Merge remote-tracking branch 'upstream/master' into path-info

src/libstore/binary-cache-store.cc

@@ -11,6 +11,7 @@
 #include "nar-accessor.hh"
 #include "json.hh"
 #include "thread-pool.hh"
+#include "callback.hh"
 
 #include <chrono>
 #include <future>
@@ -22,7 +23,8 @@
 namespace nix {
 
 BinaryCacheStore::BinaryCacheStore(const Params & params)
-    : Store(params)
+    : BinaryCacheStoreConfig(params)
+    , Store(params)
 {
     if (secretKeyFile != "")
         secretKey = std::unique_ptr<SecretKey>(new SecretKey(readFile(secretKeyFile)));
@@ -143,7 +145,7 @@ struct FileSource : FdSource
 void BinaryCacheStore::addToStore(const ValidPathInfo & info, Source & narSource,
     RepairFlag repair, CheckSigsFlag checkSigs)
 {
-    assert(info.narHash && info.narSize);
+    assert(info.narSize);
 
     if (!repair && isValidPath(info.path)) {
         // FIXME: copyNAR -> null sink
@@ -218,7 +220,7 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, Source & narSource
             }
         }
 
-        upsertFile(std::string(info.path.to_string()) + ".ls", jsonOut.str(), "application/json");
+        upsertFile(std::string(info.path.hashPart()) + ".ls", jsonOut.str(), "application/json");
     }
 
     /* Optionally maintain an index of DWARF debug info files
@@ -311,14 +313,10 @@ void BinaryCacheStore::narFromPath(const StorePath & storePath, Sink & sink)
 {
     auto info = queryPathInfo(storePath).cast<const NarInfo>();
 
-    uint64_t narSize = 0;
+    LengthSink narSize;
+    TeeSink tee { sink, narSize };
 
-    LambdaSink wrapperSink([&](const unsigned char * data, size_t len) {
-        sink(data, len);
-        narSize += len;
-    });
-
-    auto decompressor = makeDecompressionSink(info->compression, wrapperSink);
+    auto decompressor = makeDecompressionSink(info->compression, tee);
 
     try {
         getFile(info->url, *decompressor);
@@ -330,7 +328,7 @@ void BinaryCacheStore::narFromPath(const StorePath & storePath, Sink & sink)
 
     stats.narRead++;
     //stats.narReadCompressedBytes += nar->size(); // FIXME
-    stats.narReadBytes += narSize;
+    stats.narReadBytes += narSize.length;
 }
 
 void BinaryCacheStore::queryPathInfoUncached(const StorePath & storePath,
@@ -384,7 +382,10 @@ StorePath BinaryCacheStore::addToStore(const string & name, const Path & srcPath
         h = hashString(hashAlgo, s);
     }
 
-    ValidPathInfo info(makeFixedOutputPath(method, *h, name));
+    ValidPathInfo info {
+        makeFixedOutputPath(method, *h, name),
+        Hash::dummy, // Will be fixed in addToStore, which recomputes nar hash
+    };
 
     auto source = StringSource { *sink.s };
     addToStore(info, source, repair, CheckSigs);
@@ -395,7 +396,10 @@ StorePath BinaryCacheStore::addToStore(const string & name, const Path & srcPath
 StorePath BinaryCacheStore::addTextToStore(const string & name, const string & s,
     const StorePathSet & references, RepairFlag repair)
 {
-    ValidPathInfo info(computeStorePathForText(name, s, references));
+    ValidPathInfo info {
+        computeStorePathForText(name, s, references),
+        Hash::dummy, // Will be fixed in addToStore, which recomputes nar hash
+    };
     info.references = references;
 
     if (repair || !isValidPath(info.path)) {

src/libstore/binary-cache-store.hh

@@ -11,17 +11,21 @@ namespace nix {
 
 struct NarInfo;
 
-class BinaryCacheStore : public Store
+struct BinaryCacheStoreConfig : virtual StoreConfig
 {
-public:
+    using StoreConfig::StoreConfig;
 
-    const Setting<std::string> compression{this, "xz", "compression", "NAR compression method ('xz', 'bzip2', or 'none')"};
-    const Setting<bool> writeNARListing{this, false, "write-nar-listing", "whether to write a JSON file listing the files in each NAR"};
-    const Setting<bool> writeDebugInfo{this, false, "index-debug-info", "whether to index DWARF debug info files by build ID"};
-    const Setting<Path> secretKeyFile{this, "", "secret-key", "path to secret key used to sign the binary cache"};
-    const Setting<Path> localNarCache{this, "", "local-nar-cache", "path to a local cache of NARs"};
-    const Setting<bool> parallelCompression{this, false, "parallel-compression",
+    const Setting<std::string> compression{(StoreConfig*) this, "xz", "compression", "NAR compression method ('xz', 'bzip2', or 'none')"};
+    const Setting<bool> writeNARListing{(StoreConfig*) this, false, "write-nar-listing", "whether to write a JSON file listing the files in each NAR"};
+    const Setting<bool> writeDebugInfo{(StoreConfig*) this, false, "index-debug-info", "whether to index DWARF debug info files by build ID"};
+    const Setting<Path> secretKeyFile{(StoreConfig*) this, "", "secret-key", "path to secret key used to sign the binary cache"};
+    const Setting<Path> localNarCache{(StoreConfig*) this, "", "local-nar-cache", "path to a local cache of NARs"};
+    const Setting<bool> parallelCompression{(StoreConfig*) this, false, "parallel-compression",
         "enable multi-threading compression, available for xz only currently"};
+};
+
+class BinaryCacheStore : public Store, public virtual BinaryCacheStoreConfig
+{
 
 private:
 
@@ -58,7 +62,7 @@ public:
 
 public:
 
-    virtual void init();
+    virtual void init() override;
 
 private:
 

src/libstore/builtins/fetchurl.cc

@@ -58,6 +58,20 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         }
     };
 
+    /* Try the hashed mirrors first. */
+    if (getAttr("outputHashMode") == "flat")
+        for (auto hashedMirror : settings.hashedMirrors.get())
+            try {
+                if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
+                std::optional<HashType> ht = parseHashTypeOpt(getAttr("outputHashAlgo"));
+                Hash h = newHashAllowEmpty(getAttr("outputHash"), ht);
+                fetch(hashedMirror + printHashType(h.type) + "/" + h.to_string(Base16, false));
+                return;
+            } catch (Error & e) {
+                debug(e.what());
+            }
+
+    /* Otherwise try the specified URL. */
     fetch(mainUrl);
 }
 

src/libstore/content-address.cc

@@ -1,12 +1,16 @@
+#include "args.hh"
 #include "content-address.hh"
+#include "split.hh"
 
 namespace nix {
 
-std::string FixedOutputHash::printMethodAlgo() const {
+std::string FixedOutputHash::printMethodAlgo() const
+{
     return makeFileIngestionPrefix(method) + printHashType(hash.type);
 }
 
-std::string makeFileIngestionPrefix(const FileIngestionMethod m) {
+std::string makeFileIngestionPrefix(const FileIngestionMethod m)
+{
     switch (m) {
     case FileIngestionMethod::Flat:
         return "";
@@ -24,11 +28,8 @@ std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash)
         + hash.to_string(Base32, true);
 }
 
-// FIXME Put this somewhere?
-template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
-template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
-
-std::string renderContentAddress(ContentAddress ca) {
+std::string renderContentAddress(ContentAddress ca)
+{
     return std::visit(overloaded {
         [](TextHash th) {
             return "text:" + th.hash.to_string(Base32, true);
@@ -39,46 +40,97 @@ std::string renderContentAddress(ContentAddress ca) {
     }, ca);
 }
 
