state.coerceToStorePath(attr.pos, *attr.value, context,
                     "while evaluating the 'fromPath' attribute passed to builtins.fetchClosure");
         }
@@ -27,7 +27,7 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
             state.forceValue(*attr.value, attr.pos);
             toCA = true;
             if (attr.value->type() != nString || attr.value->string.s != std::string("")) {
-                PathSet context;
+                NixStringContext context;
                 toPath = state.coerceToStorePath(attr.pos, *attr.value, context,
                         "while evaluating the 'toPath' attribute passed to builtins.fetchClosure");
             }
@@ -114,8 +114,7 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
             });
     }
 
-    auto toPathS = state.store->printStorePath(*toPath);
-    v.mkString(toPathS, {toPathS});
+    state.mkStorePathString(*toPath, v);
 }
 
 static RegisterPrimOp primop_fetchClosure({

src/libexpr/primops/fetchMercurial.cc

@@ -13,7 +13,7 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
     std::optional<Hash> rev;
     std::optional<std::string> ref;
     std::string_view name = "source";
-    PathSet context;
+    NixStringContext context;
 
     state.forceValue(*args[0], pos);
 
@@ -73,8 +73,7 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
     auto [tree, input2] = input.fetch(state.store);
 
     auto attrs2 = state.buildBindings(8);
-    auto storePath = state.store->printStorePath(tree.storePath);
-    attrs2.alloc(state.sOutPath).mkString(storePath, {storePath});
+    state.mkStorePathString(tree.storePath, attrs2.alloc(state.sOutPath));
     if (input2.getRef())
         attrs2.alloc("branch").mkString(*input2.getRef());
     // Backward compatibility: set 'rev' to

src/libexpr/primops/fetchTree.cc

@@ -24,9 +24,8 @@ void emitTreeAttrs(
 
     auto attrs = state.buildBindings(8);
 
-    auto storePath = state.store->printStorePath(tree.storePath);
 
-    attrs.alloc(state.sOutPath).mkString(storePath, {storePath});
+    state.mkStorePathString(tree.storePath, attrs.alloc(state.sOutPath));
 
     // FIXME: support arbitrary input attributes.
 
@@ -107,7 +106,7 @@ static void fetchTree(
     const FetchTreeParams & params = FetchTreeParams{}
 ) {
     fetchers::Input input;
-    PathSet context;
+    NixStringContext context;
 
     state.forceValue(*args[0], pos);
 
@@ -243,10 +242,15 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
 
     // early exit if pinned and already in the store
     if (expectedHash && expectedHash->type == htSHA256) {
-        auto expectedPath =
-            unpack
-            ? state.store->makeFixedOutputPath(FileIngestionMethod::Recursive, *expectedHash, name, {})
-            : state.store->makeFixedOutputPath(FileIngestionMethod::Flat, *expectedHash, name, {});
+        auto expectedPath = state.store->makeFixedOutputPath(
+            name,
+            FixedOutputInfo {
+                .hash = {
+                    .method = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat,
+                    .hash = *expectedHash,
+                },
+                .references = {}
+            });
 
         if (state.store->isValidPath(expectedPath)) {
             state.allowAndSetStorePathString(expectedPath, v);

src/libexpr/print.cc

@@ -0,0 +1,78 @@
+#include "print.hh"
+
+namespace nix {
+
+std::ostream &
+printLiteralString(std::ostream & str, const std::string_view string)
+{
+    str << "\"";
+    for (auto i = string.begin(); i != string.end(); ++i) {
+        if (*i == '\"' || *i == '\\') str << "\\" << *i;
+        else if (*i == '\n') str << "\\n";
+        else if (*i == '\r') str << "\\r";
+        else if (*i == '\t') str << "\\t";
+        else if (*i == '$' && *(i+1) == '{') str << "\\" << *i;
+        else str << *i;
+    }
+    str << "\"";
+    return str;
+}
+
+std::ostream &
+printLiteralBool(std::ostream & str, bool boolean)
+{
+    str << (boolean ? "true" : "false");
+    return str;
+}
+
+std::ostream &
+printIdentifier(std::ostream & str, std::string_view s) {
+    if (s.empty())
+        str << "\"\"";
+    else if (s == "if") // FIXME: handle other keywords
+        str << '"' << s << '"';
+    else {
+        char c = s[0];
+        if (!((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_')) {
+            printLiteralString(str, s);
+            return str;
+        }
+        for (auto c : s)
+            if (!((c >= 'a' && c <= 'z') ||
+                  (c >= 'A' && c <= 'Z') ||
+                  (c >= '0' && c <= '9') ||
+                  c == '_' || c == '\'' || c == '-')) {
+                printLiteralString(str, s);
+                return str;
+            }
+        str << s;
+    }
+    return str;
+}
+
+// FIXME: keywords
+static bool isVarName(std::string_view s)
+{
+    if (s.size() == 0) return false;
+    char c = s[0];
+    if ((c >= '0' && c <= '9') || c == '-' || c == '\'') return false;
+    for (auto & i : s)
+        if (!((i >= 'a' && i <= 'z') ||
+              (i >= 'A' && i <= 'Z') ||
+              (i >= '0' && i <= '9') ||
+              i == '_' || i == '-' || i == '\''))
+            return false;
+    return true;
+}
+
+std::ostream &
+printAttributeName(std::ostream & str, std::string_view name) {
+    if (isVarName(name))
+        str << name;
+    else
+        printLiteralString(str, name);
+    return str;
+}
+
+
+}

src/libexpr/print.hh

@@ -0,0 +1,48 @@
+#pragma once
+/**
+ * @file
+ * @brief Common printing functions for the Nix language
+ *
+ * While most types come with their own methods for printing, they share some
+ * functions that are placed here.
+ */
+
+#include <iostream>
+
+namespace nix {
+    /**
+     * Print a string as a Nix string literal.
+     *
+     * Quotes and fairly minimal escaping are added.
+     *
+     * @param s The logical string
+     */
+    std::ostream & printLiteralString(std::ostream & o, std::string_view s);
+    inline std::ostream & printLiteralString(std::ostream & o, const char * s) {
+        return printLiteralString(o, std::string_view(s));
+    }
+    inline std::ostream & printLiteralString(std::ostream & o, const std::string & s) {
+        return printLiteralString(o, std::string_view(s));
+    }
+
+    /** Print `true` or `false`. */
+    std::ostream & printLiteralBool(std::ostream & o, bool b);
+
+    /**
+     * Print a string as an attribute name in the Nix expression language syntax.
+     *
+     * Prints a quoted string if necessary.
+     */
+    std::ostream & printAttributeName(std::ostream & o, std::string_view s);
+
+    /**
+     * Print a string as an identifier in the Nix expression language syntax.
+     *
+     * FIXME: "identifier" is ambiguous. Identifiers do not have a single
+     *        textual representation. They can be used in variable references,
+     *        let bindings, left-hand sides or attribute names in a select
+     *        expression, or something else entirely, like JSON. Use one of the
+     *        `print*` functions instead.
+     */
+    std::ostream & printIdentifier(std::ostream & o, std::string_view s);
+}

src/libexpr/tests/json.cc

@@ -8,7 +8,7 @@ namespace nix {
         protected:
             std::string getJSONValue(Value& value) {
                 std::stringstream ss;
-                PathSet ps;
+                NixStringContext ps;
                 printValueAsJSON(state, true, value, noPos, ss, ps);
                 return ss.str();
             }

src/libexpr/tests/libexpr.hh

@@ -28,7 +28,7 @@ namespace nix {
             }
             Value eval(std::string input, bool forceValue = true) {
                 Value v;
-                Expr * e = state.parseExprFromString(input, "");
+                Expr * e = state.parseExprFromString(input, state.rootPath(CanonPath::root));
                 assert(e);
                 state.eval(e, v);
                 if (forceValue)

src/libexpr/tests/local.mk

@@ -12,7 +12,7 @@ libexpr-tests_SOURCES := \
     $(wildcard $(d)/*.cc) \
     $(wildcard $(d)/value/*.cc)
 
-libexpr-tests_CXXFLAGS += -I src/libexpr -I src/libutil -I src/libstore -I src/libexpr/tests
+libexpr-tests_CXXFLAGS += -I src/libexpr -I src/libutil -I src/libstore -I src/libexpr/tests -I src/libfetchers
 
 libexpr-tests_LIBS = libstore-tests libutils-tests libexpr libutil libstore libfetchers
 

src/libexpr/tests/value/context.cc

@@ -8,69 +8,62 @@
 
 namespace nix {
 
-// Testing of trivial expressions
-struct NixStringContextElemTest : public LibExprTest {
-   const Store & store() const {
-       return *LibExprTest::store;
-   }
-};
-
-TEST_F(NixStringContextElemTest, empty_invalid) {
+TEST(NixStringContextElemTest, empty_invalid) {
     EXPECT_THROW(
-        NixStringContextElem::parse(store(), ""),
+        NixStringContextElem::parse(""),
         BadNixStringContextElem);
 }
 
-TEST_F(NixStringContextElemTest, single_bang_invalid) {
+TEST(NixStringContextElemTest, single_bang_invalid) {
     EXPECT_THROW(
-        NixStringContextElem::parse(store(), "!"),
+        NixStringContextElem::parse("!"),
         BadNixStringContextElem);
 }
 
-TEST_F(NixStringContextElemTest, double_bang_invalid) {
+TEST(NixStringContextElemTest, double_bang_invalid) {
     EXPECT_THROW(
-        NixStringContextElem::parse(store(), "!!/"),
+        NixStringContextElem::parse("!!/"),
         BadStorePath);
 }
 
-TEST_F(NixStringContextElemTest, eq_slash_invalid) {
+TEST(NixStringContextElemTest, eq_slash_invalid) {
     EXPECT_THROW(
-        NixStringContextElem::parse(store(), "=/"),
+        NixStringContextElem::parse("=/"),
         BadStorePath);
 }
 
-TEST_F(NixStringContextElemTest, slash_invalid) {
+TEST(NixStringContextElemTest, slash_invalid) {
     EXPECT_THROW(
-        NixStringContextElem::parse(store(), "/"),
+        NixStringContextElem::parse("/"),
         BadStorePath);
 }
 
-TEST_F(NixStringContextElemTest, opaque) {
-    std::string_view opaque = "/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x";
-    auto elem = NixStringContextElem::parse(store(), opaque);
+TEST(NixStringContextElemTest, opaque) {
+    std::string_view opaque = "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x";
+    auto elem = NixStringContextElem::parse(opaque);
     auto * p = std::get_if<NixStringContextElem::Opaque>(&elem);
     ASSERT_TRUE(p);
-    ASSERT_EQ(p->path, store().parseStorePath(opaque));
-    ASSERT_EQ(elem.to_string(store()), opaque);
+    ASSERT_EQ(p->path, StorePath { opaque });
+    ASSERT_EQ(elem.to_string(), opaque);
 }
 
-TEST_F(NixStringContextElemTest, drvDeep) {
-    std::string_view drvDeep = "=/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
-    auto elem = NixStringContextElem::parse(store(), drvDeep);
+TEST(NixStringContextElemTest, drvDeep) {
+    std::string_view drvDeep = "=g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
+    auto elem = NixStringContextElem::parse(drvDeep);
     auto * p = std::get_if<NixStringContextElem::DrvDeep>(&elem);
     ASSERT_TRUE(p);
-    ASSERT_EQ(p->drvPath, store().parseStorePath(drvDeep.substr(1)));
-    ASSERT_EQ(elem.to_string(store()), drvDeep);
+    ASSERT_EQ(p->drvPath, StorePath { drvDeep.substr(1) });
+    ASSERT_EQ(elem.to_string(), drvDeep);
 }
 
-TEST_F(NixStringContextElemTest, built) {
-    std::string_view built = "!foo!/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
-    auto elem = NixStringContextElem::parse(store(), built);
+TEST(NixStringContextElemTest, built) {
+    std::string_view built = "!foo!g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
+    auto elem = NixStringContextElem::parse(built);
     auto * p = std::get_if<NixStringContextElem::Built>(&elem);
     ASSERT_TRUE(p);
     ASSERT_EQ(p->output, "foo");
-    ASSERT_EQ(p->drvPath, store().parseStorePath(built.substr(5)));
-    ASSERT_EQ(elem.to_string(store()), built);
+    ASSERT_EQ(p->drvPath, StorePath { built.substr(5) });
+    ASSERT_EQ(elem.to_string(), built);
 }
 
 }
@@ -116,12 +109,12 @@ Gen<NixStringContextElem> Arbitrary<NixStringContextElem>::arbitrary()
 
 namespace nix {
 
-RC_GTEST_FIXTURE_PROP(
+RC_GTEST_PROP(
     NixStringContextElemTest,
     prop_round_rip,
     (const NixStringContextElem & o))
 {
-    RC_ASSERT(o == NixStringContextElem::parse(store(), o.to_string(store())));
+    RC_ASSERT(o == NixStringContextElem::parse(o.to_string()));
 }
 
 }

src/libexpr/value-to-json.cc

@@ -11,7 +11,7 @@
 namespace nix {
 using json = nlohmann::json;
 json printValueAsJSON(EvalState & state, bool strict,
-    Value & v, const PosIdx pos, PathSet & context, bool copyToStore)
+    Value & v, const PosIdx pos, NixStringContext & context, bool copyToStore)
 {
     checkInterrupt();
 
@@ -36,9 +36,10 @@ json printValueAsJSON(EvalState & state, bool strict,
 
         case nPath:
             if (copyToStore)
-                out = state.store->printStorePath(state.copyPathToStore(context, v.path));
+                out = state.store->printStorePath(
+                    state.copyPathToStore(context, v.path()));
             else
-                out = v.path;
+                out = v.path().path.abs();
             break;
 
         case nNull:
@@ -94,13 +95,13 @@ json printValueAsJSON(EvalState & state, bool strict,
 }
 
 void printValueAsJSON(EvalState & state, bool strict,
-    Value & v, const PosIdx pos, std::ostream & str, PathSet & context, bool copyToStore)
+    Value & v, const PosIdx pos, std::ostream & str, NixStringContext & context, bool copyToStore)
 {
     str << printValueAsJSON(state, strict, v, pos, context, copyToStore);
 }
 
 json ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
-    PathSet & context, bool copyToStore) const
+    NixStringContext & context, bool copyToStore) const
 {
     state.debugThrowLastTrace(TypeError("cannot convert %1% to JSON", showType()));
 }

src/libexpr/value-to-json.hh

@@ -11,9 +11,9 @@
 namespace nix {
 
 nlohmann::json printValueAsJSON(EvalState & state, bool strict,
-    Value & v, const PosIdx pos, PathSet & context, bool copyToStore = true);
+    Value & v, const PosIdx pos, NixStringContext & context, bool copyToStore = true);
 
 void printValueAsJSON(EvalState & state, bool strict,
-    Value & v, const PosIdx pos, std::ostream & str, PathSet & context, bool copyToStore = true);
+    Value & v, const PosIdx pos, std::ostream & str, NixStringContext & context, bool copyToStore = true);
 
 }

src/libexpr/value-to-xml.cc

@@ -18,21 +18,21 @@ static XMLAttrs singletonAttrs(const std::string & name, const std::string & val
 
 
 static void printValueAsXML(EvalState & state, bool strict, bool location,
-    Value & v, XMLWriter & doc, PathSet & context, PathSet & drvsSeen,
+    Value & v, XMLWriter & doc, NixStringContext & context, PathSet & drvsSeen,
     const PosIdx pos);
 
 
 static void posToXML(EvalState & state, XMLAttrs & xmlAttrs, const Pos & pos)
 {
-    if (auto path = std::get_if<Path>(&pos.origin))
-        xmlAttrs["path"] = *path;
+    if (auto path = std::get_if<SourcePath>(&pos.origin))
+        xmlAttrs["path"] = path->path.abs();
     xmlAttrs["line"] = fmt("%1%", pos.line);
     xmlAttrs["column"] = fmt("%1%", pos.column);
 }
 
 
 static void showAttrs(EvalState & state, bool strict, bool location,
-    Bindings & attrs, XMLWriter & doc, PathSet & context, PathSet & drvsSeen)
+    Bindings & attrs, XMLWriter & doc, NixStringContext & context, PathSet & drvsSeen)
 {
     StringSet names;
 
@@ -54,7 +54,7 @@ static void showAttrs(EvalState & state, bool strict, bool location,
 
 
 static void printValueAsXML(EvalState & state, bool strict, bool location,
-    Value & v, XMLWriter & doc, PathSet & context, PathSet & drvsSeen,
+    Value & v, XMLWriter & doc, NixStringContext & context, PathSet & drvsSeen,
     const PosIdx pos)
 {
     checkInterrupt();
@@ -78,7 +78,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
             break;
 
         case nPath:
-            doc.writeEmptyElement("path", singletonAttrs("value", v.path));
+            doc.writeEmptyElement("path", singletonAttrs("value", v.path().to_string()));
             break;
 
         case nNull:
@@ -166,7 +166,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
 
 void ExternalValueBase::printValueAsXML(EvalState & state, bool strict,
-    bool location, XMLWriter & doc, PathSet & context, PathSet & drvsSeen,
+    bool location, XMLWriter & doc, NixStringContext & context, PathSet & drvsSeen,
     const PosIdx pos) const
 {
     doc.writeEmptyElement("unevaluated");
@@ -174,7 +174,7 @@ void ExternalValueBase::printValueAsXML(EvalState & state, bool strict,
 
 
 void printValueAsXML(EvalState & state, bool strict, bool location,
-    Value & v, std::ostream & out, PathSet & context, const PosIdx pos)
+    Value & v, std::ostream & out, NixStringContext & context, const PosIdx pos)
 {
     XMLWriter doc(true, out);
     XMLOpenElement root(doc, "expr");

src/libexpr/value-to-xml.hh

@@ -10,6 +10,6 @@
 namespace nix {
 
 void printValueAsXML(EvalState & state, bool strict, bool location,
-    Value & v, std::ostream & out, PathSet & context, const PosIdx pos);
+    Value & v, std::ostream & out, NixStringContext & context, const PosIdx pos);
 
 }

src/libexpr/value.hh

@@ -5,6 +5,7 @@
 
 #include "symbol-table.hh"
 #include "value/context.hh"
+#include "input-accessor.hh"
 
 #if HAVE_BOEHMGC
 #include <gc/gc_allocator.h>
@@ -100,7 +101,7 @@ class ExternalValueBase
      * Coerce the value to a string. Defaults to uncoercable, i.e. throws an
      * error.
      */
-    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const;
+    virtual std::string coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const;
 
     /**
      * Compare to another value of the same type. Defaults to uncomparable,
@@ -112,13 +113,13 @@ class ExternalValueBase
      * Print the value as JSON. Defaults to unconvertable, i.e. throws an error
      */
     virtual nlohmann::json printValueAsJSON(EvalState & state, bool strict,
-        PathSet & context, bool copyToStore = true) const;
+        NixStringContext & context, bool copyToStore = true) const;
 
     /**
      * Print the value as XML. Defaults to unevaluated
      */
     virtual void printValueAsXML(EvalState & state, bool strict, bool location,
-        XMLWriter & doc, PathSet & context, PathSet & drvsSeen,
+        XMLWriter & doc, NixStringContext & context, PathSet & drvsSeen,
         const PosIdx pos) const;
 
     virtual ~ExternalValueBase()
@@ -188,7 +189,7 @@ public:
             const char * * context; // must be in sorted order
         } string;
 
-        const char * path;
+        const char * _path;
         Bindings * attrs;
         struct {
             size_t size;
@@ -268,19 +269,24 @@ public:
 
     void mkString(std::string_view s);
 
-    void mkString(std::string_view s, const PathSet & context);
+    void mkString(std::string_view s, const NixStringContext & context);
 
-    void mkStringMove(const char * s, const PathSet & context);
+    void mkStringMove(const char * s, const NixStringContext & context);
 
-    inline void mkPath(const char * s)
+    inline void mkString(const Symbol & s)
+    {
+        mkString(((const std::string &) s).c_str());
+    }
+
+    void mkPath(const SourcePath & path);
+
+    inline void mkPath(const char * path)
     {
         clearValue();
         internalType = tPath;
-        path = s;
+        _path = path;
     }
 
-    void mkPath(std::string_view s);
-
     inline void mkNull()
     {
         clearValue();
@@ -394,8 +400,6 @@ public:
      */
     bool isTrivial() const;
 
-    NixStringContext getContext(const Store &);
-
     auto listItems()
     {
         struct ListIterable
@@ -423,6 +427,18 @@ public:
         auto begin = listElems();
         return ConstListIterable { begin, begin + listSize() };
     }
+
+    SourcePath path() const
+    {
+        assert(internalType == tPath);
+        return SourcePath{CanonPath(_path)};
+    }
+
+    std::string_view str() const
+    {
+        assert(internalType == tString);
+        return std::string_view(string.s);
+    }
 };
 
 

src/libexpr/value/context.cc

@@ -1,11 +1,10 @@
 #include "value/context.hh"
-#include "store-api.hh"
 