-ContentAddress parseContentAddress(std::string_view rawCa) {
-    auto prefixSeparator = rawCa.find(':');
-    if (prefixSeparator != string::npos) {
-        auto prefix = string(rawCa, 0, prefixSeparator);
-        if (prefix == "text") {
-            auto hashTypeAndHash = rawCa.substr(prefixSeparator+1, string::npos);
-            Hash hash = Hash(string(hashTypeAndHash));
-            if (hash.type != htSHA256) {
-                throw Error("parseContentAddress: the text hash should have type SHA256");
-            }
-            return TextHash { hash };
-        } else if (prefix == "fixed") {
-            // This has to be an inverse of makeFixedOutputCA
-            auto methodAndHash = rawCa.substr(prefixSeparator+1, string::npos);
-            if (methodAndHash.substr(0,2) == "r:") {
-                std::string_view hashRaw = methodAndHash.substr(2,string::npos);
-                return FixedOutputHash {
-                    .method = FileIngestionMethod::Recursive,
-                    .hash = Hash(string(hashRaw)),
-                };
-            } else {
-                std::string_view hashRaw = methodAndHash;
-                return FixedOutputHash {
-                    .method = FileIngestionMethod::Flat,
-                    .hash = Hash(string(hashRaw)),
-                };
-            }
-        } else {
-            throw Error("parseContentAddress: format not recognized; has to be text or fixed");
+std::string renderContentAddressMethod(ContentAddressMethod cam)
+{
+    return std::visit(overloaded {
+        [](TextHashMethod &th) {
+            return std::string{"text:"} + printHashType(htSHA256);
+        },
+        [](FixedOutputHashMethod &fshm) {
+            return "fixed:" + makeFileIngestionPrefix(fshm.fileIngestionMethod) + printHashType(fshm.hashType);
         }
-    } else {
-        throw Error("Not a content address because it lacks an appropriate prefix");
+    }, cam);
+}
+
+/*
+  Parses content address strings up to the hash.
+ */
+static ContentAddressMethod parseContentAddressMethodPrefix(std::string_view & rest)
+{
+    std::string_view wholeInput { rest };
+
+    std::string_view prefix;
+    {
+        auto optPrefix = splitPrefixTo(rest, ':');
+        if (!optPrefix)
+            throw UsageError("not a content address because it is not in the form '<prefix>:<rest>': %s", wholeInput);
+        prefix = *optPrefix;
     }
+
+    auto parseHashType_ = [&](){
+        auto hashTypeRaw = splitPrefixTo(rest, ':');
+        if (!hashTypeRaw)
+            throw UsageError("content address hash must be in form '<algo>:<hash>', but found: %s", wholeInput);
+        HashType hashType = parseHashType(*hashTypeRaw);
+        return std::move(hashType);
+    };
+
+    // Switch on prefix
+    if (prefix == "text") {
+        // No parsing of the ingestion method, "text" only support flat.
+        HashType hashType = parseHashType_();
+        if (hashType != htSHA256)
+            throw Error("text content address hash should use %s, but instead uses %s",
+                printHashType(htSHA256), printHashType(hashType));
+        return TextHashMethod {};
+    } else if (prefix == "fixed") {
+        // Parse method
+        auto method = FileIngestionMethod::Flat;
+        if (splitPrefix(rest, "r:"))
+            method = FileIngestionMethod::Recursive;
+        HashType hashType = parseHashType_();
+        return FixedOutputHashMethod {
+            .fileIngestionMethod = method,
+            .hashType = std::move(hashType),
+        };
+    } else
+        throw UsageError("content address prefix '%s' is unrecognized. Recogonized prefixes are 'text' or 'fixed'", prefix);
+}
+
+ContentAddress parseContentAddress(std::string_view rawCa) {
+    auto rest = rawCa;
+
+    ContentAddressMethod caMethod = parseContentAddressMethodPrefix(rest);
+
+    return std::visit(
+        overloaded {
+            [&](TextHashMethod thm) {
+                return ContentAddress(TextHash {
+                    .hash = Hash::parseNonSRIUnprefixed(rest, htSHA256)
+                });
+            },
+            [&](FixedOutputHashMethod fohMethod) {
+                return ContentAddress(FixedOutputHash {
+                    .method = fohMethod.fileIngestionMethod,
+                    .hash = Hash::parseNonSRIUnprefixed(rest, std::move(fohMethod.hashType)),
+                });
+            },
+        }, caMethod);
+}
+
+ContentAddressMethod parseContentAddressMethod(std::string_view caMethod)
+{
+    std::string_view asPrefix {std::string{caMethod} + ":"};
+    return parseContentAddressMethodPrefix(asPrefix);
+}
+
+std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt)
+{
+    return rawCaOpt == "" ? std::optional<ContentAddress>() : parseContentAddress(rawCaOpt);
 };
 
-std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt) {
-    return rawCaOpt == "" ? std::optional<ContentAddress> {} : parseContentAddress(rawCaOpt);
-};
-
-std::string renderContentAddress(std::optional<ContentAddress> ca) {
+std::string renderContentAddress(std::optional<ContentAddress> ca)
+{
     return ca ? renderContentAddress(*ca) : "";
 }
 

src/libstore/content-address.hh

@@ -55,4 +55,23 @@ 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;
+};
+
+typedef std::variant<
+    TextHashMethod,
+    FixedOutputHashMethod
+  > ContentAddressMethod;
+
+ContentAddressMethod parseContentAddressMethod(std::string_view rawCaMethod);
+
+std::string renderContentAddressMethod(ContentAddressMethod caMethod);
+
 }

src/libstore/daemon.cc

@@ -2,7 +2,6 @@
 #include "monitor-fd.hh"
 #include "worker-protocol.hh"
 #include "store-api.hh"
-#include "local-store.hh"
 #include "finally.hh"
 #include "affinity.hh"
 #include "archive.hh"
@@ -232,7 +231,7 @@ struct ClientSettings
                 else if (setSubstituters(settings.extraSubstituters))
                     ;
                 else
-                    warn("ignoring the user-specified setting '%s', because it is a restricted setting and you are not a trusted user", name);
+                    debug("ignoring the client-specified setting '%s', because it is a restricted setting and you are not a trusted user", name);
             } catch (UsageError & e) {
                 warn(e.what());
             }
@@ -240,6 +239,23 @@ struct ClientSettings
     }
 };
 
+static void writeValidPathInfo(
+    ref<Store> store,
+    unsigned int clientVersion,
+    Sink & to,
+    std::shared_ptr<const ValidPathInfo> info)
+{
+    to << (info->deriver ? store->printStorePath(*info->deriver) : "")
+       << info->narHash.to_string(Base16, false);
+    writeStorePaths(*store, to, info->referencesPossiblyToSelf());
+    to << info->registrationTime << info->narSize;
+    if (GET_PROTOCOL_MINOR(clientVersion) >= 16) {
+        to << info->ultimate
+           << info->sigs
+           << renderContentAddress(info->ca);
+    }
+}
+
 static void performOp(TunnelLogger * logger, ref<Store> store,
     TrustedFlag trusted, RecursiveFlag recursive, unsigned int clientVersion,
     Source & from, BufferedSink & to, unsigned int op)
@@ -289,7 +305,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         logger->startWork();
         auto hash = store->queryPathInfo(path)->narHash;
         logger->stopWork();
-        to << hash->to_string(Base16, false);
+        to << hash.to_string(Base16, false);
         break;
     }
 
@@ -325,9 +341,9 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     case wopQueryDerivationOutputMap: {
         auto path = store->parseStorePath(readString(from));
         logger->startWork();
-        OutputPathMap outputs = store->queryDerivationOutputMap(path);
+        auto outputs = store->queryPartialDerivationOutputMap(path);
         logger->stopWork();
-        writeOutputPathMap(*store, to, outputs);
+        worker_proto::write(*store, to, outputs);
         break;
     }
 
@@ -350,47 +366,83 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     case wopAddToStore: {
-        HashType hashAlgo;
-        std::string baseName;
-        FileIngestionMethod method;
-        {
-            bool fixed;
-            uint8_t recursive;
-            std::string hashAlgoRaw;
-            from >> baseName >> fixed /* obsolete */ >> recursive >> hashAlgoRaw;
-            if (recursive > (uint8_t) FileIngestionMethod::Recursive)
-                throw Error("unsupported FileIngestionMethod with value of %i; you may need to upgrade nix-daemon", recursive);
-            method = FileIngestionMethod { recursive };
-            /* Compatibility hack. */
-            if (!fixed) {
-                hashAlgoRaw = "sha256";
-                method = FileIngestionMethod::Recursive;
+        if (GET_PROTOCOL_MINOR(clientVersion) >= 25) {
+            auto name = readString(from);
+            auto camStr = readString(from);
+            auto refs = readStorePaths<StorePathSet>(*store, from);
+            bool repairBool;
+            from >> repairBool;
+            auto repair = RepairFlag{repairBool};
+
+            logger->startWork();
+            auto pathInfo = [&]() {
+                // NB: FramedSource must be out of scope before logger->stopWork();
+                ContentAddressMethod contentAddressMethod = parseContentAddressMethod(camStr);
+                FramedSource source(from);
+                // TODO this is essentially RemoteStore::addCAToStore. Move it up to Store.
+                return std::visit(overloaded {
+                    [&](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) {
+                        if (!refs.empty())
+                            throw UnimplementedError("cannot yet have refs with flat or nar-hashed data");
+                        auto path = store->addToStoreFromDump(source, name, fohm.fileIngestionMethod, fohm.hashType, repair);
+                        return store->queryPathInfo(path);
+                    },
+                }, contentAddressMethod);
+            }();
+            logger->stopWork();
+
+            to << store->printStorePath(pathInfo->path);
+            writeValidPathInfo(store, clientVersion, to, pathInfo);
+
+        } else {
+            HashType hashAlgo;
+            std::string baseName;
+            FileIngestionMethod method;
+            {
+                bool fixed;
+                uint8_t recursive;
+                std::string hashAlgoRaw;
+                from >> baseName >> fixed /* obsolete */ >> recursive >> hashAlgoRaw;
+                if (recursive > (uint8_t) FileIngestionMethod::Recursive)
+                    throw Error("unsupported FileIngestionMethod with value of %i; you may need to upgrade nix-daemon", recursive);
+                method = FileIngestionMethod { recursive };
+                /* Compatibility hack. */
+                if (!fixed) {
+                    hashAlgoRaw = "sha256";
+                    method = FileIngestionMethod::Recursive;
+                }
+                hashAlgo = parseHashType(hashAlgoRaw);
             }
-            hashAlgo = parseHashType(hashAlgoRaw);
+
+            StringSink saved;
+            TeeSource savedNARSource(from, saved);
+            RetrieveRegularNARSink savedRegular { saved };
+
+            if (method == FileIngestionMethod::Recursive) {
+                /* Get the entire NAR dump from the client and save it to
+                  a string so that we can pass it to
+                  addToStoreFromDump(). */
+                ParseSink sink; /* null sink; just parse the NAR */
+                parseDump(sink, savedNARSource);
+            } else
+                parseDump(savedRegular, from);
+
+            logger->startWork();
+            if (!savedRegular.regular) throw Error("regular file expected");
+
+            // FIXME: try to stream directly from `from`.
+            StringSource dumpSource { *saved.s };
+            auto path = store->addToStoreFromDump(dumpSource, baseName, method, hashAlgo);
+            logger->stopWork();
+
+            to << store->printStorePath(path);
         }
-
-        StringSink saved;
-        TeeSource savedNARSource(from, saved);
-        RetrieveRegularNARSink savedRegular { saved };
-
-        if (method == FileIngestionMethod::Recursive) {
-            /* Get the entire NAR dump from the client and save it to
-               a string so that we can pass it to
-               addToStoreFromDump(). */
-            ParseSink sink; /* null sink; just parse the NAR */
-            parseDump(sink, savedNARSource);
-        } else
-            parseDump(savedRegular, from);
-
-        logger->startWork();
-        if (!savedRegular.regular) throw Error("regular file expected");
-
-        // FIXME: try to stream directly from `from`.
-        StringSource dumpSource { *saved.s };
-        auto path = store->addToStoreFromDump(dumpSource, baseName, method, hashAlgo);
-        logger->stopWork();
-
-        to << store->printStorePath(path);
         break;
     }
 
@@ -454,8 +506,46 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         readDerivation(from, *store, drv, Derivation::nameFromPath(drvPath));
         BuildMode buildMode = (BuildMode) readInt(from);
         logger->startWork();
-        if (!trusted)
-            throw Error("you are not privileged to build derivations");
+
+        /* Content-addressed derivations are trustless because their output paths
+           are verified by their content alone, so any derivation is free to
+           try to produce such a path.
+
+           Input-addressed derivation output paths, however, are calculated
+           from the derivation closure that produced them---even knowing the
+           root derivation is not enough. That the output data actually came
+           from those derivations is fundamentally unverifiable, but the daemon
+           trusts itself on that matter. The question instead is whether the
+           submitted plan has rights to the output paths it wants to fill, and
+           at least the derivation closure proves that.
+
+           It would have been nice if input-address algorithm merely depended
+           on the build time closure, rather than depending on the derivation
+           closure. That would mean input-addressed paths used at build time
+           would just be trusted and not need their own evidence. This is in
+           fact fine as the same guarantees would hold *inductively*: either
+           the remote builder has those paths and already trusts them, or it
+           needs to build them too and thus their evidence must be provided in
+           turn.  The advantage of this variant algorithm is that the evidence
+           for input-addressed paths which the remote builder already has
+           doesn't need to be sent again.
+
+           That said, now that we have floating CA derivations, it is better
+           that people just migrate to those which also solve this problem, and
+           others. It's the same migration difficulty with strictly more
+           benefit.
+
+           Lastly, do note that when we parse fixed-output content-addressed
+           derivations, we throw out the precomputed output paths and just
+           store the hashes, so there aren't two competing sources of truth an
+           attacker could exploit. */
+        if (drv.type() == DerivationType::InputAddressed && !trusted)
+            throw Error("you are not privileged to build input-addressed derivations");
+
+        /* Make sure that the non-input-addressed derivations that got this far
+           are in fact content-addressed if we don't trust them. */
+        assert(derivationIsCA(drv.type()) || trusted);
+
         auto res = store->buildDerivation(drvPath, drv, buildMode);
         logger->stopWork();
         to << res.status << res.errorMsg;
@@ -637,15 +727,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         if (info) {
             if (GET_PROTOCOL_MINOR(clientVersion) >= 17)
                 to << 1;
-            to << (info->deriver ? store->printStorePath(*info->deriver) : "")
-               << info->narHash->to_string(Base16, false);
-            writeStorePaths(*store, to, info->referencesPossiblyToSelf());
-            to << info->registrationTime << info->narSize;
-            if (GET_PROTOCOL_MINOR(clientVersion) >= 16) {
-                to << info->ultimate
-                   << info->sigs
-                   << renderContentAddress(info->ca);
-            }
+            writeValidPathInfo(store, clientVersion, to, info);
         } else {
             assert(GET_PROTOCOL_MINOR(clientVersion) >= 17);
             to << 0;
@@ -688,17 +770,18 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         auto path = store->parseStorePath(readString(from));
         logger->startWork();
         logger->stopWork();
-        dumpPath(store->printStorePath(path), to);
+        dumpPath(store->toRealPath(path), to);
         break;
     }
 
     case wopAddToStoreNar: {
         bool repair, dontCheckSigs;
-        ValidPathInfo info(store->parseStorePath(readString(from)));
+        auto path = store->parseStorePath(readString(from));
         auto deriver = readString(from);
+        auto narHash = Hash::parseAny(readString(from), htSHA256);
+        ValidPathInfo info { path, narHash };
         if (deriver != "")
             info.deriver = store->parseStorePath(deriver);
-        info.narHash = Hash(readString(from), htSHA256);
         info.setReferencesPossiblyToSelf(readStorePaths<StorePathSet>(*store, from));
         from >> info.registrationTime >> info.narSize >> info.ultimate;
         info.sigs = readStrings<StringSet>(from);
@@ -710,59 +793,12 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             info.ultimate = false;
 
         if (GET_PROTOCOL_MINOR(clientVersion) >= 23) {
-
-            struct FramedSource : Source
-            {
-                Source & from;
-                bool eof = false;
-                std::vector<unsigned char> pending;
-                size_t pos = 0;
-
-                FramedSource(Source & from) : from(from)
-                { }
-
-                ~FramedSource()
-                {
-                    if (!eof) {
-                        while (true) {
-                            auto n = readInt(from);
-                            if (!n) break;
-                            std::vector<unsigned char> data(n);
-                            from(data.data(), n);
-                        }
-                    }
-                }
-
-                size_t read(unsigned char * data, size_t len) override
-                {
-                    if (eof) throw EndOfFile("reached end of FramedSource");
-
-                    if (pos >= pending.size()) {
-                        size_t len = readInt(from);
-                        if (!len) {
-                            eof = true;
-                            return 0;
-                        }
-                        pending = std::vector<unsigned char>(len);
-                        pos = 0;
-                        from(pending.data(), len);
-                    }
-
-                    auto n = std::min(len, pending.size() - pos);
-                    memcpy(data, pending.data() + pos, n);
-                    pos += n;
-                    return n;
-                }
-            };
-
             logger->startWork();
-
             {
                 FramedSource source(from);
                 store->addToStore(info, source, (RepairFlag) repair,
                     dontCheckSigs ? NoCheckSigs : CheckSigs);
             }
-
             logger->stopWork();
         }
 
@@ -817,8 +853,7 @@ void processConnection(
     FdSink & to,
     TrustedFlag trusted,
     RecursiveFlag recursive,
-    const std::string & userName,
-    uid_t userId)
+    std::function<void(Store &)> authHook)
 {
     auto monitor = !recursive ? std::make_unique<MonitorFdHup>(from.fd) : nullptr;
 
@@ -859,15 +894,7 @@ void processConnection(
 
         /* If we can't accept clientVersion, then throw an error
            *here* (not above). */
-
-#if 0
-        /* Prevent users from doing something very dangerous. */
-        if (geteuid() == 0 &&
-            querySetting("build-users-group", "") == "")
-            throw Error("if you run 'nix-daemon' as root, then you MUST set 'build-users-group'!");
-#endif
-
-        store->createUser(userName, userId);
+        authHook(*store);
 
         tunnelLogger->stopWork();
         to.flush();

src/libstore/daemon.hh

@@ -12,7 +12,10 @@ void processConnection(
     FdSink & to,
     TrustedFlag trusted,
     RecursiveFlag recursive,
-    const std::string & userName,
-    uid_t userId);
+    /* Arbitrary hook to check authorization / initialize user data / whatever
+       after the protocol has been negotiated. The idea is that this function
+       and everything it calls doesn't know about this stuff, and the
+       `nix-daemon` handles that instead. */
+    std::function<void(Store &)> authHook);
 
 }

src/libstore/derivations.cc

@@ -7,31 +7,69 @@
 
 namespace nix {
 
-// FIXME Put this somewhere?
-template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
-template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
-
-StorePath DerivationOutput::path(const Store & store, std::string_view drvName) const
+std::optional<StorePath> DerivationOutput::path(const Store & store, std::string_view drvName, std::string_view outputName) const
 {
     return std::visit(overloaded {
-        [](DerivationOutputInputAddressed doi) {
-            return doi.path;
+        [](DerivationOutputInputAddressed doi) -> std::optional<StorePath> {
+            return { doi.path };
+        },
+        [&](DerivationOutputCAFixed dof) -> std::optional<StorePath> {
+            return {
+                dof.path(store, drvName, outputName)
+            };
+        },
+        [](DerivationOutputCAFloating dof) -> std::optional<StorePath> {
+            return std::nullopt;
         },
-        [&](DerivationOutputFixed dof) {
-            return store.makeFixedOutputPath(dof.hash.method, dof.hash.hash, drvName);
-        }
     }, output);
 }
 
 
+StorePath DerivationOutputCAFixed::path(const Store & store, std::string_view drvName, std::string_view outputName) const {
+    return store.makeFixedOutputPath(
+        hash.method, hash.hash,
+        outputPathName(drvName, outputName));
+}
+
+
+bool derivationIsCA(DerivationType dt) {
+    switch (dt) {
+    case DerivationType::InputAddressed: return false;
+    case DerivationType::CAFixed: return true;
+    case DerivationType::CAFloating: return true;
+    };
+    // Since enums can have non-variant values, but making a `default:` would
+    // disable exhaustiveness warnings.
+    assert(false);
+}
+
+bool derivationIsFixed(DerivationType dt) {
+    switch (dt) {
+    case DerivationType::InputAddressed: return false;
+    case DerivationType::CAFixed: return true;
+    case DerivationType::CAFloating: return false;
+    };
+    assert(false);
+}
+
+bool derivationIsImpure(DerivationType dt) {
+    switch (dt) {
+    case DerivationType::InputAddressed: return false;
+    case DerivationType::CAFixed: return true;
+    case DerivationType::CAFloating: return false;
+    };
+    assert(false);
+}
+
+
 bool BasicDerivation::isBuiltin() const
 {
     return string(builder, 0, 8) == "builtin:";
 }
 