 #include <optional>
 
 namespace nix {
 
-NixStringContextElem NixStringContextElem::parse(const Store & store, std::string_view s0)
+NixStringContextElem NixStringContextElem::parse(std::string_view s0)
 {
     std::string_view s = s0;
 
@@ -25,41 +24,41 @@ NixStringContextElem NixStringContextElem::parse(const Store & store, std::strin
                 "String content element beginning with '!' should have a second '!'");
         }
         return NixStringContextElem::Built {
-            .drvPath = store.parseStorePath(s.substr(index + 1)),
+            .drvPath = StorePath { s.substr(index + 1) },
             .output = std::string(s.substr(0, index)),
         };
     }
     case '=': {
         return NixStringContextElem::DrvDeep {
-            .drvPath = store.parseStorePath(s.substr(1)),
+            .drvPath = StorePath { s.substr(1) },
         };
     }
     default: {
         return NixStringContextElem::Opaque {
-            .path = store.parseStorePath(s),
+            .path = StorePath { s },
         };
     }
     }
 }
 
-std::string NixStringContextElem::to_string(const Store & store) const {
+std::string NixStringContextElem::to_string() const {
     return std::visit(overloaded {
         [&](const NixStringContextElem::Built & b) {
             std::string res;
             res += '!';
             res += b.output;
             res += '!';
-            res += store.printStorePath(b.drvPath);
+            res += b.drvPath.to_string();
             return res;
         },
         [&](const NixStringContextElem::DrvDeep & d) {
             std::string res;
             res += '=';
-            res += store.printStorePath(d.drvPath);
+            res += d.drvPath.to_string();
             return res;
         },
         [&](const NixStringContextElem::Opaque & o) {
-            return store.printStorePath(o.path);
+            return std::string { o.path.to_string() };
         },
     }, raw());
 }

src/libexpr/value/context.hh

@@ -26,8 +26,6 @@ public:
     }
 };
 
-class Store;
-
 /**
  * Plain opaque path to some store object.
  *
@@ -80,12 +78,15 @@ struct NixStringContextElem : _NixStringContextElem_Raw {
     using DrvDeep = NixStringContextElem_DrvDeep;
     using Built = NixStringContextElem_Built;
 
-    inline const Raw & raw() const {
+    inline const Raw & raw() const & {
         return static_cast<const Raw &>(*this);
     }
-    inline Raw & raw() {
+    inline Raw & raw() & {
         return static_cast<Raw &>(*this);
     }
+    inline Raw && raw() && {
+        return static_cast<Raw &&>(*this);
+    }
 
     /**
      * Decode a context string, one of:
@@ -93,10 +94,10 @@ struct NixStringContextElem : _NixStringContextElem_Raw {
      * - ‘=<path>’
      * - ‘!<name>!<path>’
      */
-    static NixStringContextElem parse(const Store & store, std::string_view s);
-    std::string to_string(const Store & store) const;
+    static NixStringContextElem parse(std::string_view s);
+    std::string to_string() const;
 };
 
-typedef std::vector<NixStringContextElem> NixStringContext;
+typedef std::set<NixStringContextElem> NixStringContext;
 
 }

src/libfetchers/fetchers.cc

@@ -210,7 +210,13 @@ StorePath Input::computeStorePath(Store & store) const
     auto narHash = getNarHash();
     if (!narHash)
         throw Error("cannot compute store path for unlocked input '%s'", to_string());
-    return store.makeFixedOutputPath(FileIngestionMethod::Recursive, *narHash, getName());
+    return store.makeFixedOutputPath(getName(), FixedOutputInfo {
+        .hash = {
+            .method = FileIngestionMethod::Recursive,
+            .hash = *narHash,
+        },
+        .references = {},
+    });
 }
 
 std::string Input::getType() const

src/libfetchers/github.cc

@@ -21,7 +21,7 @@ struct DownloadUrl
 };
 
 // A github, gitlab, or sourcehut host
-const static std::string hostRegexS = "[a-zA-Z0-9.]*"; // FIXME: check
+const static std::string hostRegexS = "[a-zA-Z0-9.-]*"; // FIXME: check
 std::regex hostRegex(hostRegexS, std::regex::ECMAScript);
 
 struct GitArchiveInputScheme : InputScheme

src/libfetchers/input-accessor.cc

@@ -0,0 +1,100 @@
+#include "input-accessor.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+std::ostream & operator << (std::ostream & str, const SourcePath & path)
+{
+    str << path.to_string();
+    return str;
+}
+
+std::string_view SourcePath::baseName() const
+{
+    return path.baseName().value_or("source");
+}
+
+SourcePath SourcePath::parent() const
+{
+    auto p = path.parent();
+    assert(p);
+    return std::move(*p);
+}
+
+InputAccessor::Stat SourcePath::lstat() const
+{
+    auto st = nix::lstat(path.abs());
+    return InputAccessor::Stat {
+        .type =
+            S_ISREG(st.st_mode) ? InputAccessor::tRegular :
+            S_ISDIR(st.st_mode) ? InputAccessor::tDirectory :
+            S_ISLNK(st.st_mode) ? InputAccessor::tSymlink :
+            InputAccessor::tMisc,
+        .isExecutable = S_ISREG(st.st_mode) && st.st_mode & S_IXUSR
+    };
+}
+
+std::optional<InputAccessor::Stat> SourcePath::maybeLstat() const
+{
+    // FIXME: merge these into one operation.
+    if (!pathExists())
+        return {};
+    return lstat();
+}
+
+InputAccessor::DirEntries SourcePath::readDirectory() const
+{
+    InputAccessor::DirEntries res;
+    for (auto & entry : nix::readDirectory(path.abs())) {
+        std::optional<InputAccessor::Type> type;
+        switch (entry.type) {
+        case DT_REG: type = InputAccessor::Type::tRegular; break;
+        case DT_LNK: type = InputAccessor::Type::tSymlink; break;
+        case DT_DIR: type = InputAccessor::Type::tDirectory; break;
+        }
+        res.emplace(entry.name, type);
+    }
+    return res;
+}
+
+StorePath SourcePath::fetchToStore(
+    ref<Store> store,
+    std::string_view name,
+    PathFilter * filter,
+    RepairFlag repair) const
+{
+    return
+        settings.readOnlyMode
+        ? store->computeStorePathForPath(name, path.abs(), FileIngestionMethod::Recursive, htSHA256, filter ? *filter : defaultPathFilter).first
+        : store->addToStore(name, path.abs(), FileIngestionMethod::Recursive, htSHA256, filter ? *filter : defaultPathFilter, repair);
+}
+
+SourcePath SourcePath::resolveSymlinks() const
+{
+    SourcePath res(CanonPath::root);
+
+    int linksAllowed = 1024;
+
+    for (auto & component : path) {
+        res.path.push(component);
+        while (true) {
+            if (auto st = res.maybeLstat()) {
+                if (!linksAllowed--)
+                    throw Error("infinite symlink recursion in path '%s'", path);
+                if (st->type != InputAccessor::tSymlink) break;
+                auto target = res.readLink();
+                if (hasPrefix(target, "/"))
+                    res = CanonPath(target);
+                else {
+                    res.path.pop();
+                    res.path.extend(CanonPath(target));
+                }
+            } else
+                break;
+        }
+    }
+
+    return res;
+}
+
+}

src/libfetchers/input-accessor.hh

@@ -0,0 +1,167 @@
+#pragma once
+
+#include "ref.hh"
+#include "types.hh"
+#include "archive.hh"
+#include "canon-path.hh"
+#include "repair-flag.hh"
+
+namespace nix {
+
+class StorePath;
+class Store;
+
+struct InputAccessor
+{
+    enum Type {
+      tRegular, tSymlink, tDirectory,
+      /**
+        Any other node types that may be encountered on the file system, such as device nodes, sockets, named pipe, and possibly even more exotic things.
+
+        Responsible for `"unknown"` from `builtins.readFileType "/dev/null"`.
+
+        Unlike `DT_UNKNOWN`, this must not be used for deferring the lookup of types.
+      */
+      tMisc
+    };
+
+    struct Stat
+    {
+        Type type = tMisc;
+        //uint64_t fileSize = 0; // regular files only
+        bool isExecutable = false; // regular files only
+    };
+
+    typedef std::optional<Type> DirEntry;
+
+    typedef std::map<std::string, DirEntry> DirEntries;
+};
+
+/**
+ * An abstraction for accessing source files during
+ * evaluation. Currently, it's just a wrapper around `CanonPath` that
+ * accesses files in the regular filesystem, but in the future it will
+ * support fetching files in other ways.
+ */
+struct SourcePath
+{
+    CanonPath path;
+
+    SourcePath(CanonPath path)
+        : path(std::move(path))
+    { }
+
+    std::string_view baseName() const;
+
+    /**
+     * Construct the parent of this `SourcePath`. Aborts if `this`
+     * denotes the root.
+     */
+    SourcePath parent() const;
+
+    /**
+     * If this `SourcePath` denotes a regular file (not a symlink),
+     * return its contents; otherwise throw an error.
+     */
+    std::string readFile() const
+    { return nix::readFile(path.abs()); }
+
+    /**
+     * Return whether this `SourcePath` denotes a file (of any type)
+     * that exists
+    */
+    bool pathExists() const
+    { return nix::pathExists(path.abs()); }
+
+    /**
+     * Return stats about this `SourcePath`, or throw an exception if
+     * it doesn't exist.
+     */
+    InputAccessor::Stat lstat() const;
+
+    /**
+     * Return stats about this `SourcePath`, or std::nullopt if it
+     * doesn't exist.
+     */
+    std::optional<InputAccessor::Stat> maybeLstat() const;
+
+    /**
+     * If this `SourcePath` denotes a directory (not a symlink),
+     * return its directory entries; otherwise throw an error.
+     */
+    InputAccessor::DirEntries readDirectory() const;
+
+    /**
+     * If this `SourcePath` denotes a symlink, return its target;
+     * otherwise throw an error.
+     */
+    std::string readLink() const
+    { return nix::readLink(path.abs()); }
+
+    /**
+     * Dump this `SourcePath` to `sink` as a NAR archive.
+     */
+    void dumpPath(
+        Sink & sink,
+        PathFilter & filter = defaultPathFilter) const
+    { return nix::dumpPath(path.abs(), sink, filter); }
+
+    /**
+     * Copy this `SourcePath` to the Nix store.
+     */
+    StorePath fetchToStore(
+        ref<Store> store,
+        std::string_view name = "source",
+        PathFilter * filter = nullptr,
+        RepairFlag repair = NoRepair) const;
+
+    /**
+     * Return the location of this path in the "real" filesystem, if
+     * it has a physical location.
+     */
+    std::optional<CanonPath> getPhysicalPath() const
+    { return path; }
+
+    std::string to_string() const
+    { return path.abs(); }
+
+    /**
+     * Append a `CanonPath` to this path.
+     */
+    SourcePath operator + (const CanonPath & x) const
+    { return {path + x}; }
+
+    /**
+     * Append a single component `c` to this path. `c` must not
+     * contain a slash. A slash is implicitly added between this path
+     * and `c`.
+     */
+    SourcePath operator + (std::string_view c) const
+    {  return {path + c}; }
+
+    bool operator == (const SourcePath & x) const
+    {
+        return path == x.path;
+    }
+
+    bool operator != (const SourcePath & x) const
+    {
+        return path != x.path;
+    }
+
+    bool operator < (const SourcePath & x) const
+    {
+        return path < x.path;
+    }
+
+    /**
+     * Resolve any symlinks in this `SourcePath` (including its
+     * parents). The result is a `SourcePath` in which no element is a
+     * symlink.
+     */
+    SourcePath resolveSymlinks() const;
+};
+
+std::ostream & operator << (std::ostream & str, const SourcePath & path);
+
+}

src/libfetchers/tarball.cc

@@ -71,15 +71,19 @@ DownloadFileResult downloadFile(
         dumpString(res.data, sink);
         auto hash = hashString(htSHA256, res.data);
         ValidPathInfo info {
-            store->makeFixedOutputPath(FileIngestionMethod::Flat, hash, name),
+            *store,
+            name,
+            FixedOutputInfo {
+                .hash = {
+                    .method = FileIngestionMethod::Flat,
+                    .hash = hash,
+                },
+                .references = {},
+            },
             hashString(htSHA256, sink.s),
         };
         info.narSize = sink.s.size();
-        info.ca = FixedOutputHash {
-            .method = FileIngestionMethod::Flat,
-            .hash = hash,
-        };
-        auto source = StringSource(sink.s);
+        auto source = StringSource { sink.s };
         store->addToStore(info, source, NoRepair, NoCheckSigs);
         storePath = std::move(info.path);
     }

src/libmain/common-args.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include "args.hh"
+#include "repair-flag.hh"
 
 namespace nix {
 
@@ -49,4 +50,21 @@ struct MixJSON : virtual Args
     }
 };
 
+struct MixRepair : virtual Args
+{
+    RepairFlag repair = NoRepair;
+
+    MixRepair()
+    {
+        addFlag({
+            .longName = "repair",
+            .description =
+                "During evaluation, rewrite missing or corrupted files in the Nix store. "
+                "During building, rebuild missing or corrupted store paths.",
+            .category = miscCategory,
+            .handler = {&repair, Repair},
+        });
+    }
+};
+
 }

src/libmain/shared.cc

@@ -10,7 +10,6 @@
 #include <cctype>
 #include <exception>
 #include <iostream>
-#include <mutex>
 
 #include <cstdlib>
 #include <sys/time.h>
@@ -20,16 +19,9 @@
 #ifdef __linux__
 #include <features.h>
 #endif
-#ifdef __GLIBC__
-#include <gnu/lib-names.h>
-#include <nss.h>
-#include <dlfcn.h>
-#endif
 
 #include <openssl/crypto.h>
 
-#include <sodium.h>
-
 
 namespace nix {
 
@@ -115,57 +107,6 @@ std::string getArg(const std::string & opt,
     return *i;
 }
 
-
-#if OPENSSL_VERSION_NUMBER < 0x10101000L
-/* OpenSSL is not thread-safe by default - it will randomly crash
-   unless the user supplies a mutex locking function. So let's do
-   that. */
-static std::vector<std::mutex> opensslLocks;
-
-static void opensslLockCallback(int mode, int type, const char * file, int line)
-{
-    if (mode & CRYPTO_LOCK)
-        opensslLocks[type].lock();
-    else
-        opensslLocks[type].unlock();
-}
-#endif
-
-static std::once_flag dns_resolve_flag;
-
-static void preloadNSS() {
-    /* builtin:fetchurl can trigger a DNS lookup, which with glibc can trigger a dynamic library load of
-       one of the glibc NSS libraries in a sandboxed child, which will fail unless the library's already
-       been loaded in the parent. So we force a lookup of an invalid domain to force the NSS machinery to
-       load its lookup libraries in the parent before any child gets a chance to. */
-    std::call_once(dns_resolve_flag, []() {
-#ifdef __GLIBC__
-        /* On linux, glibc will run every lookup through the nss layer.
-         * That means every lookup goes, by default, through nscd, which acts as a local
-         * cache.
-         * Because we run builds in a sandbox, we also remove access to nscd otherwise
-         * lookups would leak into the sandbox.
-         *
-         * But now we have a new problem, we need to make sure the nss_dns backend that
-         * does the dns lookups when nscd is not available is loaded or available.
-         *
-         * We can't make it available without leaking nix's environment, so instead we'll
-         * load the backend, and configure nss so it does not try to run dns lookups
-         * through nscd.
-         *
-         * This is technically only used for builtins:fetch* functions so we only care
-         * about dns.
-         *
-         * All other platforms are unaffected.
-         */
-        if (!dlopen(LIBNSS_DNS_SO, RTLD_NOW))
-            warn("unable to load nss_dns backend");
-        // FIXME: get hosts entry from nsswitch.conf.
-        __nss_configure_lookup("hosts", "files dns");
-#endif
-    });
-}
-
 static void sigHandler(int signo) { }
 
 
@@ -177,16 +118,7 @@ void initNix()
     std::cerr.rdbuf()->pubsetbuf(buf, sizeof(buf));
 #endif
 
-#if OPENSSL_VERSION_NUMBER < 0x10101000L
-    /* Initialise OpenSSL locking. */
-    opensslLocks = std::vector<std::mutex>(CRYPTO_num_locks());
-    CRYPTO_set_locking_callback(opensslLockCallback);
-#endif
-
-    if (sodium_init() == -1)
-        throw Error("could not initialise libsodium");
-
-    loadConfFile();
+    initLibStore();
 
     startSignalHandlerThread();
 
@@ -223,7 +155,10 @@ void initNix()
     if (sigaction(SIGTRAP, &act, 0)) throw SysError("handling SIGTRAP");
 #endif
 
-    /* Register a SIGSEGV handler to detect stack overflows. */
+    /* Register a SIGSEGV handler to detect stack overflows.
+       Why not initLibExpr()? initGC() is essentially that, but
+       detectStackOverflow is not an instance of the init function concept, as
+       it may have to be invoked more than once per process. */
     detectStackOverflow();
 
     /* There is no privacy in the Nix system ;-)  At least not for
@@ -236,16 +171,6 @@ void initNix()
     gettimeofday(&tv, 0);
     srandom(tv.tv_usec);
 
-    /* On macOS, don't use the per-session TMPDIR (as set e.g. by
-       sshd). This breaks build users because they don't have access
-       to the TMPDIR, in particular in ‘nix-store --serve’. */
-#if __APPLE__
-    if (hasPrefix(getEnv("TMPDIR").value_or("/tmp"), "/var/folders/"))
-        unsetenv("TMPDIR");
-#endif
-
-    preloadNSS();
-    initLibStore();
 }
 
 

src/libstore/binary-cache-store.cc

@@ -306,11 +306,22 @@ StorePath BinaryCacheStore::addToStoreFromDump(Source & dump, std::string_view n
         unsupported("addToStoreFromDump");
     return addToStoreCommon(dump, repair, CheckSigs, [&](HashResult nar) {
         ValidPathInfo info {
-            makeFixedOutputPath(method, nar.first, name, references),
+            *this,
+            name,
+            FixedOutputInfo {
+                .hash = {
+                    .method = method,
+                    .hash = nar.first,
+                },
+                .references = {
+                    .others = references,
+                    // caller is not capable of creating a self-reference, because this is content-addressed without modulus
+                    .self = false,
+                },
+            },
             nar.first,
         };
         info.narSize = nar.second;
-        info.references = references;
         return info;
     })->path;
 }
@@ -414,15 +425,22 @@ StorePath BinaryCacheStore::addToStore(
     });
     return addToStoreCommon(*source, repair, CheckSigs, [&](HashResult nar) {
         ValidPathInfo info {
-            makeFixedOutputPath(method, h, name, references),
+            *this,
+            name,
+            FixedOutputInfo {
+                .hash = {
+                    .method = method,
+                    .hash = h,
+                },
+                .references = {
+                    .others = references,
+                    // caller is not capable of creating a self-reference, because this is content-addressed without modulus
+                    .self = false,
+                },
+            },
             nar.first,
         };
         info.narSize = nar.second;
-        info.references = references;
-        info.ca = FixedOutputHash {
-            .method = method,
-            .hash = h,
-        };
         return info;
     })->path;
 }
@@ -434,7 +452,7 @@ StorePath BinaryCacheStore::addTextToStore(
     RepairFlag repair)
 {
     auto textHash = hashString(htSHA256, s);
-    auto path = makeTextPath(name, textHash, references);
+    auto path = makeTextPath(name, TextInfo { { textHash }, references });
 
     if (!repair && isValidPath(path))
         return path;
@@ -443,10 +461,16 @@ StorePath BinaryCacheStore::addTextToStore(
     dumpString(s, sink);
     StringSource source(sink.s);
     return addToStoreCommon(source, repair, CheckSigs, [&](HashResult nar) {
-        ValidPathInfo info { path, nar.first };
+        ValidPathInfo info {
+            *this,
+            std::string { name },
+            TextInfo {
+                { .hash = textHash },
+                references,
+            },
+            nar.first,
+        };
         info.narSize = nar.second;
-        info.ca = TextHash { textHash };
-        info.references = references;
         return info;
     })->path;
 }

src/libstore/build-result.hh

@@ -83,16 +83,11 @@ struct BuildResult
      */
     bool isNonDeterministic = false;
 
-    /**
-     * The derivation we built or the store path we substituted.
-     */
-    DerivedPath path;
-
     /**
      * For derivations, a mapping from the names of the wanted outputs
      * to actual paths.
      */
-    DrvOutputs builtOutputs;
+    SingleDrvOutputs builtOutputs;
 
     /**
      * The start/stop times of the build (or one of the rounds, if it
@@ -116,4 +111,15 @@ struct BuildResult
     }
 };
 