 
-StorePath writeDerivation(ref<Store> store,
-    const Derivation & drv, std::string_view name, RepairFlag repair)
+StorePath writeDerivation(Store & store,
+    const Derivation & drv, RepairFlag repair)
 {
     auto references = drv.inputSrcs;
     for (auto & i : drv.inputDrvs)
@@ -39,11 +77,11 @@ StorePath writeDerivation(ref<Store> store,
     /* Note that the outputs of a derivation are *not* references
        (that can be missing (of course) and should not necessarily be
        held during a garbage collection). */
-    auto suffix = std::string(name) + drvExtension;
-    auto contents = drv.unparse(*store, false);
+    auto suffix = std::string(drv.name) + drvExtension;
+    auto contents = drv.unparse(store, false);
     return settings.readOnlyMode
-        ? store->computeStorePathForText(suffix, contents, references)
-        : store->addTextToStore(suffix, contents, references, repair);
+        ? store.computeStorePathForText(suffix, contents, references)
+        : store.addTextToStore(suffix, contents, references, repair);
 }
 
 
@@ -75,12 +113,15 @@ static string parseString(std::istream & str)
     return res;
 }
 
+static void validatePath(std::string_view s) {
+    if (s.size() == 0 || s[0] != '/')
+        throw FormatError("bad path '%1%' in derivation", s);
+}
 
 static Path parsePath(std::istream & str)
 {
-    string s = parseString(str);
-    if (s.size() == 0 || s[0] != '/')
-        throw FormatError("bad path '%1%' in derivation", s);
+    auto s = parseString(str);
+    validatePath(s);
     return s;
 }
 
@@ -108,39 +149,58 @@ static StringSet parseStrings(std::istream & str, bool arePaths)
 }
 
 
-static DerivationOutput parseDerivationOutput(const Store & store, std::istringstream & str)
+static DerivationOutput parseDerivationOutput(const Store & store,
+    std::string_view pathS, std::string_view hashAlgo, std::string_view hash)
 {
-    expect(str, ","); auto path = store.parseStorePath(parsePath(str));
-    expect(str, ","); auto hashAlgo = parseString(str);
-    expect(str, ","); const auto hash = parseString(str);
-    expect(str, ")");
-
     if (hashAlgo != "") {
         auto method = FileIngestionMethod::Flat;
         if (string(hashAlgo, 0, 2) == "r:") {
             method = FileIngestionMethod::Recursive;
-            hashAlgo = string(hashAlgo, 2);
+            hashAlgo = hashAlgo.substr(2);
         }
-        const HashType hashType = parseHashType(hashAlgo);
-
-        return DerivationOutput {
-            .output = DerivationOutputFixed {
-                .hash = FixedOutputHash {
-                    .method = std::move(method),
-                    .hash = Hash(hash, hashType),
+        const auto hashType = parseHashType(hashAlgo);
+        if (hash != "") {
+            validatePath(pathS);
+            return DerivationOutput {
+                .output = DerivationOutputCAFixed {
+                    .hash = FixedOutputHash {
+                        .method = std::move(method),
+                        .hash = Hash::parseNonSRIUnprefixed(hash, hashType),
+                    },
                 },
-            }
-        };
-    } else
+            };
+        } else {
+            settings.requireExperimentalFeature("ca-derivations");
+            assert(pathS == "");
+            return DerivationOutput {
+                .output = DerivationOutputCAFloating {
+                    .method = std::move(method),
+                    .hashType = std::move(hashType),
+                },
+            };
+        }
+    } else {
+        validatePath(pathS);
         return DerivationOutput {
             .output = DerivationOutputInputAddressed {
-                .path = std::move(path),
+                .path = store.parseStorePath(pathS),
             }
         };
+    }
+}
+
+static DerivationOutput parseDerivationOutput(const Store & store, std::istringstream & str)
+{
+    expect(str, ","); const auto pathS = parseString(str);
+    expect(str, ","); const auto hashAlgo = parseString(str);
+    expect(str, ","); const auto hash = parseString(str);
+    expect(str, ")");
+
+    return parseDerivationOutput(store, pathS, hashAlgo, hash);
 }
 
 
-static Derivation parseDerivation(const Store & store, std::string && s, std::string_view name)
+Derivation parseDerivation(const Store & store, std::string && s, std::string_view name)
 {
     Derivation drv;
     drv.name = name;
@@ -188,34 +248,6 @@ static Derivation parseDerivation(const Store & store, std::string && s, std::st
 }
 
 
-Derivation readDerivation(const Store & store, const Path & drvPath, std::string_view name)
-{
-    try {
-        return parseDerivation(store, readFile(drvPath), name);
-    } catch (FormatError & e) {
-        throw Error("error parsing derivation '%1%': %2%", drvPath, e.msg());
-    }
-}
-
-
-Derivation Store::derivationFromPath(const StorePath & drvPath)
-{
-    ensurePath(drvPath);
-    return readDerivation(drvPath);
-}
-
-
-Derivation Store::readDerivation(const StorePath & drvPath)
-{
-    auto accessor = getFSAccessor();
-    try {
-        return parseDerivation(*this, accessor->readFile(printStorePath(drvPath)), Derivation::nameFromPath(drvPath));
-    } catch (FormatError & e) {
-        throw Error("error parsing derivation '%s': %s", printStorePath(drvPath), e.msg());
-    }
-}
-
-
 static void printString(string & res, std::string_view s)
 {
     char buf[s.size() * 2 + 2];
@@ -277,14 +309,23 @@ string Derivation::unparse(const Store & store, bool maskOutputs,
     for (auto & i : outputs) {
         if (first) first = false; else s += ',';
         s += '('; printUnquotedString(s, i.first);
-        s += ','; printUnquotedString(s, maskOutputs ? "" : store.printStorePath(i.second.path(store, name)));
-        if (auto hash = std::get_if<DerivationOutputFixed>(&i.second.output)) {
-            s += ','; printUnquotedString(s, hash->hash.printMethodAlgo());
-            s += ','; printUnquotedString(s, hash->hash.hash.to_string(Base16, false));
-        } else {
-            s += ','; printUnquotedString(s, "");
-            s += ','; printUnquotedString(s, "");
-        }
+        std::visit(overloaded {
+            [&](DerivationOutputInputAddressed doi) {
+                s += ','; printUnquotedString(s, maskOutputs ? "" : store.printStorePath(doi.path));
+                s += ','; printUnquotedString(s, "");
+                s += ','; printUnquotedString(s, "");
+            },
+            [&](DerivationOutputCAFixed dof) {
+                s += ','; printUnquotedString(s, maskOutputs ? "" : store.printStorePath(dof.path(store, name, i.first)));
+                s += ','; printUnquotedString(s, dof.hash.printMethodAlgo());
+                s += ','; printUnquotedString(s, dof.hash.hash.to_string(Base16, false));
+            },
+            [&](DerivationOutputCAFloating dof) {
+                s += ','; printUnquotedString(s, "");
+                s += ','; printUnquotedString(s, makeFileIngestionPrefix(dof.method) + printHashType(dof.hashType));
+                s += ','; printUnquotedString(s, "");
+            },
+        }, i.second.output);
         s += ')';
     }
 
@@ -336,60 +377,144 @@ bool isDerivation(const string & fileName)
 }
 
 
-bool BasicDerivation::isFixedOutput() const
+std::string outputPathName(std::string_view drvName, std::string_view outputName) {
+    std::string res { drvName };
+    if (outputName != "out") {
+        res += "-";
+        res += outputName;
+    }
+    return res;
+}
+
+
+DerivationType BasicDerivation::type() const
 {
-    return outputs.size() == 1 &&
-        outputs.begin()->first == "out" &&
-        std::holds_alternative<DerivationOutputFixed>(outputs.begin()->second.output);
+    std::set<std::string_view> inputAddressedOutputs, fixedCAOutputs, floatingCAOutputs;
+    std::optional<HashType> floatingHashType;
+    for (auto & i : outputs) {
+        std::visit(overloaded {
+            [&](DerivationOutputInputAddressed _) {
+               inputAddressedOutputs.insert(i.first);
+            },
+            [&](DerivationOutputCAFixed _) {
+                fixedCAOutputs.insert(i.first);
+            },
+            [&](DerivationOutputCAFloating dof) {
+                floatingCAOutputs.insert(i.first);
+                if (!floatingHashType) {
+                    floatingHashType = dof.hashType;
+                } else {
+                    if (*floatingHashType != dof.hashType)
+                        throw Error("All floating outputs must use the same hash type");
+                }
+            },
+        }, i.second.output);
+    }
+
+    if (inputAddressedOutputs.empty() && fixedCAOutputs.empty() && floatingCAOutputs.empty()) {
+        throw Error("Must have at least one output");
+    } else if (! inputAddressedOutputs.empty() && fixedCAOutputs.empty() && floatingCAOutputs.empty()) {
+        return DerivationType::InputAddressed;
+    } else if (inputAddressedOutputs.empty() && ! fixedCAOutputs.empty() && floatingCAOutputs.empty()) {
+        if (fixedCAOutputs.size() > 1)
+            // FIXME: Experimental feature?
+            throw Error("Only one fixed output is allowed for now");
+        if (*fixedCAOutputs.begin() != "out")
+            throw Error("Single fixed output must be named \"out\"");
+        return DerivationType::CAFixed;
+    } else if (inputAddressedOutputs.empty() && fixedCAOutputs.empty() && ! floatingCAOutputs.empty()) {
+        return DerivationType::CAFloating;
+    } else {
+        throw Error("Can't mix derivation output types");
+    }
 }
 
 
 DrvHashes drvHashes;
 
+/* pathDerivationModulo and hashDerivationModulo are mutually recursive
+ */
 
-/* Returns the hash of a derivation modulo fixed-output
-   subderivations.  A fixed-output derivation is a derivation with one
-   output (`out') for which an expected hash and hash algorithm are
-   specified (using the `outputHash' and `outputHashAlgo'
-   attributes).  We don't want changes to such derivations to
-   propagate upwards through the dependency graph, changing output
-   paths everywhere.
+/* Look up the derivation by value and memoize the
+   `hashDerivationModulo` call.
+ */
+static const DrvHashModulo & pathDerivationModulo(Store & store, const StorePath & drvPath)
+{
+    auto h = drvHashes.find(drvPath);
+    if (h == drvHashes.end()) {
+        assert(store.isValidPath(drvPath));
+        // Cache it
+        h = drvHashes.insert_or_assign(
+            drvPath,
+            hashDerivationModulo(
+                store,
+                store.readDerivation(drvPath),
+                false)).first;
+    }
+    return h->second;
+}
 
-   For instance, if we change the url in a call to the `fetchurl'
-   function, we do not want to rebuild everything depending on it
-   (after all, (the hash of) the file being downloaded is unchanged).
-   So the *output paths* should not change.  On the other hand, the
-   *derivation paths* should change to reflect the new dependency
-   graph.
+/* See the header for interface details. These are the implementation details.
 
-   That's what this function does: it returns a hash which is just the
-   hash of the derivation ATerm, except that any input derivation
-   paths have been replaced by the result of a recursive call to this
-   function, and that for fixed-output derivations we return a hash of
-   its output path. */
-Hash hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutputs)
+   For fixed-output derivations, each hash in the map is not the
+   corresponding output's content hash, but a hash of that hash along
+   with other constant data. The key point is that the value is a pure
+   function of the output's contents, and there are no preimage attacks
+   either spoofing an output's contents for a derivation, or
+   spoofing a derivation for an output's contents.
+
+   For regular derivations, it looks up each subderivation from its hash
+   and recurs. If the subderivation is also regular, it simply
+   substitutes the derivation path with its hash. If the subderivation
+   is fixed-output, however, it takes each output hash and pretends it
+   is a derivation hash producing a single "out" output. This is so we
+   don't leak the provenance of fixed outputs, reducing pointless cache
+   misses as the build itself won't know this.
+ */
+DrvHashModulo hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutputs)
 {
     /* Return a fixed hash for fixed-output derivations. */
-    if (drv.isFixedOutput()) {
-        DerivationOutputs::const_iterator i = drv.outputs.begin();
-        auto hash = std::get<DerivationOutputFixed>(i->second.output);
-        return hashString(htSHA256, "fixed:out:"
-            + hash.hash.printMethodAlgo() + ":"
-            + hash.hash.hash.to_string(Base16, false) + ":"
-            + store.printStorePath(i->second.path(store, drv.name)));
+    switch (drv.type()) {
+    case DerivationType::CAFloating:
+        throw Error("Regular input-addressed derivations are not yet allowed to depend on CA derivations");
+    case DerivationType::CAFixed: {
+        std::map<std::string, Hash> outputHashes;
+        for (const auto & i : drv.outputs) {
+            auto & dof = std::get<DerivationOutputCAFixed>(i.second.output);
+            auto hash = hashString(htSHA256, "fixed:out:"
+                + dof.hash.printMethodAlgo() + ":"
+                + dof.hash.hash.to_string(Base16, false) + ":"
+                + store.printStorePath(dof.path(store, drv.name, i.first)));
+            outputHashes.insert_or_assign(i.first, std::move(hash));
+        }
+        return outputHashes;
+    }
+    case DerivationType::InputAddressed:
+        break;
     }
 
     /* For other derivations, replace the inputs paths with recursive
-       calls to this function.*/
+       calls to this function. */
     std::map<std::string, StringSet> inputs2;
     for (auto & i : drv.inputDrvs) {
-        auto h = drvHashes.find(i.first);
-        if (h == drvHashes.end()) {
-            assert(store.isValidPath(i.first));
-            h = drvHashes.insert_or_assign(i.first, hashDerivationModulo(store,
-                store.readDerivation(i.first), false)).first;
-        }
-        inputs2.insert_or_assign(h->second.to_string(Base16, false), i.second);
+        const auto & res = pathDerivationModulo(store, i.first);
+        std::visit(overloaded {
+            // Regular non-CA derivation, replace derivation
+            [&](Hash drvHash) {
+                inputs2.insert_or_assign(drvHash.to_string(Base16, false), i.second);
+            },
+            // CA derivation's output hashes
+            [&](CaOutputHashes outputHashes) {
+                std::set<std::string> justOut = { "out" };
+                for (auto & output : i.second) {
+                    /* Put each one in with a single "out" output.. */
+                    const auto h = outputHashes.at(output);
+                    inputs2.insert_or_assign(
+                        h.to_string(Base16, false),
+                        justOut);
+                }
+            },
+        }, res);
     }
 
     return hashString(htSHA256, drv.unparse(store, maskOutputs, &inputs2));
@@ -410,41 +535,13 @@ bool wantOutput(const string & output, const std::set<string> & wanted)
 }
 
 
-StorePathSet BasicDerivation::outputPaths(const Store & store) const
-{
-    StorePathSet paths;
-    for (auto & i : outputs)
-        paths.insert(i.second.path(store, name));
-    return paths;
-}
-
 static DerivationOutput readDerivationOutput(Source & in, const Store & store)
 {
-    auto path = store.parseStorePath(readString(in));
-    auto hashAlgo = readString(in);
-    auto hash = readString(in);
+    const auto pathS = readString(in);
+    const auto hashAlgo = readString(in);
+    const auto hash = readString(in);
 
-    if (hashAlgo != "") {
-        auto method = FileIngestionMethod::Flat;
-        if (string(hashAlgo, 0, 2) == "r:") {
-            method = FileIngestionMethod::Recursive;
-            hashAlgo = string(hashAlgo, 2);
-        }
-        auto hashType = parseHashType(hashAlgo);
-        return DerivationOutput {
-            .output = DerivationOutputFixed {
-                .hash = FixedOutputHash {
-                    .method = std::move(method),
-                    .hash = Hash(hash, hashType),
-                },
-            }
-        };
-    } else
-        return DerivationOutput {
-            .output = DerivationOutputInputAddressed {
-                .path = std::move(path),
-            }
-        };
+    return parseDerivationOutput(store, pathS, hashAlgo, hash);
 }
 
 StringSet BasicDerivation::outputNames() const
@@ -455,6 +552,16 @@ StringSet BasicDerivation::outputNames() const
     return names;
 }
 
+DerivationOutputsAndOptPaths BasicDerivation::outputsAndOptPaths(const Store & store) const {
+    DerivationOutputsAndOptPaths outsAndOptPaths;
+    for (auto output : outputs)
+        outsAndOptPaths.insert(std::make_pair(
+            output.first,
+            std::make_pair(output.second, output.second.path(store, name, output.first))
+            )
+        );
+    return outsAndOptPaths;
+}
 
 std::string_view BasicDerivation::nameFromPath(const StorePath & drvPath) {
     auto nameWithSuffix = drvPath.name();
@@ -496,14 +603,24 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
 {
     out << drv.outputs.size();
     for (auto & i : drv.outputs) {
-        out << i.first
-            << store.printStorePath(i.second.path(store, drv.name));
-        if (auto hash = std::get_if<DerivationOutputFixed>(&i.second.output)) {
-            out << hash->hash.printMethodAlgo()
-                << hash->hash.hash.to_string(Base16, false);
-        } else {
-            out << "" << "";
-        }
+        out << i.first;
+        std::visit(overloaded {
+            [&](DerivationOutputInputAddressed doi) {
+                out << store.printStorePath(doi.path)
+                    << ""
+                    << "";
+            },
+            [&](DerivationOutputCAFixed dof) {
+                out << store.printStorePath(dof.path(store, drv.name, i.first))
+                    << dof.hash.printMethodAlgo()
+                    << dof.hash.hash.to_string(Base16, false);
+            },
+            [&](DerivationOutputCAFloating dof) {
+                out << ""
+                    << (makeFileIngestionPrefix(dof.method) + printHashType(dof.hashType))
+                    << "";
+            },
+        }, i.second.output);
     }
     writeStorePaths(store, out, drv.inputSrcs);
     out << drv.platform << drv.builder << drv.args;
@@ -519,5 +636,12 @@ std::string hashPlaceholder(const std::string & outputName)
     return "/" + hashString(htSHA256, "nix-output:" + outputName).to_string(Base32, false);
 }
 
+std::string downstreamPlaceholder(const Store & store, const StorePath & drvPath, std::string_view outputName)
+{
+    auto drvNameWithExtension = drvPath.name();
+    auto drvName = drvNameWithExtension.substr(0, drvNameWithExtension.size() - 4);
+    auto clearText = "nix-upstream-output:" + std::string { drvPath.hashPart() } + ":" + outputPathName(drvName, outputName);
+    return "/" + hashString(htSHA256, clearText).to_string(Base32, false);
+}
 
 }

src/libstore/derivations.hh

@@ -6,6 +6,7 @@
 #include "content-address.hh"
 
 #include <map>
+#include <variant>
 
 
 namespace nix {
@@ -13,30 +14,82 @@ namespace nix {
 
 /* Abstract syntax of derivations. */
 
+/* The traditional non-fixed-output derivation type. */
 struct DerivationOutputInputAddressed
 {
+    /* Will need to become `std::optional<StorePath>` once input-addressed
+       derivations are allowed to depend on cont-addressed derivations */
     StorePath path;
 };
 
-struct DerivationOutputFixed
+/* Fixed-output derivations, whose output paths are content addressed
+   according to that fixed output. */
+struct DerivationOutputCAFixed
 {
     FixedOutputHash hash; /* hash used for expected hash computation */
+    StorePath path(const Store & store, std::string_view drvName, std::string_view outputName) const;
+};
+
+/* Floating-output derivations, whose output paths are content addressed, but
+   not fixed, and so are dynamically calculated from whatever the output ends
+   up being. */
+struct DerivationOutputCAFloating
+{
+    /* information used for expected hash computation */
+    FileIngestionMethod method;
+    HashType hashType;
 };
 
 struct DerivationOutput
 {
-    std::variant<DerivationOutputInputAddressed, DerivationOutputFixed> output;
-    StorePath path(const Store & store, std::string_view drvName) const;
+    std::variant<
+        DerivationOutputInputAddressed,
+        DerivationOutputCAFixed,
+        DerivationOutputCAFloating
+    > output;
+    std::optional<HashType> hashAlgoOpt(const Store & store) const;
+    /* Note, when you use this function you should make sure that you're passing
+       the right derivation name. When in doubt, you should use the safer
+       interface provided by BasicDerivation::outputsAndOptPaths */
+    std::optional<StorePath> path(const Store & store, std::string_view drvName, std::string_view outputName) const;
 };
 
 typedef std::map<string, DerivationOutput> DerivationOutputs;
 