+/**
+ * A `BuildResult` together with its "primary key".
+ */
+struct KeyedBuildResult : BuildResult
+{
+    /**
+     * The derivation we built or the store path we substituted.
+     */
+    DerivedPath path;
+};
+
 }

src/libstore/build/derivation-goal.cc

@@ -145,8 +145,20 @@ void DerivationGoal::work()
 void DerivationGoal::addWantedOutputs(const OutputsSpec & outputs)
 {
     auto newWanted = wantedOutputs.union_(outputs);
-    if (!newWanted.isSubsetOf(wantedOutputs))
-        needRestart = true;
+    switch (needRestart) {
+    case NeedRestartForMoreOutputs::OutputsUnmodifedDontNeed:
+        if (!newWanted.isSubsetOf(wantedOutputs))
+            needRestart = NeedRestartForMoreOutputs::OutputsAddedDoNeed;
+        break;
+    case NeedRestartForMoreOutputs::OutputsAddedDoNeed:
+        /* No need to check whether we added more outputs, because a
+           restart is already queued up. */
+        break;
+    case NeedRestartForMoreOutputs::BuildInProgressWillNotNeed:
+        /* We are already building all outputs, so it doesn't matter if
+           we now want more. */
+        break;
+    };
     wantedOutputs = newWanted;
 }
 
@@ -297,12 +309,29 @@ void DerivationGoal::outputsSubstitutionTried()
         In particular, it may be the case that the hole in the closure is
         an output of the current derivation, which causes a loop if retried.
      */
-    if (nrIncompleteClosure > 0 && nrIncompleteClosure == nrFailed) retrySubstitution = true;
+    {
+        bool substitutionFailed =
+            nrIncompleteClosure > 0 &&
+            nrIncompleteClosure == nrFailed;
+        switch (retrySubstitution) {
+        case RetrySubstitution::NoNeed:
+            if (substitutionFailed)
+                retrySubstitution = RetrySubstitution::YesNeed;
+            break;
+        case RetrySubstitution::YesNeed:
+            // Should not be able to reach this state from here.
+            assert(false);
+            break;
+        case RetrySubstitution::AlreadyRetried:
+            debug("substitution failed again, but we already retried once. Not retrying again.");
+            break;
+        }
+    }
 
     nrFailed = nrNoSubstituters = nrIncompleteClosure = 0;
 
-    if (needRestart) {
-        needRestart = false;
+    if (needRestart == NeedRestartForMoreOutputs::OutputsAddedDoNeed) {
+        needRestart = NeedRestartForMoreOutputs::OutputsUnmodifedDontNeed;
         haveDerivation();
         return;
     }
@@ -330,6 +359,10 @@ void DerivationGoal::outputsSubstitutionTried()
    produced using a substitute.  So we have to build instead. */
 void DerivationGoal::gaveUpOnSubstitution()
 {
+    /* At this point we are building all outputs, so if more are wanted there
+       is no need to restart. */
+    needRestart = NeedRestartForMoreOutputs::BuildInProgressWillNotNeed;
+
     /* The inputs must be built before we can build this goal. */
     inputDrvOutputs.clear();
     if (useDerivation)
@@ -451,8 +484,8 @@ void DerivationGoal::inputsRealised()
         return;
     }
 
-    if (retrySubstitution && !retriedSubstitution) {
-        retriedSubstitution = true;
+    if (retrySubstitution == RetrySubstitution::YesNeed) {
+        retrySubstitution = RetrySubstitution::AlreadyRetried;
         haveDerivation();
         return;
     }
@@ -570,8 +603,6 @@ void DerivationGoal::inputsRealised()
        build hook. */
     state = &DerivationGoal::tryToBuild;
     worker.wakeUp(shared_from_this());
-
-    buildResult = BuildResult { .path = buildResult.path };
 }
 
 void DerivationGoal::started()
@@ -982,7 +1013,7 @@ void DerivationGoal::resolvedFinished()
     auto resolvedDrv = *resolvedDrvGoal->drv;
     auto & resolvedResult = resolvedDrvGoal->buildResult;
 
-    DrvOutputs builtOutputs;
+    SingleDrvOutputs builtOutputs;
 
     if (resolvedResult.success()) {
         auto resolvedHashes = staticOutputHashes(worker.store, resolvedDrv);
@@ -1008,7 +1039,7 @@ void DerivationGoal::resolvedFinished()
                     worker.store.printStorePath(drvPath), wantedOutput);
 
             auto realisation = [&]{
-              auto take1 = get(resolvedResult.builtOutputs, DrvOutput { *resolvedHash, wantedOutput });
+              auto take1 = get(resolvedResult.builtOutputs, wantedOutput);
               if (take1) return *take1;
 
               /* The above `get` should work. But sateful tracking of
@@ -1033,7 +1064,7 @@ void DerivationGoal::resolvedFinished()
                 worker.store.registerDrvOutput(newRealisation);
             }
             outputPaths.insert(realisation.outPath);
-            builtOutputs.emplace(realisation.id, realisation);
+            builtOutputs.emplace(wantedOutput, realisation);
         }
 
         runPostBuildHook(
@@ -1158,7 +1189,7 @@ HookReply DerivationGoal::tryBuildHook()
 }
 
 
-DrvOutputs DerivationGoal::registerOutputs()
+SingleDrvOutputs DerivationGoal::registerOutputs()
 {
     /* When using a build hook, the build hook can register the output
        as valid (by doing `nix-store --import').  If so we don't have
@@ -1320,7 +1351,7 @@ OutputPathMap DerivationGoal::queryDerivationOutputMap()
 }
 
 
-std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
+std::pair<bool, SingleDrvOutputs> DerivationGoal::checkPathValidity()
 {
     if (!drv->type().isPure()) return { false, {} };
 
@@ -1333,7 +1364,7 @@ std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
             return static_cast<StringSet>(names);
         },
     }, wantedOutputs.raw());
-    DrvOutputs validOutputs;
+    SingleDrvOutputs validOutputs;
 
     for (auto & i : queryPartialDerivationOutputMap()) {
         auto initialOutput = get(initialOutputs, i.first);
@@ -1376,7 +1407,7 @@ std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
             }
         }
         if (info.wanted && info.known && info.known->isValid())
-            validOutputs.emplace(drvOutput, Realisation { drvOutput, info.known->path });
+            validOutputs.emplace(i.first, Realisation { drvOutput, info.known->path });
     }
 
     // If we requested all the outputs, we are always fine.
@@ -1400,7 +1431,7 @@ std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
 }
 
 
-DrvOutputs DerivationGoal::assertPathValidity()
+SingleDrvOutputs DerivationGoal::assertPathValidity()
 {
     auto [allValid, validOutputs] = checkPathValidity();
     if (!allValid)
@@ -1411,7 +1442,7 @@ DrvOutputs DerivationGoal::assertPathValidity()
 
 void DerivationGoal::done(
     BuildResult::Status status,
-    DrvOutputs builtOutputs,
+    SingleDrvOutputs builtOutputs,
     std::optional<Error> ex)
 {
     buildResult.status = status;
@@ -1452,12 +1483,28 @@ void DerivationGoal::waiteeDone(GoalPtr waitee, ExitCode result)
 {
     Goal::waiteeDone(waitee, result);
 
-    if (waitee->buildResult.success())
-        if (auto bfd = std::get_if<DerivedPath::Built>(&waitee->buildResult.path))
-            for (auto & [output, realisation] : waitee->buildResult.builtOutputs)
+    if (!useDerivation) return;
+    auto & fullDrv = *dynamic_cast<Derivation *>(drv.get());
+
+    auto * dg = dynamic_cast<DerivationGoal *>(&*waitee);
+    if (!dg) return;
+
+    auto outputs = fullDrv.inputDrvs.find(dg->drvPath);
+    if (outputs == fullDrv.inputDrvs.end()) return;
+
+    for (auto & outputName : outputs->second) {
+        auto buildResult = dg->getBuildResult(DerivedPath::Built {
+            .drvPath = dg->drvPath,
+            .outputs = OutputsSpec::Names { outputName },
+        });
+        if (buildResult.success()) {
+            auto i = buildResult.builtOutputs.find(outputName);
+            if (i != buildResult.builtOutputs.end())
                 inputDrvOutputs.insert_or_assign(
-                    { bfd->drvPath, output.outputName },
-                    realisation.outPath);
+                    { dg->drvPath, outputName },
+                    i->second.outPath);
+        }
+    }
 }
 
 }

src/libstore/build/derivation-goal.hh

@@ -78,22 +78,58 @@ struct DerivationGoal : public Goal
      */
     std::map<std::pair<StorePath, std::string>, StorePath> inputDrvOutputs;
 
+    /**
+     * See `needRestart`; just for that field.
+     */
+    enum struct NeedRestartForMoreOutputs {
+        /**
+         * The goal state machine is progressing based on the current value of
+         * `wantedOutputs. No actions are needed.
+         */
+        OutputsUnmodifedDontNeed,
+        /**
+         * `wantedOutputs` has been extended, but the state machine is
+         * proceeding according to its old value, so we need to restart.
+         */
+        OutputsAddedDoNeed,
+        /**
+         * The goal state machine has progressed to the point of doing a build,
+         * in which case all outputs will be produced, so extensions to
+         * `wantedOutputs` no longer require a restart.
+         */
+        BuildInProgressWillNotNeed,
+    };
+
     /**
      * Whether additional wanted outputs have been added.
      */
-    bool needRestart = false;
+    NeedRestartForMoreOutputs needRestart = NeedRestartForMoreOutputs::OutputsUnmodifedDontNeed;
+
+    /**
+     * See `retrySubstitution`; just for that field.
+     */
+    enum RetrySubstitution {
+        /**
+         * No issues have yet arose, no need to restart.
+         */
+        NoNeed,
+        /**
+         * Something failed and there is an incomplete closure. Let's retry
+         * substituting.
+         */
+        YesNeed,
+        /**
+         * We are current or have already retried substitution, and whether or
+         * not something goes wrong we will not retry again.
+         */
+        AlreadyRetried,
+    };
 
     /**
      * Whether to retry substituting the outputs after building the
      * inputs. This is done in case of an incomplete closure.
      */
-    bool retrySubstitution = false;
-
-    /**
-     * Whether we've retried substitution, in which case we won't try
-     * again.
-     */
-    bool retriedSubstitution = false;
+    RetrySubstitution retrySubstitution = RetrySubstitution::NoNeed;
 
     /**
      * The derivation stored at drvPath.
@@ -217,7 +253,7 @@ struct DerivationGoal : public Goal
      * Check that the derivation outputs all exist and register them
      * as valid.
      */
-    virtual DrvOutputs registerOutputs();
+    virtual SingleDrvOutputs registerOutputs();
 
     /**
      * Open a log file and a pipe to it.
@@ -270,17 +306,17 @@ struct DerivationGoal : public Goal
      * Update 'initialOutputs' to determine the current status of the
      * outputs of the derivation. Also returns a Boolean denoting
      * whether all outputs are valid and non-corrupt, and a
-     * 'DrvOutputs' structure containing the valid and wanted
+     * 'SingleDrvOutputs' structure containing the valid and wanted
      * outputs.
      */
-    std::pair<bool, DrvOutputs> checkPathValidity();
+    std::pair<bool, SingleDrvOutputs> checkPathValidity();
 
     /**
      * Aborts if any output is not valid or corrupt, and otherwise
-     * returns a 'DrvOutputs' structure containing the wanted
+     * returns a 'SingleDrvOutputs' structure containing the wanted
      * outputs.
      */
-    DrvOutputs assertPathValidity();
+    SingleDrvOutputs assertPathValidity();
 
     /**
      * Forcibly kill the child process, if any.
@@ -293,7 +329,7 @@ struct DerivationGoal : public Goal
 
     void done(
         BuildResult::Status status,
-        DrvOutputs builtOutputs = {},
+        SingleDrvOutputs builtOutputs = {},
         std::optional<Error> ex = {});
 
     void waiteeDone(GoalPtr waitee, ExitCode result) override;

src/libstore/build/entry-points.cc

@@ -10,16 +10,8 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
     Worker worker(*this, evalStore ? *evalStore : *this);
 
     Goals goals;
-    for (const auto & br : reqs) {
-        std::visit(overloaded {
-            [&](const DerivedPath::Built & bfd) {
-                goals.insert(worker.makeDerivationGoal(bfd.drvPath, bfd.outputs, buildMode));
-            },
-            [&](const DerivedPath::Opaque & bo) {
-                goals.insert(worker.makePathSubstitutionGoal(bo.path, buildMode == bmRepair ? Repair : NoRepair));
-            },
-        }, br.raw());
-    }
+    for (auto & br : reqs)
+        goals.insert(worker.makeGoal(br, buildMode));
 
     worker.run(goals);
 
@@ -47,7 +39,7 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
     }
 }
 
-std::vector<BuildResult> Store::buildPathsWithResults(
+std::vector<KeyedBuildResult> Store::buildPathsWithResults(
     const std::vector<DerivedPath> & reqs,
     BuildMode buildMode,
     std::shared_ptr<Store> evalStore)
@@ -55,23 +47,23 @@ std::vector<BuildResult> Store::buildPathsWithResults(
     Worker worker(*this, evalStore ? *evalStore : *this);
 
     Goals goals;
-    for (const auto & br : reqs) {
-        std::visit(overloaded {
-            [&](const DerivedPath::Built & bfd) {
-                goals.insert(worker.makeDerivationGoal(bfd.drvPath, bfd.outputs, buildMode));
-            },
-            [&](const DerivedPath::Opaque & bo) {
-                goals.insert(worker.makePathSubstitutionGoal(bo.path, buildMode == bmRepair ? Repair : NoRepair));
-            },
-        }, br.raw());
+    std::vector<std::pair<const DerivedPath &, GoalPtr>> state;
+
+    for (const auto & req : reqs) {
+        auto goal = worker.makeGoal(req, buildMode);
+        goals.insert(goal);
+        state.push_back({req, goal});
     }
 
     worker.run(goals);
 
-    std::vector<BuildResult> results;
+    std::vector<KeyedBuildResult> results;
 
-    for (auto & i : goals)
-        results.push_back(i->buildResult);
+    for (auto & [req, goalPtr] : state)
+        results.emplace_back(KeyedBuildResult {
+            goalPtr->getBuildResult(req),
+            /* .path = */ req,
+        });
 
     return results;
 }
@@ -84,15 +76,14 @@ BuildResult Store::buildDerivation(const StorePath & drvPath, const BasicDerivat
 
     try {
         worker.run(Goals{goal});
-        return goal->buildResult;
+        return goal->getBuildResult(DerivedPath::Built {
+            .drvPath = drvPath,
+            .outputs = OutputsSpec::All {},
+        });
     } catch (Error & e) {
         return BuildResult {
             .status = BuildResult::MiscFailure,
             .errorMsg = e.msg(),
-            .path = DerivedPath::Built {
-                .drvPath = drvPath,
-                .outputs = OutputsSpec::All { },
-            },
         };
     };
 }

src/libstore/build/goal.cc

@@ -11,6 +11,29 @@ bool CompareGoalPtrs::operator() (const GoalPtr & a, const GoalPtr & b) const {
 }
 
 
+BuildResult Goal::getBuildResult(const DerivedPath & req) {
+    BuildResult res { buildResult };
+
+    if (auto pbp = std::get_if<DerivedPath::Built>(&req)) {
+        auto & bp = *pbp;
+
+        /* Because goals are in general shared between derived paths
+           that share the same derivation, we need to filter their
+           results to get back just the results we care about.
+         */
+
+        for (auto it = res.builtOutputs.begin(); it != res.builtOutputs.end();) {
+            if (bp.outputs.contains(it->first))
+                ++it;
+            else
+                it = res.builtOutputs.erase(it);
+        }
+    }
+
+    return res;
+}
+
+
 void addToWeakGoals(WeakGoals & goals, GoalPtr p)
 {
     if (goals.find(p) != goals.end())

src/libstore/build/goal.hh

@@ -81,11 +81,26 @@ struct Goal : public std::enable_shared_from_this<Goal>
      */
     ExitCode exitCode = ecBusy;
 
+protected:
     /**
      * Build result.
      */
     BuildResult buildResult;
 
+public:
+
+    /**
+     * Project a `BuildResult` with just the information that pertains
+     * to the given request.
+     *
+     * In general, goals may be aliased between multiple requests, and
+     * the stored `BuildResult` has information for the union of all
+     * requests. We don't want to leak what the other request are for
+     * sake of both privacy and determinism, and this "safe accessor"
+     * ensures we don't.
+     */
+    BuildResult getBuildResult(const DerivedPath &);
+
     /**
      * Exception containing an error message, if any.
      */
@@ -93,7 +108,6 @@ struct Goal : public std::enable_shared_from_this<Goal>
 
     Goal(Worker & worker, DerivedPath path)
         : worker(worker)
-        , buildResult { .path = std::move(path) }
     { }
 
     virtual ~Goal()

src/libstore/build/local-derivation-goal.cc

@@ -1335,7 +1335,7 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
                 result.rethrow();
     }
 
-    std::vector<BuildResult> buildPathsWithResults(
+    std::vector<KeyedBuildResult> buildPathsWithResults(
         const std::vector<DerivedPath> & paths,
         BuildMode buildMode = bmNormal,
         std::shared_ptr<Store> evalStore = nullptr) override
@@ -2174,7 +2174,7 @@ void LocalDerivationGoal::runChild()
 }
 
 
-DrvOutputs LocalDerivationGoal::registerOutputs()
+SingleDrvOutputs LocalDerivationGoal::registerOutputs()
 {
     /* When using a build hook, the build hook can register the output
        as valid (by doing `nix-store --import').  If so we don't have
@@ -2395,27 +2395,26 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
             }
         };
 
-        auto rewriteRefs = [&]() -> std::pair<bool, StorePathSet> {
+        auto rewriteRefs = [&]() -> StoreReferences {
             /* In the CA case, we need the rewritten refs to calculate the
                final path, therefore we look for a *non-rewritten
                self-reference, and use a bool rather try to solve the
                computationally intractable fixed point. */
-            std::pair<bool, StorePathSet> res {
-                false,
-                {},
+            StoreReferences res {
+                .self = false,
             };
             for (auto & r : references) {
                 auto name = r.name();
                 auto origHash = std::string { r.hashPart() };
                 if (r == *scratchPath) {
-                    res.first = true;
+                    res.self = true;
                 } else if (auto outputRewrite = get(outputRewrites, origHash)) {
                     std::string newRef = *outputRewrite;
                     newRef += '-';
                     newRef += name;
-                    res.second.insert(StorePath { newRef });
+                    res.others.insert(StorePath { newRef });
                 } else {
-                    res.second.insert(r);
+                    res.others.insert(r);
                 }
             }
             return res;
@@ -2448,18 +2447,22 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 break;
             }
             auto got = caSink.finish().first;
-            auto refs = rewriteRefs();
-
-            auto finalPath = worker.store.makeFixedOutputPath(
-                    outputHash.method,
-                    got,
-                    outputPathName(drv->name, outputName),
-                    refs.second,
-                    refs.first);
-            if (*scratchPath != finalPath) {
+            ValidPathInfo newInfo0 {
+                worker.store,
+                outputPathName(drv->name, outputName),
+                FixedOutputInfo {
+                    .hash = {
+                        .method = outputHash.method,
+                        .hash = got,
+                    },
+                    .references = rewriteRefs(),
+                },
+                Hash::dummy,
+            };
+            if (*scratchPath != newInfo0.path) {
                 // Also rewrite the output path
                 auto source = sinkToSource([&](Sink & nextSink) {
-                    RewritingSink rsink2(oldHashPart, std::string(finalPath.hashPart()), nextSink);
+                    RewritingSink rsink2(oldHashPart, std::string(newInfo0.path.hashPart()), nextSink);
                     dumpPath(actualPath, rsink2);
                     rsink2.flush();
                 });
@@ -2470,19 +2473,8 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
             }
 