+/* These are analogues to the previous DerivationOutputs data type, but they
+   also contains, for each output, the (optional) store path in which it would
+   be written. To calculate values of these types, see the corresponding
+   functions in BasicDerivation */
+typedef std::map<string, std::pair<DerivationOutput, StorePath>>
+  DerivationOutputsAndPaths;
+typedef std::map<string, std::pair<DerivationOutput, std::optional<StorePath>>>
+  DerivationOutputsAndOptPaths;
+
 /* For inputs that are sub-derivations, we specify exactly which
    output IDs we are interested in. */
 typedef std::map<StorePath, StringSet> DerivationInputs;
 
 typedef std::map<string, string> StringPairs;
 
+enum struct DerivationType : uint8_t {
+    InputAddressed,
+    CAFixed,
+    CAFloating,
+};
+
+/* Do the outputs of the derivation have paths calculated from their content,
+   or from the derivation itself? */
+bool derivationIsCA(DerivationType);
+
+/* Is the content of the outputs fixed a-priori via a hash? Never true for
+   non-CA derivations. */
+bool derivationIsFixed(DerivationType);
+
+/* Is the derivation impure and needs to access non-deterministic resources, or
+   pure and can be sandboxed? Note that whether or not we actually sandbox the
+   derivation is controlled separately. Never true for non-CA derivations. */
+bool derivationIsImpure(DerivationType);
+
 struct BasicDerivation
 {
     DerivationOutputs outputs; /* keyed on symbolic IDs */
@@ -53,14 +106,16 @@ struct BasicDerivation
     bool isBuiltin() const;
 
     /* Return true iff this is a fixed-output derivation. */
-    bool isFixedOutput() const;
-
-    /* Return the output paths of a derivation. */
-    StorePathSet outputPaths(const Store & store) const;
+    DerivationType type() const;
 
     /* Return the output names of a derivation. */
     StringSet outputNames() const;
 
+    /* Calculates the maps that contains all the DerivationOutputs, but
+       augmented with knowledge of the Store paths they would be written
+       into. */
+    DerivationOutputsAndOptPaths outputsAndOptPaths(const Store & store) const;
+
     static std::string_view nameFromPath(const StorePath & storePath);
 };
 
@@ -81,19 +136,58 @@ class Store;
 enum RepairFlag : bool { NoRepair = false, Repair = true };
 
 /* Write a derivation to the Nix store, and return its path. */
-StorePath writeDerivation(ref<Store> store,
-    const Derivation & drv, std::string_view name, RepairFlag repair = NoRepair);
+StorePath writeDerivation(Store & store,
+    const Derivation & drv, RepairFlag repair = NoRepair);
 
 /* Read a derivation from a file. */
-Derivation readDerivation(const Store & store, const Path & drvPath, std::string_view name);
+Derivation parseDerivation(const Store & store, std::string && s, std::string_view name);
 
 // FIXME: remove
 bool isDerivation(const string & fileName);
 
-Hash hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutputs);
+/* Calculate the name that will be used for the store path for this
+   output.
+
+   This is usually <drv-name>-<output-name>, but is just <drv-name> when
+   the output name is "out". */
+std::string outputPathName(std::string_view drvName, std::string_view outputName);
+
+// known CA drv's output hashes, current just for fixed-output derivations
+// whose output hashes are always known since they are fixed up-front.
+typedef std::map<std::string, Hash> CaOutputHashes;
+
+typedef std::variant<
+    Hash, // regular DRV normalized hash
+    CaOutputHashes
+> DrvHashModulo;
+
+/* Returns hashes with the details of fixed-output subderivations
+   expunged.
+
+   A fixed-output derivation is a derivation whose outputs have a
+   specified content hash and hash algorithm. (Currently they must have
+   exactly one output (`out'), which is specified using the `outputHash'
+   and `outputHashAlgo' attributes, but the algorithm doesn't assume
+   this.) We don't want changes to such derivations to propagate upwards
+   through the dependency graph, changing output paths everywhere.
+
+   For instance, if we change the url in a call to the `fetchurl'
+   function, we do not want to rebuild everything depending on it---after
+   all, (the hash of) the file being downloaded is unchanged.  So the
+   *output paths* should not change. On the other hand, the *derivation
+   paths* should change to reflect the new dependency graph.
+
+   For fixed-output derivations, this returns a map from the name of
+   each output to its hash, unique up to the output's contents.
+
+   For regular derivations, it returns a single hash of the derivation
+   ATerm, after subderivations have been likewise expunged from that
+   derivation.
+ */
+DrvHashModulo hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutputs);
 
 /* Memoisation of hashDerivationModulo(). */
-typedef std::map<StorePath, Hash> DrvHashes;
+typedef std::map<StorePath, DrvHashModulo> DrvHashes;
 
 extern DrvHashes drvHashes; // FIXME: global, not thread-safe
 
@@ -105,6 +199,21 @@ struct Sink;
 Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv, std::string_view name);
 void writeDerivation(Sink & out, const Store & store, const BasicDerivation & drv);
 
+/* This creates an opaque and almost certainly unique string
+   deterministically from the output name.
+
+   It is used as a placeholder to allow derivations to refer to their
+   own outputs without needing to use the hash of a derivation in
+   itself, making the hash near-impossible to calculate. */
 std::string hashPlaceholder(const std::string & outputName);
 
+/* This creates an opaque and almost certainly unique string
+   deterministically from a derivation path and output name.
+
+   It is used as a placeholder to allow derivations to refer to
+   content-addressed paths whose content --- and thus the path
+   themselves --- isn't yet known. This occurs when a derivation has a
+   dependency which is a CA derivation. */
+std::string downstreamPlaceholder(const Store & store, const StorePath & drvPath, std::string_view outputName);
+
 }

src/libstore/dummy-store.cc

@@ -0,0 +1,68 @@
+#include "store-api.hh"
+#include "callback.hh"
+
+namespace nix {
+
+struct DummyStoreConfig : virtual StoreConfig {
+    using StoreConfig::StoreConfig;
+
+    const std::string name() override { return "Dummy Store"; }
+};
+
+struct DummyStore : public Store, public virtual DummyStoreConfig
+{
+    DummyStore(const std::string scheme, const std::string uri, const Params & params)
+        : DummyStore(params)
+    { }
+
+    DummyStore(const Params & params)
+        : StoreConfig(params)
+        , Store(params)
+    {
+    }
+
+    string getUri() override
+    {
+        return *uriSchemes().begin();
+    }
+
+    void queryPathInfoUncached(const StorePath & path,
+        Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept override
+    {
+        callback(nullptr);
+    }
+
+    static std::set<std::string> uriSchemes() {
+        return {"dummy"};
+    }
+
+    std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override
+    { unsupported("queryPathFromHashPart"); }
+
+    void addToStore(const ValidPathInfo & info, Source & source,
+        RepairFlag repair, CheckSigsFlag checkSigs) override
+    { unsupported("addToStore"); }
+
+    StorePath addToStore(const string & name, const Path & srcPath,
+        FileIngestionMethod method, HashType hashAlgo,
+        PathFilter & filter, RepairFlag repair) override
+    { unsupported("addToStore"); }
+
+    StorePath addTextToStore(const string & name, const string & s,
+        const StorePathSet & references, RepairFlag repair) override
+    { unsupported("addTextToStore"); }
+
+    void narFromPath(const StorePath & path, Sink & sink) override
+    { unsupported("narFromPath"); }
+
+    void ensurePath(const StorePath & path) override
+    { unsupported("ensurePath"); }
+
+    BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
+        BuildMode buildMode) override
+    { unsupported("buildDerivation"); }
+};
+
+static RegisterStoreImplementation<DummyStore, DummyStoreConfig> regStore;
+
+}

src/libstore/export-import.cc

@@ -38,9 +38,9 @@ void Store::exportPath(const StorePath & path, Sink & sink)
        filesystem corruption from spreading to other machines.
        Don't complain if the stored hash is zero (unknown). */
     Hash hash = hashSink.currentHash().first;
-    if (hash != info->narHash && info->narHash != Hash(info->narHash->type))
+    if (hash != info->narHash && info->narHash != Hash(info->narHash.type))
         throw Error("hash of path '%s' has changed from '%s' to '%s'!",
-            printStorePath(path), info->narHash->to_string(Base32, true), hash.to_string(Base32, true));
+            printStorePath(path), info->narHash.to_string(Base32, true), hash.to_string(Base32, true));
 
     teeSink
         << exportMagic
@@ -69,17 +69,18 @@ StorePaths Store::importPaths(Source & source, CheckSigsFlag checkSigs)
         if (magic != exportMagic)
             throw Error("Nix archive cannot be imported; wrong format");
 
-        ValidPathInfo info(parseStorePath(readString(source)));
+        auto path = parseStorePath(readString(source));
 
         //Activity act(*logger, lvlInfo, format("importing path '%s'") % info.path);
 
-        info.setReferencesPossiblyToSelf(readStorePaths<StorePathSet>(*this, source));
-
+        auto references = readStorePaths<StorePathSet>(*this, source);
         auto deriver = readString(source);
+        auto narHash = hashString(htSHA256, *saved.s);
+
+        ValidPathInfo info { path, narHash };
         if (deriver != "")
             info.deriver = parseStorePath(deriver);
-
-        info.narHash = hashString(htSHA256, *saved.s);
+        info.setReferencesPossiblyToSelf(std::move(references));
         info.narSize = saved.s->size();
 