             HashResult narHashAndSize = hashPath(htSHA256, actualPath);
-            ValidPathInfo newInfo0 {
-                finalPath,
-                narHashAndSize.first,
-            };
-
+            newInfo0.narHash = narHashAndSize.first;
             newInfo0.narSize = narHashAndSize.second;
-            newInfo0.ca = FixedOutputHash {
-                .method = outputHash.method,
-                .hash = got,
-            };
-            newInfo0.references = refs.second;
-            if (refs.first)
-                newInfo0.references.insert(newInfo0.path);
 
             assert(newInfo0.ca);
             return newInfo0;
@@ -2504,8 +2496,8 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 ValidPathInfo newInfo0 { requiredFinalPath, narHashAndSize.first };
                 newInfo0.narSize = narHashAndSize.second;
                 auto refs = rewriteRefs();
-                newInfo0.references = refs.second;
-                if (refs.first)
+                newInfo0.references = std::move(refs.others);
+                if (refs.self)
                     newInfo0.references.insert(newInfo0.path);
                 return newInfo0;
             },
@@ -2519,7 +2511,7 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 /* Check wanted hash */
                 const Hash & wanted = dof.hash.hash;
                 assert(newInfo0.ca);
-                auto got = getContentAddressHash(*newInfo0.ca);
+                auto got = newInfo0.ca->getHash();
                 if (wanted != got) {
                     /* Throw an error after registering the path as
                        valid. */
@@ -2691,7 +2683,7 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
        means it's safe to link the derivation to the output hash. We must do
        that for floating CA derivations, which otherwise couldn't be cached,
        but it's fine to do in all cases. */
-    DrvOutputs builtOutputs;
+    SingleDrvOutputs builtOutputs;
 
     for (auto & [outputName, newInfo] : infos) {
         auto oldinfo = get(initialOutputs, outputName);
@@ -2710,7 +2702,7 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
             worker.store.registerDrvOutput(thisRealisation);
         }
         if (wantedOutputs.contains(outputName))
-            builtOutputs.emplace(thisRealisation.id, thisRealisation);
+            builtOutputs.emplace(outputName, thisRealisation);
     }
 
     return builtOutputs;

src/libstore/build/local-derivation-goal.hh

@@ -237,7 +237,7 @@ struct LocalDerivationGoal : public DerivationGoal
      * Check that the derivation outputs all exist and register them
      * as valid.
      */
-    DrvOutputs registerOutputs() override;
+    SingleDrvOutputs registerOutputs() override;
 
     void signRealisation(Realisation &) override;
 

src/libstore/build/substitution-goal.cc

@@ -95,7 +95,9 @@ void PathSubstitutionGoal::tryNext()
     subs.pop_front();
 
     if (ca) {
-        subPath = sub->makeFixedOutputPathFromCA(storePath.name(), *ca);
+        subPath = sub->makeFixedOutputPathFromCA(
+            std::string { storePath.name() },
+            ContentAddressWithReferences::withoutRefs(*ca));
         if (sub->storeDir == worker.store.storeDir)
             assert(subPath == storePath);
     } else if (sub->storeDir != worker.store.storeDir) {

src/libstore/build/worker.cc

@@ -92,6 +92,7 @@ std::shared_ptr<PathSubstitutionGoal> Worker::makePathSubstitutionGoal(const Sto
     return goal;
 }
 
+
 std::shared_ptr<DrvOutputSubstitutionGoal> Worker::makeDrvOutputSubstitutionGoal(const DrvOutput& id, RepairFlag repair, std::optional<ContentAddress> ca)
 {
     std::weak_ptr<DrvOutputSubstitutionGoal> & goal_weak = drvOutputSubstitutionGoals[id];
@@ -104,6 +105,20 @@ std::shared_ptr<DrvOutputSubstitutionGoal> Worker::makeDrvOutputSubstitutionGoal
     return goal;
 }
 
+
+GoalPtr Worker::makeGoal(const DerivedPath & req, BuildMode buildMode)
+{
+    return std::visit(overloaded {
+        [&](const DerivedPath::Built & bfd) -> GoalPtr {
+            return makeDerivationGoal(bfd.drvPath, bfd.outputs, buildMode);
+        },
+        [&](const DerivedPath::Opaque & bo) -> GoalPtr {
+            return makePathSubstitutionGoal(bo.path, buildMode == bmRepair ? Repair : NoRepair);
+        },
+    }, req.raw());
+}
+
+
 template<typename K, typename G>
 static void removeGoal(std::shared_ptr<G> goal, std::map<K, std::weak_ptr<G>> & goalMap)
 {

src/libstore/build/worker.hh

@@ -181,7 +181,7 @@ public:
      */
 
     /**
-     * derivation goal
+     * @ref DerivationGoal "derivation goal"
      */
 private:
     std::shared_ptr<DerivationGoal> makeDerivationGoalCommon(
@@ -196,11 +196,19 @@ public:
         const OutputsSpec & wantedOutputs, BuildMode buildMode = bmNormal);
 
     /**
-     * substitution goal
+     * @ref SubstitutionGoal "substitution goal"
      */
     std::shared_ptr<PathSubstitutionGoal> makePathSubstitutionGoal(const StorePath & storePath, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
     std::shared_ptr<DrvOutputSubstitutionGoal> makeDrvOutputSubstitutionGoal(const DrvOutput & id, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
 
+    /**
+     * Make a goal corresponding to the `DerivedPath`.
+     *
+     * It will be a `DerivationGoal` for a `DerivedPath::Built` or
+     * a `SubstitutionGoal` for a `DerivedPath::Opaque`.
+     */
+    GoalPtr makeGoal(const DerivedPath & req, BuildMode buildMode = bmNormal);
+
     /**
      * Remove a dead goal.
      */

src/libstore/content-address.cc

@@ -9,7 +9,7 @@ std::string FixedOutputHash::printMethodAlgo() const
     return makeFileIngestionPrefix(method) + printHashType(hash.type);
 }
 
-std::string makeFileIngestionPrefix(const FileIngestionMethod m)
+std::string makeFileIngestionPrefix(FileIngestionMethod m)
 {
     switch (m) {
     case FileIngestionMethod::Flat:
@@ -21,39 +21,35 @@ std::string makeFileIngestionPrefix(const FileIngestionMethod m)
     }
 }
 
-std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash)
-{
-    return "fixed:"
-        + makeFileIngestionPrefix(method)
-        + hash.to_string(Base32, true);
-}
-
-std::string renderContentAddress(ContentAddress ca)
+std::string ContentAddress::render() const
 {
     return std::visit(overloaded {
-        [](TextHash & th) {
-            return "text:" + th.hash.to_string(Base32, true);
+        [](const TextHash & th) {
+            return "text:"
+                + th.hash.to_string(Base32, true);
         },
-        [](FixedOutputHash & fsh) {
-            return makeFixedOutputCA(fsh.method, fsh.hash);
+        [](const FixedOutputHash & fsh) {
+            return "fixed:"
+                + makeFileIngestionPrefix(fsh.method)
+                + fsh.hash.to_string(Base32, true);
         }
-    }, ca);
+    }, raw);
 }
 
-std::string renderContentAddressMethod(ContentAddressMethod cam)
+std::string ContentAddressMethod::render() const
 {
     return std::visit(overloaded {
-        [](TextHashMethod & th) {
+        [](const TextHashMethod & th) {
             return std::string{"text:"} + printHashType(htSHA256);
         },
-        [](FixedOutputHashMethod & fshm) {
+        [](const FixedOutputHashMethod & fshm) {
             return "fixed:" + makeFileIngestionPrefix(fshm.fileIngestionMethod) + printHashType(fshm.hashType);
         }
-    }, cam);
+    }, raw);
 }
 
-/*
-  Parses content address strings up to the hash.
+/**
+ * Parses content address strings up to the hash.
  */
 static ContentAddressMethod parseContentAddressMethodPrefix(std::string_view & rest)
 {
@@ -97,7 +93,7 @@ static ContentAddressMethod parseContentAddressMethodPrefix(std::string_view & r
         throw UsageError("content address prefix '%s' is unrecognized. Recogonized prefixes are 'text' or 'fixed'", prefix);
 }
 
-ContentAddress parseContentAddress(std::string_view rawCa) {
+ContentAddress ContentAddress::parse(std::string_view rawCa) {
     auto rest = rawCa;
 
     ContentAddressMethod caMethod = parseContentAddressMethodPrefix(rest);
@@ -115,10 +111,10 @@ ContentAddress parseContentAddress(std::string_view rawCa) {
                     .hash = Hash::parseNonSRIUnprefixed(rest, std::move(fohMethod.hashType)),
                 });
             },
-        }, caMethod);
+        }, caMethod.raw);
 }
 
-ContentAddressMethod parseContentAddressMethod(std::string_view caMethod)
+ContentAddressMethod ContentAddressMethod::parse(std::string_view caMethod)
 {
     std::string asPrefix = std::string{caMethod} + ":";
     // parseContentAddressMethodPrefix takes its argument by reference
@@ -126,26 +122,55 @@ ContentAddressMethod parseContentAddressMethod(std::string_view caMethod)
     return parseContentAddressMethodPrefix(asPrefixView);
 }
 
-std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt)
+std::optional<ContentAddress> ContentAddress::parseOpt(std::string_view rawCaOpt)
 {
-    return rawCaOpt == "" ? std::optional<ContentAddress>() : parseContentAddress(rawCaOpt);
+    return rawCaOpt == ""
+        ? std::nullopt
+        : std::optional { ContentAddress::parse(rawCaOpt) };
 };
 
 std::string renderContentAddress(std::optional<ContentAddress> ca)
 {
-    return ca ? renderContentAddress(*ca) : "";
+    return ca ? ca->render() : "";
 }
 
-Hash getContentAddressHash(const ContentAddress & ca)
+const Hash & ContentAddress::getHash() const
 {
     return std::visit(overloaded {
-        [](const TextHash & th) {
+        [](const TextHash & th) -> auto & {
             return th.hash;
         },
-        [](const FixedOutputHash & fsh) {
+        [](const FixedOutputHash & fsh) -> auto & {
             return fsh.hash;
-        }
-    }, ca);
+        },
+    }, raw);
+}
+
+bool StoreReferences::empty() const
+{
+    return !self && others.empty();
+}
+
+size_t StoreReferences::size() const
+{
+    return (self ? 1 : 0) + others.size();
+}
+
+ContentAddressWithReferences ContentAddressWithReferences::withoutRefs(const ContentAddress & ca) {
+    return std::visit(overloaded {
+        [&](const TextHash & h) -> ContentAddressWithReferences {
+            return TextInfo {
+                .hash = h,
+                .references = {},
+            };
+        },
+        [&](const FixedOutputHash & h) -> ContentAddressWithReferences {
+            return FixedOutputInfo {
+                .hash = h,
+                .references = {},
+            };
+        },
+    }, ca.raw);
 }
 
 }

src/libstore/content-address.hh

@@ -3,12 +3,30 @@
 
 #include <variant>
 #include "hash.hh"
+#include "path.hh"
 #include "comparator.hh"
 
 namespace nix {
 
+/*
+ * Content addressing method
+ */
+
+/* We only have one way to hash text with references, so this is a single-value
+   type, mainly useful with std::variant.
+*/
+
 /**
- * An enumeration of the ways we can serialize file system objects.
+ * The single way we can serialize "text" file system objects.
+ *
+ * Somewhat obscure, used by \ref Derivation derivations and
+ * `builtins.toFile` currently.
+ */
+struct TextHashMethod : std::monostate { };
+
+/**
+ * An enumeration of the main ways we can serialize file system
+ * objects.
  */
 enum struct FileIngestionMethod : uint8_t {
     /**
@@ -22,6 +40,53 @@ enum struct FileIngestionMethod : uint8_t {
     Recursive = true
 };
 
+/**
+ * Compute the prefix to the hash algorithm which indicates how the
+ * files were ingested.
+ */
+std::string makeFileIngestionPrefix(FileIngestionMethod m);
+
+struct FixedOutputHashMethod {
+    FileIngestionMethod fileIngestionMethod;
+    HashType hashType;
+
+    GENERATE_CMP(FixedOutputHashMethod, me->fileIngestionMethod, me->hashType);
+};
+
+/**
+ * An enumeration of all the ways we can serialize file system objects.
+ *
+ * Just the type of a content address. Combine with the hash itself, and
+ * we have a `ContentAddress` as defined below. Combine that, in turn,
+ * with info on references, and we have `ContentAddressWithReferences`,
+ * as defined further below.
+ */
+struct ContentAddressMethod
+{
+    typedef std::variant<
+        TextHashMethod,
+        FixedOutputHashMethod
+    > Raw;
+
+    Raw raw;
+
+    GENERATE_CMP(ContentAddressMethod, me->raw);
+
+    /* The moral equivalent of `using Raw::Raw;` */
+    ContentAddressMethod(auto &&... arg)
+        : raw(std::forward<decltype(arg)>(arg)...)
+    { }
+
+    static ContentAddressMethod parse(std::string_view rawCaMethod);
+
+    std::string render() const;
+};
+
+
+/*
+ * Mini content address
+ */
+
 /**
  * Somewhat obscure, used by \ref Derivation derivations and
  * `builtins.toFile` currently.
@@ -36,7 +101,7 @@ struct TextHash {
 };
 
 /**
- * For path computed by makeFixedOutputPath.
+ * Used by most store objects that are content-addressed.
  */
 struct FixedOutputHash {
     /**
@@ -65,41 +130,96 @@ struct FixedOutputHash {
  * - ‘fixed:<r?>:<ht>:<h>’: For paths computed by
  *   Store::makeFixedOutputPath() / Store::addToStore().
  */
-typedef std::variant<
-    TextHash,
-    FixedOutputHash
-> ContentAddress;
+struct ContentAddress
+{
+    typedef std::variant<
+        TextHash,
+        FixedOutputHash
+    > Raw;
 
-/**
- * Compute the prefix to the hash algorithm which indicates how the
- * files were ingested.
- */
-std::string makeFileIngestionPrefix(const FileIngestionMethod m);
+    Raw raw;
 
-/**
- * Compute the content-addressability assertion (ValidPathInfo::ca) for
- * paths created by Store::makeFixedOutputPath() / Store::addToStore().
- */
-std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash);
+    GENERATE_CMP(ContentAddress, me->raw);
 
-std::string renderContentAddress(ContentAddress ca);
+    /* The moral equivalent of `using Raw::Raw;` */
+    ContentAddress(auto &&... arg)
+        : raw(std::forward<decltype(arg)>(arg)...)
+    { }
+
+    /**
+     * Compute the content-addressability assertion (ValidPathInfo::ca) for
+     * paths created by Store::makeFixedOutputPath() / Store::addToStore().
+     */
+    std::string render() const;
+
+    static ContentAddress parse(std::string_view rawCa);
+
+    static std::optional<ContentAddress> parseOpt(std::string_view rawCaOpt);
+
+    const Hash & getHash() const;
+};
 
 std::string renderContentAddress(std::optional<ContentAddress> ca);
 
-ContentAddress parseContentAddress(std::string_view rawCa);
-
-std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt);
-
-Hash getContentAddressHash(const ContentAddress & ca);
 
 /*
-  We only have one way to hash text with references, so this is single-value
-  type is only useful in std::variant.
-*/
-struct TextHashMethod { };
-struct FixedOutputHashMethod {
-  FileIngestionMethod fileIngestionMethod;
-  HashType hashType;
+ * Full content address
+ *
+ * See the schema for store paths in store-api.cc
+ */
+
+/**
+ * A set of references to other store objects.
+ *
+ * References to other store objects are tracked with store paths, self
+ * references however are tracked with a boolean.
+ */
+struct StoreReferences {
+    /**
+     * References to other store objects
+     */
+    StorePathSet others;
+
+    /**
+     * Reference to this store object
+     */
+    bool self = false;
+
+    /**
+     * @return true iff no references, i.e. others is empty and self is
+     * false.
+     */
+    bool empty() const;
+
+    /**
+     * Returns the numbers of references, i.e. the size of others + 1
+     * iff self is true.
+     */
+    size_t size() const;
+
+    GENERATE_CMP(StoreReferences, me->self, me->others);
+};
+
+// This matches the additional info that we need for makeTextPath
+struct TextInfo {
+    TextHash hash;
+    /**
+     * References to other store objects only; self references
+     * disallowed
+     */
+    StorePathSet references;
+
+    GENERATE_CMP(TextInfo, me->hash, me->references);
+};
+
+struct FixedOutputInfo {
+    FixedOutputHash hash;
+    /**
+     * References to other store objects or this one.
+     */
+    StoreReferences references;
+
+    GENERATE_CMP(FixedOutputInfo, me->hash, me->references);
 };
 
 /**
@@ -107,13 +227,27 @@ struct FixedOutputHashMethod {
  *
  * A ContentAddress without a Hash.
  */
-typedef std::variant<
-    TextHashMethod,
-    FixedOutputHashMethod
-  > ContentAddressMethod;
+struct ContentAddressWithReferences
+{
+    typedef std::variant<
+        TextInfo,
+        FixedOutputInfo
+    > Raw;
 
-ContentAddressMethod parseContentAddressMethod(std::string_view rawCaMethod);
+    Raw raw;
 
-std::string renderContentAddressMethod(ContentAddressMethod caMethod);
+    GENERATE_CMP(ContentAddressWithReferences, me->raw);
+
+    /* The moral equivalent of `using Raw::Raw;` */
+    ContentAddressWithReferences(auto &&... arg)
+        : raw(std::forward<decltype(arg)>(arg)...)
+    { }
+
+    /**
+     * Create a ContentAddressWithReferences from a mere ContentAddress, by
+     * assuming no references in all cases.
+     */
+    static ContentAddressWithReferences withoutRefs(const ContentAddress &);
+};
 
 }

src/libstore/daemon.cc

@@ -401,21 +401,21 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             logger->startWork();
             auto pathInfo = [&]() {
                 // NB: FramedSource must be out of scope before logger->stopWork();
-                ContentAddressMethod contentAddressMethod = parseContentAddressMethod(camStr);
+                ContentAddressMethod contentAddressMethod = ContentAddressMethod::parse(camStr);
                 FramedSource source(from);
                 // TODO this is essentially RemoteStore::addCAToStore. Move it up to Store.
                 return std::visit(overloaded {
-                    [&](TextHashMethod &) {
+                    [&](const TextHashMethod &) {
                         // We could stream this by changing Store
                         std::string contents = source.drain();
                         auto path = store->addTextToStore(name, contents, refs, repair);
                         return store->queryPathInfo(path);
                     },
-                    [&](FixedOutputHashMethod & fohm) {
+                    [&](const FixedOutputHashMethod & fohm) {
                         auto path = store->addToStoreFromDump(source, name, fohm.fileIngestionMethod, fohm.hashType, repair, refs);
                         return store->queryPathInfo(path);
                     },
-                }, contentAddressMethod);
+                }, contentAddressMethod.raw);
             }();
             logger->stopWork();
 
@@ -637,7 +637,10 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             to << res.timesBuilt << res.isNonDeterministic << res.startTime << res.stopTime;
         }
         if (GET_PROTOCOL_MINOR(clientVersion) >= 28) {
-            worker_proto::write(*store, to, res.builtOutputs);
+            DrvOutputs builtOutputs;
+            for (auto & [output, realisation] : res.builtOutputs)
+                builtOutputs.insert_or_assign(realisation.id, realisation);
+            worker_proto::write(*store, to, builtOutputs);
         }
         break;
     }
@@ -880,7 +883,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         info.references = worker_proto::read(*store, from, Phantom<StorePathSet> {});
         from >> info.registrationTime >> info.narSize >> info.ultimate;
         info.sigs = readStrings<StringSet>(from);
-        info.ca = parseContentAddressOpt(readString(from));
+        info.ca = ContentAddress::parseOpt(readString(from));
         from >> repair >> dontCheckSigs;
         if (!trusted && dontCheckSigs)
             dontCheckSigs = false;
@@ -1064,6 +1067,8 @@ void processConnection(
 
             opCount++;
 
+            debug("performing daemon worker op: %d", op);
+
             try {
                 performOp(tunnelLogger, store, trusted, recursive, clientVersion, from, to, op);
             } catch (Error & e) {

src/libstore/derivations.cc

@@ -36,8 +36,8 @@ std::optional<StorePath> DerivationOutput::path(const Store & store, std::string
 StorePath DerivationOutput::CAFixed::path(const Store & store, std::string_view drvName, std::string_view outputName) const
 {
     return store.makeFixedOutputPath(
-        hash.method, hash.hash,
-        outputPathName(drvName, outputName));
+        outputPathName(drvName, outputName),
+        { hash, {} });
 }
 
 
@@ -313,6 +313,15 @@ Derivation parseDerivation(const Store & store, std::string && s, std::string_vi
 }
 
 
+/**
+ * Print a derivation string literal to an `std::string`.
+ *
+ * This syntax does not generalize to the expression language, which needs to
+ * escape `$`.
+ *
+ * @param res Where to print to
+ * @param s Which logical string to print
+ */
 static void printString(std::string & res, std::string_view s)
 {
     boost::container::small_vector<char, 64 * 1024> buffer;
@@ -933,7 +942,7 @@ void Derivation::checkInvariants(Store & store, const StorePath & drvPath) const
                 envHasRightPath(doia.path, i.first);
             },
             [&](const DerivationOutput::CAFixed & dof) {
-                StorePath path = store.makeFixedOutputPath(dof.hash.method, dof.hash.hash, drvName);
+                StorePath path = store.makeFixedOutputPath(drvName, { dof.hash, {} });
                 envHasRightPath(path, i.first);
             },
             [&](const DerivationOutput::CAFloating &) {
@@ -980,7 +989,8 @@ nlohmann::json DerivationOutput::toJSON(
 