         // Ignore optional legacy signature.

src/libstore/filetransfer.cc

@@ -5,6 +5,7 @@
 #include "s3.hh"
 #include "compression.hh"
 #include "finally.hh"
+#include "callback.hh"
 
 #ifdef ENABLE_S3
 #include <aws/core/client/ClientConfiguration.h>

src/libstore/filetransfer.hh

@@ -17,15 +17,30 @@ struct FileTransferSettings : Config
     Setting<std::string> userAgentSuffix{this, "", "user-agent-suffix",
         "String appended to the user agent in HTTP requests."};
 
-    Setting<size_t> httpConnections{this, 25, "http-connections",
-        "Number of parallel HTTP connections.",
+    Setting<size_t> httpConnections{
+        this, 25, "http-connections",
+        R"(
+          The maximum number of parallel TCP connections used to fetch
+          files from binary caches and by other downloads. It defaults
+          to 25. 0 means no limit.
+        )",
         {"binary-caches-parallel-connections"}};
 
-    Setting<unsigned long> connectTimeout{this, 0, "connect-timeout",
-        "Timeout for connecting to servers during downloads. 0 means use curl's builtin default."};
+    Setting<unsigned long> connectTimeout{
+        this, 0, "connect-timeout",
+        R"(
+          The timeout (in seconds) for establishing connections in the
+          binary cache substituter. It corresponds to `curl`’s
+          `--connect-timeout` option.
+        )"};
 
-    Setting<unsigned long> stalledDownloadTimeout{this, 300, "stalled-download-timeout",
-        "Timeout (in seconds) for receiving data from servers during download. Nix cancels idle downloads after this timeout's duration."};
+    Setting<unsigned long> stalledDownloadTimeout{
+        this, 300, "stalled-download-timeout",
+        R"(
+          The timeout (in seconds) for receiving data from servers
+          during download. Nix cancels idle downloads after this
+          timeout's duration.
+        )"};
 
     Setting<unsigned int> tries{this, 5, "download-attempts",
         "How often Nix will attempt to download a file before giving up."};

src/libstore/gc.cc

@@ -85,8 +85,7 @@ void LocalStore::addIndirectRoot(const Path & path)
 }
 
 
-Path LocalFSStore::addPermRoot(const StorePath & storePath,
-    const Path & _gcRoot, bool indirect, bool allowOutsideRootsDir)
+Path LocalFSStore::addPermRoot(const StorePath & storePath, const Path & _gcRoot)
 {
     Path gcRoot(canonPath(_gcRoot));
 
@@ -95,47 +94,12 @@ Path LocalFSStore::addPermRoot(const StorePath & storePath,
                 "creating a garbage collector root (%1%) in the Nix store is forbidden "
                 "(are you running nix-build inside the store?)", gcRoot);
 
-    if (indirect) {
-        /* Don't clobber the link if it already exists and doesn't
-           point to the Nix store. */
-        if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
-            throw Error("cannot create symlink '%1%'; already exists", gcRoot);
-        makeSymlink(gcRoot, printStorePath(storePath));
-        addIndirectRoot(gcRoot);
-    }
-
-    else {
-        if (!allowOutsideRootsDir) {
-            Path rootsDir = canonPath((format("%1%/%2%") % stateDir % gcRootsDir).str());
-
-            if (string(gcRoot, 0, rootsDir.size() + 1) != rootsDir + "/")
-                throw Error(
-                    "path '%1%' is not a valid garbage collector root; "
-                    "it's not in the directory '%2%'",
-                    gcRoot, rootsDir);
-        }
-
-        if (baseNameOf(gcRoot) == std::string(storePath.to_string()))
-            writeFile(gcRoot, "");
-        else
-            makeSymlink(gcRoot, printStorePath(storePath));
-    }
-
-    /* Check that the root can be found by the garbage collector.
-       !!! This can be very slow on machines that have many roots.
-       Instead of reading all the roots, it would be more efficient to
-       check if the root is in a directory in or linked from the
-       gcroots directory. */
-    if (settings.checkRootReachability) {
-        auto roots = findRoots(false);
-        if (roots[storePath].count(gcRoot) == 0)
-            logWarning({
-                .name = "GC root",
-                .hint = hintfmt("warning: '%1%' is not in a directory where the garbage collector looks for roots; "
-                    "therefore, '%2%' might be removed by the garbage collector",
-                    gcRoot, printStorePath(storePath))
-            });
-    }
+    /* Don't clobber the link if it already exists and doesn't
+       point to the Nix store. */
+    if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
+        throw Error("cannot create symlink '%1%'; already exists", gcRoot);
+    makeSymlink(gcRoot, printStorePath(storePath));
+    addIndirectRoot(gcRoot);
 
     /* Grab the global GC root, causing us to block while a GC is in
        progress.  This prevents the set of permanent roots from
@@ -610,9 +574,12 @@ bool LocalStore::canReachRoot(GCState & state, StorePathSet & visited, const Sto
     /* If keep-derivations is set and this is a derivation, then
        don't delete the derivation if any of the outputs are alive. */
     if (state.gcKeepDerivations && path.isDerivation()) {
-        for (auto & i : queryDerivationOutputs(path))
-            if (isValidPath(i) && queryPathInfo(i)->deriver == path)
-                incoming.insert(i);
+        for (auto & [name, maybeOutPath] : queryPartialDerivationOutputMap(path))
+            if (maybeOutPath &&
+                isValidPath(*maybeOutPath) &&
+                queryPathInfo(*maybeOutPath)->deriver == path
+                )
+                incoming.insert(*maybeOutPath);
     }
 
     /* If keep-outputs is set, then don't delete this path if there
@@ -696,9 +663,7 @@ void LocalStore::removeUnusedLinks(const GCState & state)
         if (name == "." || name == "..") continue;
         Path path = linksDir + "/" + name;
 
-        struct stat st;
-        if (lstat(path.c_str(), &st) == -1)
-            throw SysError("statting '%1%'", path);
+        auto st = lstat(path);
 
         if (st.st_nlink != 1) {
             actualSize += st.st_size;

src/libstore/globals.cc

@@ -2,6 +2,7 @@
 #include "util.hh"
 #include "archive.hh"
 #include "args.hh"
+#include "abstract-setting-to-json.hh"
 
 #include <algorithm>
 #include <map>
@@ -9,6 +10,8 @@
 #include <dlfcn.h>
 #include <sys/utsname.h>
 
+#include <nlohmann/json.hpp>
+
 
 namespace nix {
 
@@ -39,6 +42,7 @@ Settings::Settings()
 {
     buildUsersGroup = getuid() == 0 ? "nixbld" : "";
     lockCPU = getEnv("NIX_AFFINITY_HACK") == "1";
+    allowSymlinkedStore = getEnv("NIX_IGNORE_SYMLINK_STORE") == "1";
 
     caFile = getEnv("NIX_SSL_CERT_FILE").value_or(getEnv("SSL_CERT_FILE").value_or(""));
     if (caFile == "") {
@@ -144,6 +148,12 @@ bool Settings::isWSL1()
 
 const string nixVersion = PACKAGE_VERSION;
 
+NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {
+    {SandboxMode::smEnabled, true},
+    {SandboxMode::smRelaxed, "relaxed"},
+    {SandboxMode::smDisabled, false},
+});
+
 template<> void BaseSetting<SandboxMode>::set(const std::string & str)
 {
     if (str == "true") value = smEnabled;
@@ -160,11 +170,6 @@ template<> std::string BaseSetting<SandboxMode>::to_string() const
     else abort();
 }
 
-template<> void BaseSetting<SandboxMode>::toJSON(JSONPlaceholder & out)
-{
-    AbstractSetting::toJSON(out);
-}
-
 template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::string & category)
 {
     args.addFlag({

src/libstore/globals.hh

@@ -80,89 +80,209 @@ public:
     Setting<bool> keepGoing{this, false, "keep-going",
         "Whether to keep building derivations when another build fails."};
 
-    Setting<bool> tryFallback{this, false, "fallback",
-        "Whether to fall back to building when substitution fails.",
+    Setting<bool> tryFallback{
+        this, false, "fallback",
+        R"(
+          If set to `true`, Nix will fall back to building from source if a
+          binary substitute fails. This is equivalent to the `--fallback`
+          flag. The default is `false`.
+        )",
         {"build-fallback"}};
 
     /* Whether to show build log output in real time. */
     bool verboseBuild = true;
 
     Setting<size_t> logLines{this, 10, "log-lines",
-        "If verbose-build is false, the number of lines of the tail of "
+        "If `verbose-build` is false, the number of lines of the tail of "
         "the log to show if a build fails."};
 
-    MaxBuildJobsSetting maxBuildJobs{this, 1, "max-jobs",
-        "Maximum number of parallel build jobs. \"auto\" means use number of cores.",
+    MaxBuildJobsSetting maxBuildJobs{
+        this, 1, "max-jobs",
+        R"(
+          This option defines the maximum number of jobs that Nix will try to
+          build in parallel. The default is `1`. The special value `auto`
+          causes Nix to use the number of CPUs in your system. `0` is useful
+          when using remote builders to prevent any local builds (except for
+          `preferLocalBuild` derivation attribute which executes locally
+          regardless). It can be overridden using the `--max-jobs` (`-j`)
+          command line switch.
+        )",
         {"build-max-jobs"}};
 
-    Setting<unsigned int> buildCores{this, getDefaultCores(), "cores",
-        "Number of CPU cores to utilize in parallel within a build, "
-        "i.e. by passing this number to Make via '-j'. 0 means that the "
-        "number of actual CPU cores on the local host ought to be "
-        "auto-detected.", {"build-cores"}};
+    Setting<unsigned int> buildCores{
+        this, getDefaultCores(), "cores",
+        R"(
+          Sets the value of the `NIX_BUILD_CORES` environment variable in the
+          invocation of builders. Builders can use this variable at their
+          discretion to control the maximum amount of parallelism. For
+          instance, in Nixpkgs, if the derivation attribute
+          `enableParallelBuilding` is set to `true`, the builder passes the
+          `-jN` flag to GNU Make. It can be overridden using the `--cores`
+          command line switch and defaults to `1`. The value `0` means that
+          the builder should use all available CPU cores in the system.
+        )",
+        {"build-cores"}};
 
     /* Read-only mode.  Don't copy stuff to the store, don't change
        the database. */
     bool readOnlyMode = false;
 
-    Setting<std::string> thisSystem{this, SYSTEM, "system",
-        "The canonical Nix system name."};
+    Setting<std::string> thisSystem{
+        this, SYSTEM, "system",
+        R"(
+          This option specifies the canonical Nix system name of the current
+          installation, such as `i686-linux` or `x86_64-darwin`. Nix can only
+          build derivations whose `system` attribute equals the value
+          specified here. In general, it never makes sense to modify this
+          value from its default, since you can use it to ‘lie’ about the
+          platform you are building on (e.g., perform a Mac OS build on a
+          Linux machine; the result would obviously be wrong). It only makes
+          sense if the Nix binaries can run on multiple platforms, e.g.,
+          ‘universal binaries’ that run on `x86_64-linux` and `i686-linux`.
 