 DerivationOutput DerivationOutput::fromJSON(
     const Store & store, std::string_view drvName, std::string_view outputName,
-    const nlohmann::json & _json)
+    const nlohmann::json & _json,
+    const ExperimentalFeatureSettings & xpSettings)
 {
     std::set<std::string_view> keys;
     auto json = (std::map<std::string, nlohmann::json>) _json;
@@ -1019,6 +1029,7 @@ DerivationOutput DerivationOutput::fromJSON(
     }
 
     else if (keys == (std::set<std::string_view> { "hashAlgo" })) {
+        xpSettings.require(Xp::CaDerivations);
         auto [method, hashType] = methodAlgo();
         return DerivationOutput::CAFloating {
             .method = method,
@@ -1031,6 +1042,7 @@ DerivationOutput DerivationOutput::fromJSON(
     }
 
     else if (keys == (std::set<std::string_view> { "hashAlgo", "impure" })) {
+        xpSettings.require(Xp::ImpureDerivations);
         auto [method, hashType] = methodAlgo();
         return DerivationOutput::Impure {
             .method = method,

src/libstore/derivations.hh

@@ -136,11 +136,15 @@ struct DerivationOutput : _DerivationOutputRaw
         const Store & store,
         std::string_view drvName,
         std::string_view outputName) const;
+    /**
+     * @param xpSettings Stop-gap to avoid globals during unit tests.
+     */
     static DerivationOutput fromJSON(
         const Store & store,
         std::string_view drvName,
         std::string_view outputName,
-        const nlohmann::json & json);
+        const nlohmann::json & json,
+        const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 };
 
 typedef std::map<std::string, DerivationOutput> DerivationOutputs;

src/libstore/derived-path.cc

@@ -62,15 +62,31 @@ std::string DerivedPath::Opaque::to_string(const Store & store) const
 std::string DerivedPath::Built::to_string(const Store & store) const
 {
     return store.printStorePath(drvPath)
-        + "!"
+        + '^'
+        + outputs.to_string();
+}
+
+std::string DerivedPath::Built::to_string_legacy(const Store & store) const
+{
+    return store.printStorePath(drvPath)
+        + '!'
         + outputs.to_string();
 }
 
 std::string DerivedPath::to_string(const Store & store) const
 {
-    return std::visit(
-        [&](const auto & req) { return req.to_string(store); },
-        this->raw());
+    return std::visit(overloaded {
+        [&](const DerivedPath::Built & req) { return req.to_string(store); },
+        [&](const DerivedPath::Opaque & req) { return req.to_string(store); },
+    }, this->raw());
+}
+
+std::string DerivedPath::to_string_legacy(const Store & store) const
+{
+    return std::visit(overloaded {
+        [&](const DerivedPath::Built & req) { return req.to_string_legacy(store); },
+        [&](const DerivedPath::Opaque & req) { return req.to_string(store); },
+    }, this->raw());
 }
 
 
@@ -87,14 +103,24 @@ DerivedPath::Built DerivedPath::Built::parse(const Store & store, std::string_vi
     };
 }
 
-DerivedPath DerivedPath::parse(const Store & store, std::string_view s)
+static inline DerivedPath parseWith(const Store & store, std::string_view s, std::string_view separator)
 {
-    size_t n = s.find("!");
+    size_t n = s.find(separator);
     return n == s.npos
         ? (DerivedPath) DerivedPath::Opaque::parse(store, s)
         : (DerivedPath) DerivedPath::Built::parse(store, s.substr(0, n), s.substr(n + 1));
 }
 
+DerivedPath DerivedPath::parse(const Store & store, std::string_view s)
+{
+	return parseWith(store, s, "^");
+}
+
+DerivedPath DerivedPath::parseLegacy(const Store & store, std::string_view s)
+{
+	return parseWith(store, s, "!");
+}
+
 RealisedPath::Set BuiltPath::toRealisedPaths(Store & store) const
 {
     RealisedPath::Set res;

src/libstore/derived-path.hh

@@ -48,8 +48,18 @@ struct DerivedPathBuilt {
     StorePath drvPath;
     OutputsSpec outputs;
 
+    /**
+     * Uses `^` as the separator
+     */
     std::string to_string(const Store & store) const;
-    static DerivedPathBuilt parse(const Store & store, std::string_view, std::string_view);
+    /**
+     * Uses `!` as the separator
+     */
+    std::string to_string_legacy(const Store & store) const;
+    /**
+     * The caller splits on the separator, so it works for both variants.
+     */
+    static DerivedPathBuilt parse(const Store & store, std::string_view drvPath, std::string_view outputs);
     nlohmann::json toJSON(ref<Store> store) const;
 
     GENERATE_CMP(DerivedPathBuilt, me->drvPath, me->outputs);
@@ -81,8 +91,22 @@ struct DerivedPath : _DerivedPathRaw {
         return static_cast<const Raw &>(*this);
     }
 
+    /**
+     * Uses `^` as the separator
+     */
     std::string to_string(const Store & store) const;
+    /**
+     * Uses `!` as the separator
+     */
+    std::string to_string_legacy(const Store & store) const;
+    /**
+     * Uses `^` as the separator
+     */
     static DerivedPath parse(const Store & store, std::string_view);
+    /**
+     * Uses `!` as the separator
+     */
+    static DerivedPath parseLegacy(const Store & store, std::string_view);
 };
 
 /**

src/libstore/dummy-store.cc

@@ -71,6 +71,9 @@ struct DummyStore : public virtual DummyStoreConfig, public virtual Store
     void queryRealisationUncached(const DrvOutput &,
         Callback<std::shared_ptr<const Realisation>> callback) noexcept override
     { callback(nullptr); }
+
+    virtual ref<FSAccessor> getFSAccessor() override
+    { unsupported("getFSAccessor"); }
 };
 
 static RegisterStoreImplementation<DummyStore, DummyStoreConfig> regDummyStore;

src/libstore/globals.cc

@@ -7,12 +7,23 @@
 
 #include <algorithm>
 #include <map>
+#include <mutex>
 #include <thread>
 #include <dlfcn.h>
 #include <sys/utsname.h>
 
 #include <nlohmann/json.hpp>
 
+#include <sodium/core.h>
+
+#ifdef __GLIBC__
+#include <gnu/lib-names.h>
+#include <nss.h>
+#include <dlfcn.h>
+#endif
+
+#include "config-impl.hh"
+
 
 namespace nix {
 
@@ -41,7 +52,6 @@ Settings::Settings()
     , nixDaemonSocketFile(canonPath(getEnvNonEmpty("NIX_DAEMON_SOCKET_PATH").value_or(nixStateDir + DEFAULT_SOCKET_PATH)))
 {
     buildUsersGroup = getuid() == 0 ? "nixbld" : "";
-    lockCPU = getEnv("NIX_AFFINITY_HACK") == "1";
     allowSymlinkedStore = getEnv("NIX_IGNORE_SYMLINK_STORE") == "1";
 
     auto sslOverride = getEnv("NIX_SSL_CERT_FILE").value_or(getEnv("SSL_CERT_FILE").value_or(""));
@@ -185,18 +195,18 @@ NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {
     {SandboxMode::smDisabled, false},
 });
 
-template<> void BaseSetting<SandboxMode>::set(const std::string & str, bool append)
+template<> SandboxMode BaseSetting<SandboxMode>::parse(const std::string & str) const
 {
-    if (str == "true") value = smEnabled;
-    else if (str == "relaxed") value = smRelaxed;
-    else if (str == "false") value = smDisabled;
+    if (str == "true") return smEnabled;
+    else if (str == "relaxed") return smRelaxed;
+    else if (str == "false") return smDisabled;
     else throw UsageError("option '%s' has invalid value '%s'", name, str);
 }
 
-template<> bool BaseSetting<SandboxMode>::isAppendable()
+template<> struct BaseSetting<SandboxMode>::trait
 {
-    return false;
-}
+    static constexpr bool appendable = false;
+};
 
 template<> std::string BaseSetting<SandboxMode>::to_string() const
 {
@@ -228,23 +238,23 @@ template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::s
     });
 }
 
-void MaxBuildJobsSetting::set(const std::string & str, bool append)
+unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
 {
-    if (str == "auto") value = std::max(1U, std::thread::hardware_concurrency());
+    if (str == "auto") return std::max(1U, std::thread::hardware_concurrency());
     else {
         if (auto n = string2Int<decltype(value)>(str))
-            value = *n;
+            return *n;
         else
             throw UsageError("configuration setting '%s' should be 'auto' or an integer", name);
     }
 }
 
 
-void PluginFilesSetting::set(const std::string & str, bool append)
+Paths PluginFilesSetting::parse(const std::string & str) const
 {
     if (pluginsLoaded)
         throw UsageError("plugin-files set after plugins were loaded, you may need to move the flag before the subcommand");
-    BaseSetting<Paths>::set(str, append);
+    return BaseSetting<Paths>::parse(str);
 }
 
 
@@ -281,6 +291,42 @@ void initPlugins()
     settings.pluginFiles.pluginsLoaded = true;
 }
 
+static void preloadNSS()
+{
+    /* builtin:fetchurl can trigger a DNS lookup, which with glibc can trigger a dynamic library load of
+       one of the glibc NSS libraries in a sandboxed child, which will fail unless the library's already
+       been loaded in the parent. So we force a lookup of an invalid domain to force the NSS machinery to
+       load its lookup libraries in the parent before any child gets a chance to. */
+    static std::once_flag dns_resolve_flag;
+
+    std::call_once(dns_resolve_flag, []() {
+#ifdef __GLIBC__
+        /* On linux, glibc will run every lookup through the nss layer.
+         * That means every lookup goes, by default, through nscd, which acts as a local
+         * cache.
+         * Because we run builds in a sandbox, we also remove access to nscd otherwise
+         * lookups would leak into the sandbox.
+         *
+         * But now we have a new problem, we need to make sure the nss_dns backend that
+         * does the dns lookups when nscd is not available is loaded or available.
+         *
+         * We can't make it available without leaking nix's environment, so instead we'll
+         * load the backend, and configure nss so it does not try to run dns lookups
+         * through nscd.
+         *
+         * This is technically only used for builtins:fetch* functions so we only care
+         * about dns.
+         *
+         * All other platforms are unaffected.
+         */
+        if (!dlopen(LIBNSS_DNS_SO, RTLD_NOW))
+            warn("unable to load nss_dns backend");
+        // FIXME: get hosts entry from nsswitch.conf.
+        __nss_configure_lookup("hosts", "files dns");
+#endif
+    });
+}
+
 static bool initLibStoreDone = false;
 
 void assertLibStoreInitialized() {
@@ -291,6 +337,24 @@ void assertLibStoreInitialized() {
 }
 
 void initLibStore() {
+
+    initLibUtil();
+
+    if (sodium_init() == -1)
+        throw Error("could not initialise libsodium");
+
+    loadConfFile();
+
+    preloadNSS();
+
+    /* On macOS, don't use the per-session TMPDIR (as set e.g. by
+       sshd). This breaks build users because they don't have access
+       to the TMPDIR, in particular in ‘nix-store --serve’. */
+#if __APPLE__
+    if (hasPrefix(getEnv("TMPDIR").value_or("/tmp"), "/var/folders/"))
+        unsetenv("TMPDIR");
+#endif
+
     initLibStoreDone = true;
 }
 

src/libstore/globals.hh

@@ -26,7 +26,7 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    unsigned int parse(const std::string & str) const override;
 };
 
 struct PluginFilesSetting : public BaseSetting<Paths>
@@ -43,7 +43,7 @@ struct PluginFilesSetting : public BaseSetting<Paths>
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    Paths parse(const std::string & str) const override;
 };
 
 const uint32_t maxIdsPerBuild =
@@ -328,16 +328,6 @@ public:
           users in `build-users-group`.
 
           UIDs are allocated starting at 872415232 (0x34000000) on Linux and 56930 on macOS.
-
-          > **Warning**
-          > This is an experimental feature.
-
-          To enable it, add the following to [`nix.conf`](#):
-
-          ```
-          extra-experimental-features = auto-allocate-uids
-          auto-allocate-uids = true
-          ```
         )"};
 
     Setting<uint32_t> startId{this,
@@ -367,16 +357,6 @@ public:
 
           Cgroups are required and enabled automatically for derivations
           that require the `uid-range` system feature.
-
-          > **Warning**
-          > This is an experimental feature.
-
-          To enable it, add the following to [`nix.conf`](#):
-
-          ```
-          extra-experimental-features = cgroups
-          use-cgroups = true
-          ```
         )"};
     #endif
 
@@ -478,11 +458,6 @@ public:
         )",
         {"env-keep-derivations"}};
 
-    /**
-     * Whether to lock the Nix client and worker to the same CPU.
-     */
-    bool lockCPU;
-
     Setting<SandboxMode> sandboxMode{
         this,
         #if __linux__

src/libstore/legacy-ssh-store.cc

@@ -156,7 +156,7 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
                     throw Error("NAR hash is now mandatory");
                 info->narHash = Hash::parseAnyPrefixed(s);
             }
-            info->ca = parseContentAddressOpt(readString(conn->from));
+            info->ca = ContentAddress::parseOpt(readString(conn->from));
             info->sigs = readStrings<StringSet>(conn->from);
 
             auto s = readString(conn->from);
@@ -287,19 +287,18 @@ public:
 
         conn->to.flush();
 
-        BuildResult status {
-            .path = DerivedPath::Built {
-                .drvPath = drvPath,
-                .outputs = OutputsSpec::All { },
-            },
-        };
+        BuildResult status;
         status.status = (BuildResult::Status) readInt(conn->from);
         conn->from >> status.errorMsg;
 
         if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 3)
             conn->from >> status.timesBuilt >> status.isNonDeterministic >> status.startTime >> status.stopTime;
         if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 6) {
-            status.builtOutputs = worker_proto::read(*this, conn->from, Phantom<DrvOutputs> {});
+            auto builtOutputs = worker_proto::read(*this, conn->from, Phantom<DrvOutputs> {});
+            for (auto && [output, realisation] : builtOutputs)
+                status.builtOutputs.insert_or_assign(
+                    std::move(output.outputName),
+                    std::move(realisation));
         }
         return status;
     }
@@ -330,7 +329,7 @@ public:
 
         conn->to.flush();
 
-        BuildResult result { .path = DerivedPath::Opaque { StorePath::dummy } };
+        BuildResult result;
         result.status = (BuildResult::Status) readInt(conn->from);
 
         if (!result.success()) {
@@ -342,6 +341,9 @@ public:
     void ensurePath(const StorePath & path) override
     { unsupported("ensurePath"); }
 
+    virtual ref<FSAccessor> getFSAccessor() override
+    { unsupported("getFSAccessor"); }
+
     void computeFSClosure(const StorePathSet & paths,
         StorePathSet & out, bool flipDirection = false,
         bool includeOutputs = false, bool includeDerivers = false) override

src/libstore/local-store.cc

@@ -710,6 +710,7 @@ void canonicalisePathMetaData(const Path & path,
     canonicalisePathMetaData(path, uidRange, inodesSeen);
 }
 
+
 void LocalStore::registerDrvOutput(const Realisation & info, CheckSigsFlag checkSigs)
 {
     experimentalFeatureSettings.require(Xp::CaDerivations);
@@ -888,7 +889,7 @@ std::shared_ptr<const ValidPathInfo> LocalStore::queryPathInfoInternal(State & s
     if (s) info->sigs = tokenizeString<StringSet>(s, " ");
 
     s = (const char *) sqlite3_column_text(state.stmts->QueryPathInfo, 7);
-    if (s) info->ca = parseContentAddressOpt(s);
+    if (s) info->ca = ContentAddress::parseOpt(s);
 
     /* Get the references. */
     auto useQueryReferences(state.stmts->QueryReferences.use()(info->id));
@@ -1221,7 +1222,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
                     printStorePath(info.path), info.narSize, hashResult.second);
 
             if (info.ca) {
-                if (auto foHash = std::get_if<FixedOutputHash>(&*info.ca)) {
+                if (auto foHash = std::get_if<FixedOutputHash>(&info.ca->raw)) {
                     auto actualFoHash = hashCAPath(
                         foHash->method,
                         foHash->hash.type,
@@ -1234,7 +1235,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
                             actualFoHash.hash.to_string(Base32, true));
                     }
                 }
-                if (auto textHash = std::get_if<TextHash>(&*info.ca)) {
+                if (auto textHash = std::get_if<TextHash>(&info.ca->raw)) {
                     auto actualTextHash = hashString(htSHA256, readFile(realPath));
                     if (textHash->hash != actualTextHash) {
                         throw Error("ca hash mismatch importing path '%s';\n  specified: %s\n  got:       %s",
@@ -1320,7 +1321,19 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
 
     auto [hash, size] = hashSink->finish();
 
-    auto dstPath = makeFixedOutputPath(method, hash, name, references);
+    ContentAddressWithReferences desc = FixedOutputInfo {
+        .hash = {
+            .method = method,
+            .hash = hash,
+        },
+        .references = {
+            .others = references,
+            // caller is not capable of creating a self-reference, because this is content-addressed without modulus
+            .self = false,
+        },
+    };
+
+    auto dstPath = makeFixedOutputPathFromCA(name, desc);
 
     addTempRoot(dstPath);
 
@@ -1340,7 +1353,7 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
             autoGC();
 
             if (inMemory) {
-                 StringSource dumpSource { dump };
+                StringSource dumpSource { dump };
                 /* Restore from the NAR in memory. */
                 if (method == FileIngestionMethod::Recursive)
                     restorePath(realPath, dumpSource);
@@ -1364,10 +1377,13 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
 
             optimisePath(realPath, repair);
 
-            ValidPathInfo info { dstPath, narHash.first };
+            ValidPathInfo info {
+                *this,
+                name,
+                std::move(desc),
+                narHash.first
+            };
             info.narSize = narHash.second;
-            info.references = references;
-            info.ca = FixedOutputHash { .method = method, .hash = hash };
             registerValidPath(info);
         }
 
@@ -1384,7 +1400,10 @@ StorePath LocalStore::addTextToStore(
     const StorePathSet & references, RepairFlag repair)
 {
     auto hash = hashString(htSHA256, s);
-    auto dstPath = makeTextPath(name, hash, references);
+    auto dstPath = makeTextPath(name, TextInfo {
+        { .hash = hash },
+        references,
+    });
 
     addTempRoot(dstPath);
 

src/libstore/make-content-addressed.cc

@@ -27,18 +27,17 @@ std::map<StorePath, StorePath> makeContentAddressed(
 
         StringMap rewrites;
 
-        StorePathSet references;
-        bool hasSelfReference = false;
+        StoreReferences refs;
         for (auto & ref : oldInfo->references) {
             if (ref == path)
-                hasSelfReference = true;
+                refs.self = true;
             else {
                 auto i = remappings.find(ref);
                 auto replacement = i != remappings.end() ? i->second : ref;
                 // FIXME: warn about unremapped paths?
                 if (replacement != ref)
                     rewrites.insert_or_assign(srcStore.printStorePath(ref), srcStore.printStorePath(replacement));
-                references.insert(std::move(replacement));
+                refs.others.insert(std::move(replacement));
             }
         }
 
@@ -49,24 +48,28 @@ std::map<StorePath, StorePath> makeContentAddressed(
 
         auto narModuloHash = hashModuloSink.finish().first;
 
-        auto dstPath = dstStore.makeFixedOutputPath(
-            FileIngestionMethod::Recursive, narModuloHash, path.name(), references, hasSelfReference);
+        ValidPathInfo info {
+            dstStore,
+            path.name(),
+            FixedOutputInfo {
+                .hash = {
+                    .method = FileIngestionMethod::Recursive,
+                    .hash = narModuloHash,
+                },
+                .references = std::move(refs),
+            },
+            Hash::dummy,
+        };
 
-        printInfo("rewriting '%s' to '%s'", pathS, srcStore.printStorePath(dstPath));
+        printInfo("rewriting '%s' to '%s'", pathS, dstStore.printStorePath(info.path));
 
         StringSink sink2;
-        RewritingSink rsink2(oldHashPart, std::string(dstPath.hashPart()), sink2);
+        RewritingSink rsink2(oldHashPart, std::string(info.path.hashPart()), sink2);
         rsink2(sink.s);
         rsink2.flush();
 
-        ValidPathInfo info { dstPath, hashString(htSHA256, sink2.s) };
-        info.references = std::move(references);
-        if (hasSelfReference) info.references.insert(info.path);
+        info.narHash = hashString(htSHA256, sink2.s);
         info.narSize = sink.s.size();
-        info.ca = FixedOutputHash {
-            .method = FileIngestionMethod::Recursive,
-            .hash = narModuloHash,
-        };
 
         StringSource source(sink2.s);
         dstStore.addToStore(info, source);

src/libstore/nar-info-disk-cache.cc

@@ -273,7 +273,7 @@ public:
                 narInfo->deriver = StorePath(queryNAR.getStr(9));
             for (auto & sig : tokenizeString<Strings>(queryNAR.getStr(10), " "))
                 narInfo->sigs.insert(sig);
-            narInfo->ca = parseContentAddressOpt(queryNAR.getStr(11));
+            narInfo->ca = ContentAddress::parseOpt(queryNAR.getStr(11));
 
             return {oValid, narInfo};
         });

src/libstore/nar-info.cc

@@ -7,15 +7,18 @@ namespace nix {
 NarInfo::NarInfo(const Store & store, const std::string & s, const std::string & whence)
     : ValidPathInfo(StorePath(StorePath::dummy), Hash(Hash::dummy)) // FIXME: hack
 {
-    auto corrupt = [&]() {
-        return Error("NAR info file '%1%' is corrupt", whence);
+    unsigned line = 1;
+
+    auto corrupt = [&](const char * reason) {
+        return Error("NAR info file '%1%' is corrupt: %2%", whence,
+            std::string(reason) + (line > 0 ? " at line " + std::to_string(line) : ""));
     };
 
     auto parseHashField = [&](const std::string & s) {
         try {
             return Hash::parseAnyPrefixed(s);
         } catch (BadHash &) {
-            throw corrupt();
+            throw corrupt("bad hash");
         }
     };
 
@@ -26,12 +29,12 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
     while (pos < s.size()) {
 
         size_t colon = s.find(':', pos);
-        if (colon == std::string::npos) throw corrupt();
+        if (colon == std::string::npos) throw corrupt("expecting ':'");
 
         std::string name(s, pos, colon - pos);
 
         size_t eol = s.find('\n', colon + 2);
-        if (eol == std::string::npos) throw corrupt();
+        if (eol == std::string::npos) throw corrupt("expecting '\\n'");
 
         std::string value(s, colon + 2, eol - colon - 2);
 
@@ -47,7 +50,7 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
             fileHash = parseHashField(value);
         else if (name == "FileSize") {
             auto n = string2Int<decltype(fileSize)>(value);
-            if (!n) throw corrupt();
+            if (!n) throw corrupt("invalid FileSize");
             fileSize = *n;
         }
         else if (name == "NarHash") {
@@ -56,12 +59,12 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         }
         else if (name == "NarSize") {
             auto n = string2Int<decltype(narSize)>(value);
-            if (!n) throw corrupt();
+            if (!n) throw corrupt("invalid NarSize");
             narSize = *n;
         }
         else if (name == "References") {
             auto refs = tokenizeString<Strings>(value, " ");
-            if (!references.empty()) throw corrupt();
+            if (!references.empty()) throw corrupt("extra References");
             for (auto & r : refs)
                 references.insert(StorePath(r));
         }
@@ -72,17 +75,26 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         else if (name == "Sig")
             sigs.insert(value);
         else if (name == "CA") {
-            if (ca) throw corrupt();
+            if (ca) throw corrupt("extra CA");
             // FIXME: allow blank ca or require skipping field?
-            ca = parseContentAddressOpt(value);
+            ca = ContentAddress::parseOpt(value);
         }
 
         pos = eol + 1;
+        line += 1;
     }
 
     if (compression == "") compression = "bzip2";
 
-    if (!havePath || !haveNarHash || url.empty() || narSize == 0) throw corrupt();
+    if (!havePath || !haveNarHash || url.empty() || narSize == 0) {
+        line = 0; // don't include line information in the error
+        throw corrupt(
+            !havePath ? "StorePath missing" :
+            !haveNarHash ? "NarHash missing" :
+            url.empty() ? "URL missing" :
+            narSize == 0 ? "NarSize missing or zero"
+            : "?");
+    }
 }
 
 std::string NarInfo::to_string(const Store & store) const

src/libstore/nar-info.hh

@@ -17,6 +17,9 @@ struct NarInfo : ValidPathInfo
     uint64_t fileSize = 0;
 
     NarInfo() = delete;
+    NarInfo(const Store & store, std::string && name, ContentAddressWithReferences && ca, Hash narHash)
+        : ValidPathInfo(store, std::move(name), std::move(ca), narHash)
+    { }
     NarInfo(StorePath && path, Hash narHash) : ValidPathInfo(std::move(path), narHash) { }
     NarInfo(const ValidPathInfo & info) : ValidPathInfo(info) { }
     NarInfo(const Store & store, const std::string & s, const std::string & whence);

src/libstore/path-info.cc

@@ -21,25 +21,45 @@ void ValidPathInfo::sign(const Store & store, const SecretKey & secretKey)
     sigs.insert(secretKey.signDetached(fingerprint(store)));
 }
 
-
-bool ValidPathInfo::isContentAddressed(const Store & store) const
+std::optional<ContentAddressWithReferences> ValidPathInfo::contentAddressWithReferences() const
 {
-    if (! ca) return false;
+    if (! ca)
+        return std::nullopt;
 
-    auto caPath = std::visit(overloaded {
-        [&](const TextHash & th) {
-            return store.makeTextPath(path.name(), th.hash, references);
+    return std::visit(overloaded {
+        [&](const TextHash & th) -> ContentAddressWithReferences {
+            assert(references.count(path) == 0);
+            return TextInfo {
+                .hash = th,
+                .references = references,
+            };
         },
-        [&](const FixedOutputHash & fsh) {
+        [&](const FixedOutputHash & foh) -> ContentAddressWithReferences {
             auto refs = references;
             bool hasSelfReference = false;
             if (refs.count(path)) {
                 hasSelfReference = true;
                 refs.erase(path);
             }
-            return store.makeFixedOutputPath(fsh.method, fsh.hash, path.name(), refs, hasSelfReference);
-        }
-    }, *ca);
+            return FixedOutputInfo {
+                .hash = foh,
+                .references = {
+                    .others = std::move(refs),
+                    .self = hasSelfReference,
+                },
+            };
+        },
+    }, ca->raw);
+}
+
+bool ValidPathInfo::isContentAddressed(const Store & store) const
+{
+    auto fullCaOpt = contentAddressWithReferences();
+
+    if (! fullCaOpt)
+        return false;
+
+    auto caPath = store.makeFixedOutputPathFromCA(path.name(), *fullCaOpt);
 
     bool res = caPath == path;
 
@@ -77,6 +97,29 @@ Strings ValidPathInfo::shortRefs() const
 }
 
 
+ValidPathInfo::ValidPathInfo(
+    const Store & store,
+    std::string_view name,
+    ContentAddressWithReferences && ca,
+    Hash narHash)
+      : path(store.makeFixedOutputPathFromCA(name, ca))
+      , narHash(narHash)
+{
+    std::visit(overloaded {
+        [this](TextInfo && ti) {
+            this->references = std::move(ti.references);
+            this->ca = std::move((TextHash &&) ti);
+        },
+        [this](FixedOutputInfo && foi) {
+            this->references = std::move(foi.references.others);
+            if (foi.references.self)
+                this->references.insert(path);
+            this->ca = std::move((FixedOutputHash &&) foi);
+        },
+    }, std::move(ca).raw);
+}
+
+
 ValidPathInfo ValidPathInfo::read(Source & source, const Store & store, unsigned int format)
 {
     return read(source, store, format, store.parseStorePath(readString(source)));
@@ -93,7 +136,7 @@ ValidPathInfo ValidPathInfo::read(Source & source, const Store & store, unsigned
     if (format >= 16) {
         source >> info.ultimate;
         info.sigs = readStrings<StringSet>(source);
-        info.ca = parseContentAddressOpt(readString(source));
+        info.ca = ContentAddress::parseOpt(readString(source));
     }
     return info;
 }

src/libstore/path-info.hh

@@ -92,6 +92,13 @@ struct ValidPathInfo
 
     void sign(const Store & store, const SecretKey & secretKey);
 
+	/**
+	 * @return The `ContentAddressWithReferences` that determines the
+	 * store path for a content-addressed store object, `std::nullopt`
+	 * for an input-addressed store object.
+	 */
+    std::optional<ContentAddressWithReferences> contentAddressWithReferences() const;
+
     /**
      * @return true iff the path is verifiably content-addressed.
      */
@@ -118,6 +125,9 @@ struct ValidPathInfo
     ValidPathInfo(StorePath && path, Hash narHash) : path(std::move(path)), narHash(narHash) { };
     ValidPathInfo(const StorePath & path, Hash narHash) : path(path), narHash(narHash) { };
 
+    ValidPathInfo(const Store & store,
+        std::string_view name, ContentAddressWithReferences && ca, Hash narHash);
+
     virtual ~ValidPathInfo() { }
 
     static ValidPathInfo read(Source & source, const Store & store, unsigned int format);

src/libstore/realisation.hh

@@ -13,9 +13,25 @@ namespace nix {
 
 class Store;
 
+/**
+ * A general `Realisation` key.
+ *
+ * This is similar to a `DerivedPath::Opaque`, but the derivation is
+ * identified by its "hash modulo" instead of by its store path.
+ */
 struct DrvOutput {
-    // The hash modulo of the derivation
+    /**
+     * The hash modulo of the derivation.
+     *
+     * Computed from the derivation itself for most types of
+     * derivations, but computed from the (fixed) content address of the
+     * output for fixed-output derivations.
+     */
     Hash drvHash;
+
+    /**
+     * The name of the output.
+     */
     std::string outputName;
 
     std::string to_string() const;
@@ -60,6 +76,21 @@ struct Realisation {
     GENERATE_CMP(Realisation, me->id, me->outPath);
 };
 
+/**
+ * Collection type for a single derivation's outputs' `Realisation`s.
+ *
+ * Since these are the outputs of a single derivation, we know the
+ * output names are unique so we can use them as the map key.
+ */
+typedef std::map<std::string, Realisation> SingleDrvOutputs;
+
+/**
+ * Collection type for multiple derivations' outputs' `Realisation`s.
+ *
+ * `DrvOutput` is used because in general the derivations are not all
+ * the same, so we need to identify firstly which derivation, and
+ * secondly which output of that derivation.
+ */
 typedef std::map<DrvOutput, Realisation> DrvOutputs;
 
 struct OpaquePath {

src/libstore/remote-store.cc

@@ -78,7 +78,7 @@ void write(const Store & store, Sink & out, const std::optional<TrustedFlag> & o
 
 ContentAddress read(const Store & store, Source & from, Phantom<ContentAddress> _)
 {
-    return parseContentAddress(readString(from));
+    return ContentAddress::parse(readString(from));
 }
 
 void write(const Store & store, Sink & out, const ContentAddress & ca)
@@ -90,12 +90,12 @@ void write(const Store & store, Sink & out, const ContentAddress & ca)
 DerivedPath read(const Store & store, Source & from, Phantom<DerivedPath> _)
 {
     auto s = readString(from);
-    return DerivedPath::parse(store, s);
+    return DerivedPath::parseLegacy(store, s);
 }
 
 void write(const Store & store, Sink & out, const DerivedPath & req)
 {
-    out << req.to_string(store);
+    out << req.to_string_legacy(store);
 }
 
 
@@ -125,10 +125,26 @@ void write(const Store & store, Sink & out, const DrvOutput & drvOutput)
 }
 
 
-BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
+KeyedBuildResult read(const Store & store, Source & from, Phantom<KeyedBuildResult> _)
 {
     auto path = worker_proto::read(store, from, Phantom<DerivedPath> {});
-    BuildResult res { .path = path };
+    auto br = worker_proto::read(store, from, Phantom<BuildResult> {});
+    return KeyedBuildResult {
+        std::move(br),
+        /* .path = */ std::move(path),
+    };
+}
+
+void write(const Store & store, Sink & to, const KeyedBuildResult & res)
+{
+    worker_proto::write(store, to, res.path);
+    worker_proto::write(store, to, static_cast<const BuildResult &>(res));
+}
+
+
+BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
+{
+    BuildResult res;
     res.status = (BuildResult::Status) readInt(from);
     from
         >> res.errorMsg
@@ -136,13 +152,16 @@ BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
         >> res.isNonDeterministic
         >> res.startTime
         >> res.stopTime;
-    res.builtOutputs = worker_proto::read(store, from, Phantom<DrvOutputs> {});
+    auto builtOutputs = worker_proto::read(store, from, Phantom<DrvOutputs> {});
+    for (auto && [output, realisation] : builtOutputs)
+        res.builtOutputs.insert_or_assign(
+            std::move(output.outputName),
+            std::move(realisation));
     return res;
 }
 
 void write(const Store & store, Sink & to, const BuildResult & res)
 {
-    worker_proto::write(store, to, res.path);
     to
         << res.status
         << res.errorMsg
@@ -150,7 +169,10 @@ void write(const Store & store, Sink & to, const BuildResult & res)
         << res.isNonDeterministic
         << res.startTime
         << res.stopTime;
-    worker_proto::write(store, to, res.builtOutputs);
+    DrvOutputs builtOutputs;
+    for (auto & [output, realisation] : res.builtOutputs)
+        builtOutputs.insert_or_assign(realisation.id, realisation);
+    worker_proto::write(store, to, builtOutputs);
 }
 
 
@@ -168,7 +190,7 @@ void write(const Store & store, Sink & out, const std::optional<StorePath> & sto
 
 std::optional<ContentAddress> read(const Store & store, Source & from, Phantom<std::optional<ContentAddress>> _)
 {
-    return parseContentAddressOpt(readString(from));
+    return ContentAddress::parseOpt(readString(from));
 }
 
 void write(const Store & store, Sink & out, const std::optional<ContentAddress> & caOpt)
@@ -586,7 +608,7 @@ ref<const ValidPathInfo> RemoteStore::addCAToStore(
         conn->to
             << wopAddToStore
             << name
-            << renderContentAddressMethod(caMethod);
+            << caMethod.render();
         worker_proto::write(*this, conn->to, references);
         conn->to << repair;
 
@@ -644,7 +666,7 @@ ref<const ValidPathInfo> RemoteStore::addCAToStore(
                 }
 
             }
-        }, caMethod);
+        }, caMethod.raw);
         auto path = parseStorePath(readString(conn->from));
         // Release our connection to prevent a deadlock in queryPathInfo().
         conn_.reset();
@@ -865,7 +887,7 @@ void RemoteStore::buildPaths(const std::vector<DerivedPath> & drvPaths, BuildMod
     readInt(conn->from);
 }
 
-std::vector<BuildResult> RemoteStore::buildPathsWithResults(
+std::vector<KeyedBuildResult> RemoteStore::buildPathsWithResults(
     const std::vector<DerivedPath> & paths,
     BuildMode buildMode,
     std::shared_ptr<Store> evalStore)
@@ -880,7 +902,7 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
         writeDerivedPaths(*this, conn, paths);
         conn->to << buildMode;
         conn.processStderr();
-        return worker_proto::read(*this, conn->from, Phantom<std::vector<BuildResult>> {});
+        return worker_proto::read(*this, conn->from, Phantom<std::vector<KeyedBuildResult>> {});
     } else {
         // Avoid deadlock.
         conn_.reset();
@@ -889,21 +911,25 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
         // fails, but meh.
         buildPaths(paths, buildMode, evalStore);
 
-        std::vector<BuildResult> results;
+        std::vector<KeyedBuildResult> results;
 
         for (auto & path : paths) {
             std::visit(
                 overloaded {
                     [&](const DerivedPath::Opaque & bo) {
-                        results.push_back(BuildResult {
-                            .status = BuildResult::Substituted,
-                            .path = bo,
+                        results.push_back(KeyedBuildResult {
+                            {
+                                .status = BuildResult::Substituted,
+                            },
+                            /* .path = */ bo,
                         });
                     },
                     [&](const DerivedPath::Built & bfd) {
-                        BuildResult res {
-                            .status = BuildResult::Built,
-                            .path = bfd,
+                        KeyedBuildResult res {
+                            {
+                                .status = BuildResult::Built
+                            },
+                            /* .path = */ bfd,
                         };
 
                         OutputPathMap outputs;
@@ -922,10 +948,10 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
                                     queryRealisation(outputId);
                                 if (!realisation)
                                     throw MissingRealisation(outputId);
-                                res.builtOutputs.emplace(realisation->id, *realisation);
+                                res.builtOutputs.emplace(output, *realisation);
                             } else {
                                 res.builtOutputs.emplace(
-                                    outputId,
+                                    output,
                                     Realisation {
                                         .id = outputId,
                                         .outPath = outputPath,
@@ -952,12 +978,7 @@ BuildResult RemoteStore::buildDerivation(const StorePath & drvPath, const BasicD
     writeDerivation(conn->to, *this, drv);
     conn->to << buildMode;
     conn.processStderr();
-    BuildResult res {
-        .path = DerivedPath::Built {
-            .drvPath = drvPath,
-            .outputs = OutputsSpec::All { },
-        },
-    };
+    BuildResult res;
     res.status = (BuildResult::Status) readInt(conn->from);
     conn->from >> res.errorMsg;
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 29) {
@@ -965,7 +986,10 @@ BuildResult RemoteStore::buildDerivation(const StorePath & drvPath, const BasicD
     }
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 28) {
         auto builtOutputs = worker_proto::read(*this, conn->from, Phantom<DrvOutputs> {});
-        res.builtOutputs = builtOutputs;
+        for (auto && [output, realisation] : builtOutputs)
+            res.builtOutputs.insert_or_assign(
+                std::move(output.outputName),
+                std::move(realisation));
     }
     return res;
 }

src/libstore/remote-store.hh

@@ -114,7 +114,7 @@ public:
 
     void buildPaths(const std::vector<DerivedPath> & paths, BuildMode buildMode, std::shared_ptr<Store> evalStore) override;
 
-    std::vector<BuildResult> buildPathsWithResults(
+    std::vector<KeyedBuildResult> buildPathsWithResults(
         const std::vector<DerivedPath> & paths,
         BuildMode buildMode,
         std::shared_ptr<Store> evalStore) override;

src/libstore/sqlite.cc

@@ -239,14 +239,11 @@ SQLiteTxn::~SQLiteTxn()
     }
 }
 
-void handleSQLiteBusy(const SQLiteBusy & e)
+void handleSQLiteBusy(const SQLiteBusy & e, time_t & nextWarning)
 {
-    static std::atomic<time_t> lastWarned{0};
-
     time_t now = time(0);
-
-    if (now > lastWarned + 10) {
-        lastWarned = now;
+    if (now > nextWarning) {
+        nextWarning = now + 10;
         logWarning({
             .msg = hintfmt(e.what())
         });

src/libstore/sqlite.hh

@@ -139,7 +139,7 @@ protected:
 
 MakeError(SQLiteBusy, SQLiteError);
 
-void handleSQLiteBusy(const SQLiteBusy & e);
+void handleSQLiteBusy(const SQLiteBusy & e, time_t & nextWarning);
 
 /**
  * Convenience function for retrying a SQLite transaction when the
@@ -148,11 +148,13 @@ void handleSQLiteBusy(const SQLiteBusy & e);
 template<typename T, typename F>
 T retrySQLite(F && fun)
 {
+    time_t nextWarning = time(0) + 1;
+
     while (true) {
         try {
             return fun();
         } catch (SQLiteBusy & e) {
-            handleSQLiteBusy(e);
+            handleSQLiteBusy(e, nextWarning);
         }
     }
 }

src/libstore/store-api.cc

@@ -7,6 +7,7 @@
 #include "nar-info-disk-cache.hh"
 #include "thread-pool.hh"
 #include "url.hh"
+#include "references.hh"
 #include "archive.hh"
 #include "callback.hh"
 #include "remote-store.hh"
@@ -99,10 +100,12 @@ StorePath Store::followLinksToStorePath(std::string_view path) const
        silly, but it's done that way for compatibility).  <id> is the
        name of the output (usually, "out").
 
-   <h2> = base-16 representation of a SHA-256 hash of:
+   <h2> = base-16 representation of a SHA-256 hash of <s2>
+
+   <s2> =
      if <type> = "text:...":
        the string written to the resulting store path
-     if <type> = "source":
+     if <type> = "source:...":
        the serialisation of the path from which this store path is
        copied, as returned by hashPath()
      if <type> = "output:<id>":
@@ -163,63 +166,63 @@ StorePath Store::makeOutputPath(std::string_view id,
 }
 
 
+/* Stuff the references (if any) into the type.  This is a bit
+   hacky, but we can't put them in, say, <s2> (per the grammar above)
+   since that would be ambiguous. */
 static std::string makeType(
     const Store & store,
     std::string && type,
-    const StorePathSet & references,
-    bool hasSelfReference = false)
+    const StoreReferences & references)
 {
-    for (auto & i : references) {
+    for (auto & i : references.others) {
         type += ":";
         type += store.printStorePath(i);
     }
-    if (hasSelfReference) type += ":self";
+    if (references.self) type += ":self";
     return std::move(type);
 }
 
 
-StorePath Store::makeFixedOutputPath(
-    FileIngestionMethod method,
-    const Hash & hash,
-    std::string_view name,
-    const StorePathSet & references,
-    bool hasSelfReference) const
+StorePath Store::makeFixedOutputPath(std::string_view name, const FixedOutputInfo & info) const
 {
-    if (hash.type == htSHA256 && method == FileIngestionMethod::Recursive) {
-        return makeStorePath(makeType(*this, "source", references, hasSelfReference), hash, name);
+    if (info.hash.hash.type == htSHA256 && info.hash.method == FileIngestionMethod::Recursive) {
+        return makeStorePath(makeType(*this, "source", info.references), info.hash.hash, name);
     } else {
-        assert(references.empty());
+        assert(info.references.size() == 0);
         return makeStorePath("output:out",
             hashString(htSHA256,
                 "fixed:out:"
-                + makeFileIngestionPrefix(method)
-                + hash.to_string(Base16, true) + ":"),
+                + makeFileIngestionPrefix(info.hash.method)
+                + info.hash.hash.to_string(Base16, true) + ":"),
             name);
     }
 }
 
-StorePath Store::makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
-    const StorePathSet & references, bool hasSelfReference) const
+
+StorePath Store::makeTextPath(std::string_view name, const TextInfo & info) const
+{
+    assert(info.hash.hash.type == htSHA256);
+    return makeStorePath(
+        makeType(*this, "text", StoreReferences {
+            .others = info.references,
+            .self = false,
+        }),
+        info.hash.hash,
+        name);
+}
+
+
+StorePath Store::makeFixedOutputPathFromCA(std::string_view name, const ContentAddressWithReferences & ca) const
 {
     // New template
     return std::visit(overloaded {
-        [&](const TextHash & th) {
-            return makeTextPath(name, th.hash, references);
+        [&](const TextInfo & ti) {
+            return makeTextPath(name, ti);
         },
-        [&](const FixedOutputHash & fsh) {
-            return makeFixedOutputPath(fsh.method, fsh.hash, name, references, hasSelfReference);
+        [&](const FixedOutputInfo & foi) {
+            return makeFixedOutputPath(name, foi);
         }
-    }, ca);
-}
-
-StorePath Store::makeTextPath(std::string_view name, const Hash & hash,
-    const StorePathSet & references) const
-{
-    assert(hash.type == htSHA256);
-    /* Stuff the references (if any) into the type.  This is a bit
-       hacky, but we can't put them in `s' since that would be
-       ambiguous. */
-    return makeStorePath(makeType(*this, "text", references), hash, name);
+    }, ca.raw);
 }
 
 
@@ -229,7 +232,14 @@ std::pair<StorePath, Hash> Store::computeStorePathForPath(std::string_view name,
     Hash h = method == FileIngestionMethod::Recursive
         ? hashPath(hashAlgo, srcPath, filter).first
         : hashFile(hashAlgo, srcPath);
-    return std::make_pair(makeFixedOutputPath(method, h, name), h);
+    FixedOutputInfo caInfo {
+        .hash = {
+            .method = method,
+            .hash = h,
+        },
+        .references = {},
+    };
+    return std::make_pair(makeFixedOutputPath(name, caInfo), h);
 }
 
 
@@ -238,7 +248,10 @@ StorePath Store::computeStorePathForText(
     std::string_view s,
     const StorePathSet & references) const
 {
-    return makeTextPath(name, hashString(htSHA256, s), references);
+    return makeTextPath(name, TextInfo {
+        { .hash = hashString(htSHA256, s) },
+        references,
+    });
 }
 
 
@@ -426,11 +439,18 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
         throw Error("hash mismatch for '%s'", srcPath);
 
     ValidPathInfo info {
-        makeFixedOutputPath(method, hash, name),
+        *this,
+        name,
+        FixedOutputInfo {
+            .hash = {
+                .method = method,
+                .hash = hash,
+            },
+            .references = {},
+        },
         narHash,
     };
     info.narSize = narSize;
-    info.ca = FixedOutputHash { .method = method, .hash = hash };
 
     if (!isValidPath(info.path)) {
         auto source = sinkToSource([&](Sink & scratchpadSink) {
@@ -522,7 +542,9 @@ void Store::querySubstitutablePathInfos(const StorePathCAMap & paths, Substituta
 
             // Recompute store path so that we can use a different store root.
             if (path.second) {
-                subPath = makeFixedOutputPathFromCA(path.first.name(), *path.second);
+                subPath = makeFixedOutputPathFromCA(
+                    path.first.name(),
+                    ContentAddressWithReferences::withoutRefs(*path.second));
                 if (sub->storeDir == storeDir)
                     assert(subPath == path.first);
                 if (subPath != path.first)
@@ -539,10 +561,11 @@ void Store::querySubstitutablePathInfos(const StorePathCAMap & paths, Substituta
                 auto narInfo = std::dynamic_pointer_cast<const NarInfo>(
                     std::shared_ptr<const ValidPathInfo>(info));
                 infos.insert_or_assign(path.first, SubstitutablePathInfo{
-                    info->deriver,
-                    info->references,
-                    narInfo ? narInfo->fileSize : 0,
-                    info->narSize});
+                    .deriver = info->deriver,
+                    .references = info->references,
+                    .downloadSize = narInfo ? narInfo->fileSize : 0,
+                    .narSize = info->narSize,
+                });
             } catch (InvalidPath &) {
             } catch (SubstituterDisabled &) {
             } catch (Error & e) {
@@ -1026,7 +1049,9 @@ void copyStorePath(
     // recompute store path on the chance dstStore does it differently
     if (info->ca && info->references.empty()) {
         auto info2 = make_ref<ValidPathInfo>(*info);
-        info2->path = dstStore.makeFixedOutputPathFromCA(info->path.name(), *info->ca);
+        info2->path = dstStore.makeFixedOutputPathFromCA(
+            info->path.name(),
+            info->contentAddressWithReferences().value());
         if (dstStore.storeDir == srcStore.storeDir)
             assert(info->path == info2->path);
         info = info2;
@@ -1138,7 +1163,9 @@ std::map<StorePath, StorePath> copyPaths(
         auto storePathForSrc = currentPathInfo.path;
         auto storePathForDst = storePathForSrc;
         if (currentPathInfo.ca && currentPathInfo.references.empty()) {
-            storePathForDst = dstStore.makeFixedOutputPathFromCA(storePathForSrc.name(), *currentPathInfo.ca);
+            storePathForDst = dstStore.makeFixedOutputPathFromCA(
+                currentPathInfo.path.name(),
+                currentPathInfo.contentAddressWithReferences().value());
             if (dstStore.storeDir == srcStore.storeDir)
                 assert(storePathForDst == storePathForSrc);
             if (storePathForDst != storePathForSrc)

src/libstore/store-api.hh

@@ -92,6 +92,7 @@ enum BuildMode { bmNormal, bmRepair, bmCheck };
 enum TrustedFlag : bool { NotTrusted = false, Trusted = true };
 
 struct BuildResult;
+struct KeyedBuildResult;
 
 
 typedef std::map<StorePath, std::optional<ContentAddress>> StorePathCAMap;
@@ -268,17 +269,11 @@ public:
     StorePath makeOutputPath(std::string_view id,
         const Hash & hash, std::string_view name) const;
 
-    StorePath makeFixedOutputPath(FileIngestionMethod method,
-        const Hash & hash, std::string_view name,
-        const StorePathSet & references = {},
-        bool hasSelfReference = false) const;
+    StorePath makeFixedOutputPath(std::string_view name, const FixedOutputInfo & info) const;
 
-    StorePath makeTextPath(std::string_view name, const Hash & hash,
-        const StorePathSet & references = {}) const;
+    StorePath makeTextPath(std::string_view name, const TextInfo & info) const;
 
-    StorePath makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
-        const StorePathSet & references = {},
-        bool hasSelfReference = false) const;
+    StorePath makeFixedOutputPathFromCA(std::string_view name, const ContentAddressWithReferences & ca) const;
 
     /**
      * Preparatory part of addToStore().
@@ -575,7 +570,7 @@ public:
      * case of a build/substitution error, this function won't throw an
      * exception, but return a BuildResult containing an error message.
      */
-    virtual std::vector<BuildResult> buildPathsWithResults(
+    virtual std::vector<KeyedBuildResult> buildPathsWithResults(
         const std::vector<DerivedPath> & paths,
         BuildMode buildMode = bmNormal,
         std::shared_ptr<Store> evalStore = nullptr);
@@ -678,8 +673,7 @@ public:
     /**
      * @return An object to access files in the Nix store.
      */
-    virtual ref<FSAccessor> getFSAccessor()
-    { unsupported("getFSAccessor"); }
+    virtual ref<FSAccessor> getFSAccessor() = 0;
 
     /**
      * Repair the contents of the given path by redownloading it using

src/libstore/tests/derivation.cc

@@ -1,6 +1,7 @@
 #include <nlohmann/json.hpp>
 #include <gtest/gtest.h>
 
+#include "experimental-features.hh"
 #include "derivations.hh"
 
 #include "tests/libstore.hh"
@@ -9,10 +10,32 @@ namespace nix {
 
 class DerivationTest : public LibStoreTest
 {
+public:
+    /**
+     * We set these in tests rather than the regular globals so we don't have
+     * to worry about race conditions if the tests run concurrently.
+     */
+    ExperimentalFeatureSettings mockXpSettings;
 };
 
-#define TEST_JSON(NAME, STR, VAL, DRV_NAME, OUTPUT_NAME) \
-    TEST_F(DerivationTest, DerivationOutput_ ## NAME ## _to_json) {    \
+class CaDerivationTest : public DerivationTest
+{
+    void SetUp() override
+    {
+        mockXpSettings.set("experimental-features", "ca-derivations");
+    }
+};
+
+class ImpureDerivationTest : public DerivationTest
+{
+    void SetUp() override
+    {
+        mockXpSettings.set("experimental-features", "impure-derivations");
+    }
+};
+
+#define TEST_JSON(FIXTURE, NAME, STR, VAL, DRV_NAME, OUTPUT_NAME) \
+    TEST_F(FIXTURE, DerivationOutput_ ## NAME ## _to_json) {    \
         using nlohmann::literals::operator "" _json;           \
         ASSERT_EQ(                                             \
             STR ## _json,                                      \
@@ -22,7 +45,7 @@ class DerivationTest : public LibStoreTest
                 OUTPUT_NAME));                                 \
     }                                                          \
                                                                \
-    TEST_F(DerivationTest, DerivationOutput_ ## NAME ## _from_json) {  \
+    TEST_F(FIXTURE, DerivationOutput_ ## NAME ## _from_json) {  \
         using nlohmann::literals::operator "" _json;           \
         ASSERT_EQ(                                             \
             DerivationOutput { VAL },                          \
@@ -30,10 +53,11 @@ class DerivationTest : public LibStoreTest
                 *store,                                        \
                 DRV_NAME,                                      \
                 OUTPUT_NAME,                                   \
-                STR ## _json));                                \
+                STR ## _json,                                  \
+                mockXpSettings));                              \
     }
 
-TEST_JSON(inputAddressed,
+TEST_JSON(DerivationTest, inputAddressed,
     R"({
         "path": "/nix/store/c015dhfh5l0lp6wxyvdn7bmwhbbr6hr9-drv-name-output-name"
     })",
@@ -42,7 +66,7 @@ TEST_JSON(inputAddressed,
     }),
     "drv-name", "output-name")
 
-TEST_JSON(caFixed,
+TEST_JSON(DerivationTest, caFixed,
     R"({
         "hashAlgo": "r:sha256",
         "hash": "894517c9163c896ec31a2adbd33c0681fd5f45b2c0ef08a64c92a03fb97f390f",
@@ -56,7 +80,7 @@ TEST_JSON(caFixed,
     }),
     "drv-name", "output-name")
 
-TEST_JSON(caFloating,
+TEST_JSON(CaDerivationTest, caFloating,
     R"({
         "hashAlgo": "r:sha256"
     })",
@@ -66,12 +90,12 @@ TEST_JSON(caFloating,
     }),
     "drv-name", "output-name")
 
-TEST_JSON(deferred,
+TEST_JSON(DerivationTest, deferred,
     R"({ })",
     DerivationOutput::Deferred { },
     "drv-name", "output-name")
 
-TEST_JSON(impure,
+TEST_JSON(ImpureDerivationTest, impure,
     R"({
         "hashAlgo": "r:sha256",
         "impure": true

src/libstore/tests/derived-path.cc

@@ -51,6 +51,14 @@ TEST_F(DerivedPathTest, force_init)
 {
 }
 
+RC_GTEST_FIXTURE_PROP(
+    DerivedPathTest,
+    prop_legacy_round_rip,
+    (const DerivedPath & o))
+{
+    RC_ASSERT(o == DerivedPath::parseLegacy(*store, o.to_string_legacy(*store)));
+}
+
 RC_GTEST_FIXTURE_PROP(
     DerivedPathTest,
     prop_round_rip,

src/libstore/worker-protocol.hh

@@ -103,6 +103,7 @@ MAKE_WORKER_PROTO(, DerivedPath);
 MAKE_WORKER_PROTO(, Realisation);
 MAKE_WORKER_PROTO(, DrvOutput);
 MAKE_WORKER_PROTO(, BuildResult);
+MAKE_WORKER_PROTO(, KeyedBuildResult);
 MAKE_WORKER_PROTO(, std::optional<TrustedFlag>);
 
 MAKE_WORKER_PROTO(template<typename T>, std::vector<T>);

src/libutil/canon-path.cc

@@ -13,6 +13,11 @@ CanonPath::CanonPath(std::string_view raw, const CanonPath & root)
     : path(absPath((Path) raw, root.abs()))
 { }
 
+CanonPath CanonPath::fromCwd(std::string_view path)
+{
+    return CanonPath(unchecked_t(), absPath((Path) path));
+}
+
 std::optional<CanonPath> CanonPath::parent() const
 {
     if (isRoot()) return std::nullopt;

src/libutil/canon-path.hh

@@ -46,6 +46,8 @@ public:
         : path(std::move(path))
     { }
 
+    static CanonPath fromCwd(std::string_view path = ".");
+
     static CanonPath root;
 
     /**

src/libutil/compression.cc

@@ -23,7 +23,7 @@ struct ChunkedCompressionSink : CompressionSink
 {
     uint8_t outbuf[32 * 1024];
 
-    void write(std::string_view data) override
+    void writeUnbuffered(std::string_view data) override
     {
         const size_t CHUNK_SIZE = sizeof(outbuf) << 2;
         while (!data.empty()) {
@@ -103,7 +103,7 @@ struct ArchiveCompressionSink : CompressionSink
             throw Error(reason, archive_error_string(this->archive));
     }
 
-    void write(std::string_view data) override
+    void writeUnbuffered(std::string_view data) override
     {
         ssize_t result = archive_write_data(archive, data.data(), data.length());
         if (result <= 0) check(result);
@@ -136,7 +136,7 @@ struct NoneSink : CompressionSink
             warn("requested compression level '%d' not supported by compression method 'none'", level);
     }
     void finish() override { flush(); }
-    void write(std::string_view data) override { nextSink(data); }
+    void writeUnbuffered(std::string_view data) override { nextSink(data); }
 };
 
 struct BrotliDecompressionSink : ChunkedCompressionSink

src/libutil/compression.hh

@@ -12,7 +12,7 @@ namespace nix {
 struct CompressionSink : BufferedSink, FinishSink
 {
     using BufferedSink::operator ();
-    using BufferedSink::write;
+    using BufferedSink::writeUnbuffered;
     using FinishSink::finish;
 };
 

src/libutil/config-impl.hh

@@ -0,0 +1,71 @@
+#pragma once
+/**
+ * @file
+ *
+ * Template implementations (as opposed to mere declarations).
+ *
+ * One only needs to include this when one is declaring a
+ * `BaseClass<CustomType>` setting, or as derived class of such an
+ * instantiation.
+ */
+
+#include "config.hh"
+
+namespace nix {
+
+template<> struct BaseSetting<Strings>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<StringSet>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<StringMap>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<std::set<ExperimentalFeature>>::trait
+{
+    static constexpr bool appendable = true;
+};
+
+template<typename T>
+struct BaseSetting<T>::trait
+{
+    static constexpr bool appendable = false;
+};
+
+template<typename T>
+bool BaseSetting<T>::isAppendable()
+{
+    return trait::appendable;
+}
+
+template<> void BaseSetting<Strings>::appendOrSet(Strings && newValue, bool append);
+template<> void BaseSetting<StringSet>::appendOrSet(StringSet && newValue, bool append);
+template<> void BaseSetting<StringMap>::appendOrSet(StringMap && newValue, bool append);
+template<> void BaseSetting<std::set<ExperimentalFeature>>::appendOrSet(std::set<ExperimentalFeature> && newValue, bool append);
+
+template<typename T>
+void BaseSetting<T>::appendOrSet(T && newValue, bool append)
+{
+    static_assert(!trait::appendable, "using default `appendOrSet` implementation with an appendable type");
+    assert(!append);
+    value = std::move(newValue);
+}
+
+template<typename T>
+void BaseSetting<T>::set(const std::string & str, bool append)
+{
+    if (experimentalFeatureSettings.isEnabled(experimentalFeature))
+        appendOrSet(parse(str), append);
+    else {
+        assert(experimentalFeature);
+        warn("Ignoring setting '%s' because experimental feature '%s' is not enabled",
+            name,
+            showExperimentalFeature(*experimentalFeature));
+    }
+}
+
+}

src/libutil/config.cc

@@ -3,6 +3,8 @@
 #include "abstract-setting-to-json.hh"
 #include "experimental-features.hh"
 
+#include "config-impl.hh"
+
 #include <nlohmann/json.hpp>
 
 namespace nix {
@@ -70,23 +72,18 @@ void AbstractConfig::reapplyUnknownSettings()
         set(s.first, s.second);
 }
 
-// Whether we should process the option. Excludes aliases, which are handled elsewhere, and disabled features.
-static bool applicable(const Config::SettingData & sd)
-{
-    return !sd.isAlias
-        && experimentalFeatureSettings.isEnabled(sd.setting->experimentalFeature);
-}
-
 void Config::getSettings(std::map<std::string, SettingInfo> & res, bool overriddenOnly)
 {
     for (auto & opt : _settings)
-        if (applicable(opt.second) && (!overriddenOnly || opt.second.setting->overridden))
+        if (!opt.second.isAlias && (!overriddenOnly || opt.second.setting->overridden))
             res.emplace(opt.first, SettingInfo{opt.second.setting->to_string(), opt.second.setting->description});
 }
 
 void AbstractConfig::applyConfig(const std::string & contents, const std::string & path) {
     unsigned int pos = 0;
 
+    std::vector<std::pair<std::string, std::string>> parsedContents;
+
     while (pos < contents.size()) {
         std::string line;
         while (pos < contents.size() && contents[pos] != '\n')
@@ -132,8 +129,21 @@ void AbstractConfig::applyConfig(const std::string & contents, const std::string
         auto i = tokens.begin();
         advance(i, 2);
 
-        set(name, concatStringsSep(" ", Strings(i, tokens.end()))); // FIXME: slow
+        parsedContents.push_back({
+            name,
+            concatStringsSep(" ", Strings(i, tokens.end())),
+        });
     };
+
+    // First apply experimental-feature related settings
+    for (auto & [name, value] : parsedContents)
+        if (name == "experimental-features" || name == "extra-experimental-features")
+            set(name, value);
+
+    // Then apply other settings
+    for (auto & [name, value] : parsedContents)
+        if (name != "experimental-features" && name != "extra-experimental-features")
+            set(name, value);
 }
 
 void AbstractConfig::applyConfigFile(const Path & path)
@@ -154,7 +164,7 @@ nlohmann::json Config::toJSON()
 {
     auto res = nlohmann::json::object();
     for (auto & s : _settings)
-        if (applicable(s.second))
+        if (!s.second.isAlias)
             res.emplace(s.first, s.second.setting->toJSON());
     return res;
 }
@@ -163,7 +173,7 @@ std::string Config::toKeyValue()
 {
     auto res = std::string();
     for (auto & s : _settings)
-        if (applicable(s.second))
+        if (s.second.isAlias)
             res += fmt("%s = %s\n", s.first, s.second.setting->to_string());
     return res;
 }
@@ -171,9 +181,6 @@ std::string Config::toKeyValue()
 void Config::convertToArgs(Args & args, const std::string & category)
 {
     for (auto & s : _settings) {
-        /* We do include args for settings gated on disabled
-           experimental-features. The args themselves however will also be
-           gated on any experimental feature the underlying setting is. */
         if (!s.second.isAlias)
             s.second.setting->convertToArg(args, category);
     }
@@ -201,6 +208,10 @@ std::map<std::string, nlohmann::json> AbstractSetting::toJSONObject()
     std::map<std::string, nlohmann::json> obj;
     obj.emplace("description", description);
     obj.emplace("aliases", aliases);
+    if (experimentalFeature)
+        obj.emplace("experimentalFeature", *experimentalFeature);
+    else
+        obj.emplace("experimentalFeature", nullptr);
     return obj;
 }
 
@@ -208,12 +219,6 @@ void AbstractSetting::convertToArg(Args & args, const std::string & category)
 {
 }
 
-template<typename T>
-bool BaseSetting<T>::isAppendable()
-{
-    return false;
-}
-
 template<typename T>
 void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
 {
@@ -237,9 +242,9 @@ void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
         });
 }
 
-template<> void BaseSetting<std::string>::set(const std::string & str, bool append)
+template<> std::string BaseSetting<std::string>::parse(const std::string & str) const
 {
-    value = str;
+    return str;
 }
 
 template<> std::string BaseSetting<std::string>::to_string() const
@@ -248,11 +253,11 @@ template<> std::string BaseSetting<std::string>::to_string() const
 }
 
 template<typename T>
-void BaseSetting<T>::set(const std::string & str, bool append)
+T BaseSetting<T>::parse(const std::string & str) const
 {
     static_assert(std::is_integral<T>::value, "Integer required.");
     if (auto n = string2Int<T>(str))
-        value = *n;
+        return *n;
     else
         throw UsageError("setting '%s' has invalid value '%s'", name, str);
 }
@@ -264,12 +269,12 @@ std::string BaseSetting<T>::to_string() const
     return std::to_string(value);
 }
 
-template<> void BaseSetting<bool>::set(const std::string & str, bool append)
+template<> bool BaseSetting<bool>::parse(const std::string & str) const
 {
     if (str == "true" || str == "yes" || str == "1")
-        value = true;
+        return true;
     else if (str == "false" || str == "no" || str == "0")
-        value = false;
+        return false;
     else
         throw UsageError("Boolean setting '%s' has invalid value '%s'", name, str);
 }
@@ -297,16 +302,15 @@ template<> void BaseSetting<bool>::convertToArg(Args & args, const std::string &
     });
 }
 
-template<> void BaseSetting<Strings>::set(const std::string & str, bool append)
+template<> Strings BaseSetting<Strings>::parse(const std::string & str) const
 {
-    auto ss = tokenizeString<Strings>(str);
-    if (!append) value.clear();
-    for (auto & s : ss) value.push_back(std::move(s));
+    return tokenizeString<Strings>(str);
 }
 
-template<> bool BaseSetting<Strings>::isAppendable()
+template<> void BaseSetting<Strings>::appendOrSet(Strings && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue)) value.push_back(std::move(s));
 }
 
 template<> std::string BaseSetting<Strings>::to_string() const
@@ -314,16 +318,16 @@ template<> std::string BaseSetting<Strings>::to_string() const
     return concatStringsSep(" ", value);
 }
 
-template<> void BaseSetting<StringSet>::set(const std::string & str, bool append)
+template<> StringSet BaseSetting<StringSet>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
-    for (auto & s : tokenizeString<StringSet>(str))
-        value.insert(s);
+    return tokenizeString<StringSet>(str);
 }
 
-template<> bool BaseSetting<StringSet>::isAppendable()
+template<> void BaseSetting<StringSet>::appendOrSet(StringSet && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue))
+        value.insert(s);
 }
 
 template<> std::string BaseSetting<StringSet>::to_string() const
@@ -331,21 +335,24 @@ template<> std::string BaseSetting<StringSet>::to_string() const
     return concatStringsSep(" ", value);
 }
 
-template<> void BaseSetting<std::set<ExperimentalFeature>>::set(const std::string & str, bool append)
+template<> std::set<ExperimentalFeature> BaseSetting<std::set<ExperimentalFeature>>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
+    std::set<ExperimentalFeature> res;
     for (auto & s : tokenizeString<StringSet>(str)) {
         auto thisXpFeature = parseExperimentalFeature(s);
         if (thisXpFeature)
-            value.insert(thisXpFeature.value());
+            res.insert(thisXpFeature.value());
         else
             warn("unknown experimental feature '%s'", s);
     }
+    return res;
 }
 
-template<> bool BaseSetting<std::set<ExperimentalFeature>>::isAppendable()
+template<> void BaseSetting<std::set<ExperimentalFeature>>::appendOrSet(std::set<ExperimentalFeature> && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue))
+        value.insert(s);
 }
 
 template<> std::string BaseSetting<std::set<ExperimentalFeature>>::to_string() const
@@ -356,20 +363,23 @@ template<> std::string BaseSetting<std::set<ExperimentalFeature>>::to_string() c
     return concatStringsSep(" ", stringifiedXpFeatures);
 }
 
-template<> void BaseSetting<StringMap>::set(const std::string & str, bool append)
+template<> StringMap BaseSetting<StringMap>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
+    StringMap res;
     for (auto & s : tokenizeString<Strings>(str)) {
         auto eq = s.find_first_of('=');
         if (std::string::npos != eq)
-            value.emplace(std::string(s, 0, eq), std::string(s, eq + 1));
+            res.emplace(std::string(s, 0, eq), std::string(s, eq + 1));
         // else ignored
     }
+    return res;
 }
 
-template<> bool BaseSetting<StringMap>::isAppendable()
+template<> void BaseSetting<StringMap>::appendOrSet(StringMap && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && [k, v] : std::move(newValue))
+        value.emplace(std::move(k), std::move(v));
 }
 
 template<> std::string BaseSetting<StringMap>::to_string() const
@@ -393,15 +403,15 @@ template class BaseSetting<StringSet>;
 template class BaseSetting<StringMap>;
 template class BaseSetting<std::set<ExperimentalFeature>>;
 
-void PathSetting::set(const std::string & str, bool append)
+Path PathSetting::parse(const std::string & str) const
 {
     if (str == "") {
         if (allowEmpty)
-            value = "";
+            return "";
         else
             throw UsageError("setting '%s' cannot be empty", name);
     } else
-        value = canonPath(str);
+        return canonPath(str);
 }
 
 bool GlobalConfig::set(const std::string & name, const std::string & value)

src/libutil/config.hh

@@ -215,8 +215,11 @@ protected:
 
     virtual void set(const std::string & value, bool append = false) = 0;
 
-    virtual bool isAppendable()
-    { return false; }
+    /**
+     * Whether the type is appendable; i.e. whether the `append`
+     * parameter to `set()` is allowed to be `true`.
+     */
+    virtual bool isAppendable() = 0;
 
     virtual std::string to_string() const = 0;
 
@@ -241,6 +244,23 @@ protected:
     const T defaultValue;
     const bool documentDefault;
 
+    /**
+     * Parse the string into a `T`.
+     *
+     * Used by `set()`.
+     */
+    virtual T parse(const std::string & str) const;
+
+    /**
+     * Append or overwrite `value` with `newValue`.
+     *
+     * Some types to do not support appending in which case `append`
+     * should never be passed. The default handles this case.
+     *
+     * @param append Whether to append or overwrite.
+     */
+    virtual void appendOrSet(T && newValue, bool append);
+
 public:
 
     BaseSetting(const T & def,
@@ -268,9 +288,25 @@ public:
     template<typename U>
     void setDefault(const U & v) { if (!overridden) value = v; }
 
-    void set(const std::string & str, bool append = false) override;
+    /**
+     * Require any experimental feature the setting depends on
+     *
+     * Uses `parse()` to get the value from `str`, and `appendOrSet()`
+     * to set it.
+     */
+    void set(const std::string & str, bool append = false) override final;
 
-    bool isAppendable() override;
+    /**
+     * C++ trick; This is template-specialized to compile-time indicate whether
+     * the type is appendable.
+     */
+    struct trait;
+
+    /**
+     * Always defined based on the C++ magic
+     * with `trait` above.
+     */
+    bool isAppendable() override final;
 
     virtual void override(const T & v)
     {
@@ -336,7 +372,7 @@ public:
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    Path parse(const std::string & str) const override;
 
     Path operator +(const char * p) const { return value + p; }
 
@@ -371,8 +407,23 @@ extern GlobalConfig globalConfig;
 
 struct ExperimentalFeatureSettings : Config {
 
-    Setting<std::set<ExperimentalFeature>> experimentalFeatures{this, {}, "experimental-features",
-        "Experimental Nix features to enable."};
+    Setting<std::set<ExperimentalFeature>> experimentalFeatures{
+        this, {}, "experimental-features",
+        R"(
+          Experimental features that are enabled.
+
+          Example:
+
+          ```
+          experimental-features = nix-command flakes
+          ```
+
+          The following experimental features are available:
+
+          {{#include experimental-features-shortlist.md}}
+
+          Experimental features are [further documented in the manual](@docroot@/contributing/experimental-features.md).
+        )"};
 
     /**
      * Check whether the given experimental feature is enabled.

src/libutil/experimental-features.cc

@@ -5,29 +5,219 @@
 
 namespace nix {
 
-std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
-    { Xp::CaDerivations, "ca-derivations" },
-    { Xp::ImpureDerivations, "impure-derivations" },
-    { Xp::Flakes, "flakes" },
-    { Xp::NixCommand, "nix-command" },
-    { Xp::RecursiveNix, "recursive-nix" },
-    { Xp::NoUrlLiterals, "no-url-literals" },
-    { Xp::FetchClosure, "fetch-closure" },
-    { Xp::ReplFlake, "repl-flake" },
-    { Xp::AutoAllocateUids, "auto-allocate-uids" },
-    { Xp::Cgroups, "cgroups" },
-    { Xp::DiscardReferences, "discard-references" },
+struct ExperimentalFeatureDetails
+{
+    ExperimentalFeature tag;
+    std::string_view name;
+    std::string_view description;
 };
 
+constexpr std::array<ExperimentalFeatureDetails, 12> xpFeatureDetails = {{
+    {
+        .tag = Xp::CaDerivations,
+        .name = "ca-derivations",
+        .description = R"(
+            Allow derivations to be content-addressed in order to prevent
+            rebuilds when changes to the derivation do not result in changes to
+            the derivation's output. See
+            [__contentAddressed](@docroot@/language/advanced-attributes.md#adv-attr-__contentAddressed)
+            for details.
+        )",
+    },
+    {
+        .tag = Xp::ImpureDerivations,
+        .name = "impure-derivations",
+        .description = R"(
+            Allow derivations to produce non-fixed outputs by setting the
+            `__impure` derivation attribute to `true`. An impure derivation can
+            have differing outputs each time it is built.
+
+            Example:
+
+            ```
+            derivation {
+              name = "impure";
+              builder = /bin/sh;
+              __impure = true; # mark this derivation as impure
+              args = [ "-c" "read -n 10 random < /dev/random; echo $random > $out" ];
+              system = builtins.currentSystem;
+            }
+            ```
+
+            Each time this derivation is built, it can produce a different
+            output (as the builder outputs random bytes to `$out`).  Impure
+            derivations also have access to the network, and only fixed-output
+            or other impure derivations can rely on impure derivations. Finally,
+            an impure derivation cannot also be
+            [content-addressed](#xp-feature-ca-derivations).
+        )",
+    },
+    {
+        .tag = Xp::Flakes,
+        .name = "flakes",
+        .description = R"(
+            Enable flakes. See the manual entry for [`nix
+            flake`](@docroot@/command-ref/new-cli/nix3-flake.md) for details.
+        )",
+    },
+    {
+        .tag = Xp::NixCommand,
+        .name = "nix-command",
+        .description = R"(
+            Enable the new `nix` subcommands. See the manual on
+            [`nix`](@docroot@/command-ref/new-cli/nix.md) for details.
+        )",
+    },
+    {
+        .tag = Xp::RecursiveNix,
+        .name = "recursive-nix",
+        .description = R"(
+            Allow derivation builders to call Nix, and thus build derivations
+            recursively.
+
+            Example:
+
+            ```
+            with import <nixpkgs> {};
+
+            runCommand "foo"
+              {
+                 buildInputs = [ nix jq ];
+                 NIX_PATH = "nixpkgs=${<nixpkgs>}";
+              }
+              ''
+                hello=$(nix-build -E '(import <nixpkgs> {}).hello.overrideDerivation (args: { name = "recursive-hello"; })')
+
+                mkdir -p $out/bin
+                ln -s $hello/bin/hello $out/bin/hello
+              ''
+            ```
+
+            An important restriction on recursive builders is disallowing
+            arbitrary substitutions. For example, running
+
+            ```
+            nix-store -r /nix/store/kmwd1hq55akdb9sc7l3finr175dajlby-hello-2.10
+            ```
+
+            in the above `runCommand` script would be disallowed, as this could
+            lead to derivations with hidden dependencies or breaking
+            reproducibility by relying on the current state of the Nix store. An
+            exception would be if
+            `/nix/store/kmwd1hq55akdb9sc7l3finr175dajlby-hello-2.10` were
+            already in the build inputs or built by a previous recursive Nix
+            call.
+        )",
+    },
+    {
+        .tag = Xp::NoUrlLiterals,
+        .name = "no-url-literals",
+        .description = R"(
+            Disallow unquoted URLs as part of the Nix language syntax. The Nix
+            language allows for URL literals, like so:
+
+            ```
+            $ nix repl
+            Welcome to Nix 2.15.0. Type :? for help.
+
+            nix-repl> http://foo
+            "http://foo"
+            ```
+
+            But enabling this experimental feature will cause the Nix parser to
+            throw an error when encountering a URL literal:
+
+            ```
+            $ nix repl --extra-experimental-features 'no-url-literals'
+            Welcome to Nix 2.15.0. Type :? for help.
+
+            nix-repl> http://foo
+            error: URL literals are disabled
+
+            at «string»:1:1:
+
+            1| http://foo
+             | ^
+
+            ```
+
+            While this is currently an experimental feature, unquoted URLs are
+            being deprecated and their usage is discouraged.
+
+            The reason is that, as opposed to path literals, URLs have no
+            special properties that distinguish them from regular strings, URLs
+            containing parameters have to be quoted anyway, and unquoted URLs
+            may confuse external tooling.
+        )",
+    },
+    {
+        .tag = Xp::FetchClosure,
+        .name = "fetch-closure",
+        .description = R"(
+            Enable the use of the [`fetchClosure`](@docroot@/language/builtins.md#builtins-fetchClosure) built-in function in the Nix language.
+        )",
+    },
+    {
+        .tag = Xp::ReplFlake,
+        .name = "repl-flake",
+        .description = R"(
+            Allow passing [installables](@docroot@/command-ref/new-cli/nix.md#installables) to `nix repl`, making its interface consistent with the other experimental commands.
+        )",
+    },
+    {
+        .tag = Xp::AutoAllocateUids,
+        .name = "auto-allocate-uids",
+        .description = R"(
+            Allows Nix to automatically pick UIDs for builds, rather than creating
+            `nixbld*` user accounts. See the [`auto-allocate-uids`](#conf-auto-allocate-uids) setting for details.
+        )",
+    },
+    {
+        .tag = Xp::Cgroups,
+        .name = "cgroups",
+        .description = R"(
+            Allows Nix to execute builds inside cgroups. See
+            the [`use-cgroups`](#conf-use-cgroups) setting for details.
+        )",
+    },
+    {
+        .tag = Xp::DiscardReferences,
+        .name = "discard-references",
+        .description = R"(
+            Allow the use of the [`unsafeDiscardReferences`](@docroot@/language/advanced-attributes.html#adv-attr-unsafeDiscardReferences) attribute in derivations
+            that use [structured attributes](@docroot@/language/advanced-attributes.html#adv-attr-structuredAttrs). This disables scanning of outputs for
+            runtime dependencies.
+        )",
+    },
+    {
+        .tag = Xp::DaemonTrustOverride,
+        .name = "daemon-trust-override",
+        .description = R"(
+            Allow forcing trusting or not trusting clients with
+            `nix-daemon`. This is useful for testing, but possibly also
+            useful for various experiments with `nix-daemon --stdio`
+            networking.
+        )",
+    },
+}};
+
+static_assert(
+    []() constexpr {
+        for (auto [index, feature] : enumerate(xpFeatureDetails))
+            if (index != (size_t)feature.tag)
+                return false;
+        return true;
+    }(),
+    "array order does not match enum tag order");
+
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)
 {
     using ReverseXpMap = std::map<std::string_view, ExperimentalFeature>;
 
-    static auto reverseXpMap = []()
-    {
+    static std::unique_ptr<ReverseXpMap> reverseXpMap = []() {
         auto reverseXpMap = std::make_unique<ReverseXpMap>();
-        for (auto & [feature, name] : stringifiedXpFeatures)
-            (*reverseXpMap)[name] = feature;
+        for (auto & xpFeature : xpFeatureDetails)
+            (*reverseXpMap)[xpFeature.name] = xpFeature.tag;
         return reverseXpMap;
     }();
 
@@ -37,20 +227,27 @@ const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::str
         return std::nullopt;
 }
 
-std::string_view showExperimentalFeature(const ExperimentalFeature feature)
+std::string_view showExperimentalFeature(const ExperimentalFeature tag)
 {
-    const auto ret = get(stringifiedXpFeatures, feature);
-    assert(ret);
-    return *ret;
+    assert((size_t)tag < xpFeatureDetails.size());
+    return xpFeatureDetails[(size_t)tag].name;
+}
+
+nlohmann::json documentExperimentalFeatures() 
+{
+    StringMap res;
+    for (auto & xpFeature : xpFeatureDetails)
+        res[std::string { xpFeature.name }] =
+            trim(stripIndentation(xpFeature.description));
+    return (nlohmann::json) res;
 }
 
 std::set<ExperimentalFeature> parseFeatures(const std::set<std::string> & rawFeatures)
 {
     std::set<ExperimentalFeature> res;
-    for (auto & rawFeature : rawFeatures) {
+    for (auto & rawFeature : rawFeatures)
         if (auto feature = parseExperimentalFeature(rawFeature))
             res.insert(*feature);
-    }
     return res;
 }
 

src/libutil/experimental-features.hh

@@ -11,8 +11,9 @@ namespace nix {
 /**
  * The list of available experimental features.
  *
- * If you update this, don’t forget to also change the map defining their
- * string representation in the corresponding `.cc` file.
+ * If you update this, don’t forget to also change the map defining
+ * their string representation and documentation in the corresponding
+ * `.cc` file as well.
  */
 enum struct ExperimentalFeature
 {
@@ -27,6 +28,7 @@ enum struct ExperimentalFeature
     AutoAllocateUids,
     Cgroups,
     DiscardReferences,
+    DaemonTrustOverride,
 };
 
 /**
@@ -34,26 +36,52 @@ enum struct ExperimentalFeature
  */
 using Xp = ExperimentalFeature;
 
+/**
+ * Parse an experimental feature (enum value) from its name. Experimental
+ * feature flag names are hyphenated and do not contain spaces.
+ */
 const std::optional<ExperimentalFeature> parseExperimentalFeature(
         const std::string_view & name);
+
+/**
+ * Show the name of an experimental feature. This is the opposite of
+ * parseExperimentalFeature().
+ */
 std::string_view showExperimentalFeature(const ExperimentalFeature);
 
+/**
+ * Compute the documentation of all experimental features.
+ *
+ * See `doc/manual` for how this information is used.
+ */
+nlohmann::json documentExperimentalFeatures();
+
+/**
+ * Shorthand for `str << showExperimentalFeature(feature)`.
+ */
 std::ostream & operator<<(
         std::ostream & str,
         const ExperimentalFeature & feature);
 
 /**
- * Parse a set of strings to the corresponding set of experimental features,
- * ignoring (but warning for) any unkwown feature.
+ * Parse a set of strings to the corresponding set of experimental
+ * features, ignoring (but warning for) any unknown feature.
  */
 std::set<ExperimentalFeature> parseFeatures(const std::set<std::string> &);
 
+/**
+ * An experimental feature was required for some (experimental)
+ * operation, but was not enabled.
+ */
 class MissingExperimentalFeature : public Error
 {
 public:
+    /**
+     * The experimental feature that was required but not enabled.
+     */
     ExperimentalFeature missingFeature;
 
-    MissingExperimentalFeature(ExperimentalFeature);
+    MissingExperimentalFeature(ExperimentalFeature missingFeature);
 };
 
 /**

src/libutil/hash.cc

@@ -1,6 +1,7 @@
 #include <iostream>
 #include <cstring>
 
+#include <openssl/crypto.h>
 #include <openssl/md5.h>
 #include <openssl/sha.h>
 
@@ -16,7 +17,6 @@
 
 namespace nix {
 
-
 static size_t regularHashSize(HashType type) {
     switch (type) {
     case htMD5: return md5HashSize;
@@ -343,7 +343,7 @@ HashSink::~HashSink()
     delete ctx;
 }
 
-void HashSink::write(std::string_view data)
+void HashSink::writeUnbuffered(std::string_view data)
 {
     bytes += data.size();
     update(ht, *ctx, data);

src/libutil/hash.hh

@@ -197,7 +197,7 @@ public:
     HashSink(HashType ht);
     HashSink(const HashSink & h);
     ~HashSink();
-    void write(std::string_view data) override;
+    void writeUnbuffered(std::string_view data) override;
     HashResult finish() override;
     HashResult currentHash();
 };

src/libutil/serialise.cc

@@ -20,7 +20,7 @@ void BufferedSink::operator () (std::string_view data)
            buffer size. */
         if (bufPos + data.size() >= bufSize) {
             flush();
-            write(data);
+            writeUnbuffered(data);
             break;
         }
         /* Otherwise, copy the bytes to the buffer.  Flush the buffer
@@ -38,7 +38,7 @@ void BufferedSink::flush()
     if (bufPos == 0) return;
     size_t n = bufPos;
     bufPos = 0; // don't trigger the assert() in ~BufferedSink()
-    write({buffer.get(), n});
+    writeUnbuffered({buffer.get(), n});
 }
 
 
@@ -48,7 +48,7 @@ FdSink::~FdSink()
 }
 
 
-void FdSink::write(std::string_view data)
+void FdSink::writeUnbuffered(std::string_view data)
 {
     written += data.size();
     try {