-    Setting<time_t> maxSilentTime{this, 0, "max-silent-time",
-        "The maximum time in seconds that a builer can go without "
-        "producing any output on stdout/stderr before it is killed. "
-        "0 means infinity.",
+          It defaults to the canonical Nix system name detected by `configure`
+          at build time.
+        )"};
+
+    Setting<time_t> maxSilentTime{
+        this, 0, "max-silent-time",
+        R"(
+          This option defines the maximum number of seconds that a builder can
+          go without producing any data on standard output or standard error.
+          This is useful (for instance in an automated build system) to catch
+          builds that are stuck in an infinite loop, or to catch remote builds
+          that are hanging due to network problems. It can be overridden using
+          the `--max-silent-time` command line switch.
+
+          The value `0` means that there is no timeout. This is also the
+          default.
+        )",
         {"build-max-silent-time"}};
 
-    Setting<time_t> buildTimeout{this, 0, "timeout",
-        "The maximum duration in seconds that a builder can run. "
-        "0 means infinity.", {"build-timeout"}};
+    Setting<time_t> buildTimeout{
+        this, 0, "timeout",
+        R"(
+          This option defines the maximum number of seconds that a builder can
+          run. This is useful (for instance in an automated build system) to
+          catch builds that are stuck in an infinite loop but keep writing to
+          their standard output or standard error. It can be overridden using
+          the `--timeout` command line switch.
+
+          The value `0` means that there is no timeout. This is also the
+          default.
+        )",
+        {"build-timeout"}};
 
     PathSetting buildHook{this, true, nixLibexecDir + "/nix/build-remote", "build-hook",
         "The path of the helper program that executes builds to remote machines."};
 
-    Setting<std::string> builders{this, "@" + nixConfDir + "/machines", "builders",
-        "A semicolon-separated list of build machines, in the format of nix.machines."};
+    Setting<std::string> builders{
+        this, "@" + nixConfDir + "/machines", "builders",
+        "A semicolon-separated list of build machines, in the format of `nix.machines`."};
 
-    Setting<bool> buildersUseSubstitutes{this, false, "builders-use-substitutes",
-        "Whether build machines should use their own substitutes for obtaining "
-        "build dependencies if possible, rather than waiting for this host to "
-        "upload them."};
+    Setting<bool> buildersUseSubstitutes{
+        this, false, "builders-use-substitutes",
+        R"(
+          If set to `true`, Nix will instruct remote build machines to use
+          their own binary substitutes if available. In practical terms, this
+          means that remote hosts will fetch as many build dependencies as
+          possible from their own substitutes (e.g, from `cache.nixos.org`),
+          instead of waiting for this host to upload them all. This can
+          drastically reduce build times if the network connection between
+          this computer and the remote build host is slow.
+        )"};
 
     Setting<off_t> reservedSize{this, 8 * 1024 * 1024, "gc-reserved-space",
         "Amount of reserved disk space for the garbage collector."};
 
-    Setting<bool> fsyncMetadata{this, true, "fsync-metadata",
-        "Whether SQLite should use fsync()."};
+    Setting<bool> fsyncMetadata{
+        this, true, "fsync-metadata",
+        R"(
+          If set to `true`, changes to the Nix store metadata (in
+          `/nix/var/nix/db`) are synchronously flushed to disk. This improves
+          robustness in case of system crashes, but reduces performance. The
+          default is `true`.
+        )"};
 
     Setting<bool> useSQLiteWAL{this, !isWSL1(), "use-sqlite-wal",
         "Whether SQLite should use WAL mode."};
 
     Setting<bool> syncBeforeRegistering{this, false, "sync-before-registering",
-        "Whether to call sync() before registering a path as valid."};
+        "Whether to call `sync()` before registering a path as valid."};
 
-    Setting<bool> useSubstitutes{this, true, "substitute",
-        "Whether to use substitutes.",
+    Setting<bool> useSubstitutes{
+        this, true, "substitute",
+        R"(
+          If set to `true` (default), Nix will use binary substitutes if
+          available. This option can be disabled to force building from
+          source.
+        )",
         {"build-use-substitutes"}};
 
-    Setting<std::string> buildUsersGroup{this, "", "build-users-group",
-        "The Unix group that contains the build users."};
+    Setting<std::string> buildUsersGroup{
+        this, "", "build-users-group",
+        R"(
+          This options specifies the Unix group containing the Nix build user
+          accounts. In multi-user Nix installations, builds should not be
+          performed by the Nix account since that would allow users to
+          arbitrarily modify the Nix store and database by supplying specially
+          crafted builders; and they cannot be performed by the calling user
+          since that would allow him/her to influence the build result.
+
+          Therefore, if this option is non-empty and specifies a valid group,
+          builds will be performed under the user accounts that are a member
+          of the group specified here (as listed in `/etc/group`). Those user
+          accounts should not be used for any other purpose\!
+
+          Nix will never run two builds under the same user account at the
+          same time. This is to prevent an obvious security hole: a malicious
+          user writing a Nix expression that modifies the build result of a
+          legitimate Nix expression being built by another user. Therefore it
+          is good to have as many Nix build user accounts as you can spare.
+          (Remember: uids are cheap.)
+
+          The build users should have permission to create files in the Nix
+          store, but not delete them. Therefore, `/nix/store` should be owned
+          by the Nix account, its group should be the group specified here,
+          and its mode should be `1775`.
+
+          If the build users group is empty, builds will be performed under
+          the uid of the Nix process (that is, the uid of the caller if
+          `NIX_REMOTE` is empty, the uid under which the Nix daemon runs if
+          `NIX_REMOTE` is `daemon`). Obviously, this should not be used in
+          multi-user settings with untrusted users.
+        )"};
 
     Setting<bool> impersonateLinux26{this, false, "impersonate-linux-26",
         "Whether to impersonate a Linux 2.6 machine on newer kernels.",
         {"build-impersonate-linux-26"}};
 
-    Setting<bool> keepLog{this, true, "keep-build-log",
-        "Whether to store build logs.",
+    Setting<bool> keepLog{
+        this, true, "keep-build-log",
+        R"(
+          If set to `true` (the default), Nix will write the build log of a
+          derivation (i.e. the standard output and error of its builder) to
+          the directory `/nix/var/log/nix/drvs`. The build log can be
+          retrieved using the command `nix-store -l path`.
+        )",
         {"build-keep-log"}};
 
-    Setting<bool> compressLog{this, true, "compress-build-log",
-        "Whether to compress logs.",
+    Setting<bool> compressLog{
+        this, true, "compress-build-log",
+        R"(
+          If set to `true` (the default), build logs written to
+          `/nix/var/log/nix/drvs` will be compressed on the fly using bzip2.
+          Otherwise, they will not be compressed.
+        )",
         {"build-compress-log"}};
 
-    Setting<unsigned long> maxLogSize{this, 0, "max-build-log-size",
-        "Maximum number of bytes a builder can write to stdout/stderr "
-        "before being killed (0 means no limit).",
+    Setting<unsigned long> maxLogSize{
+        this, 0, "max-build-log-size",
+        R"(
+          This option defines the maximum number of bytes that a builder can
+          write to its stdout/stderr. If the builder exceeds this limit, it’s
+          killed. A value of `0` (the default) means that there is no limit.
+        )",
         {"build-max-log-size"}};
 
     /* When buildRepeat > 0 and verboseBuild == true, whether to print
@@ -173,57 +293,156 @@ public:
     Setting<unsigned int> pollInterval{this, 5, "build-poll-interval",
         "How often (in seconds) to poll for locks."};
 
-    Setting<bool> checkRootReachability{this, false, "gc-check-reachability",
-        "Whether to check if new GC roots can in fact be found by the "
-        "garbage collector."};
+    Setting<bool> gcKeepOutputs{
+        this, false, "keep-outputs",
+        R"(
+          If `true`, the garbage collector will keep the outputs of
+          non-garbage derivations. If `false` (default), outputs will be
+          deleted unless they are GC roots themselves (or reachable from other
+          roots).
 
-    Setting<bool> gcKeepOutputs{this, false, "keep-outputs",
-        "Whether the garbage collector should keep outputs of live derivations.",
+          In general, outputs must be registered as roots separately. However,
+          even if the output of a derivation is registered as a root, the
+          collector will still delete store paths that are used only at build
+          time (e.g., the C compiler, or source tarballs downloaded from the
+          network). To prevent it from doing so, set this option to `true`.
+        )",
         {"gc-keep-outputs"}};
 
-    Setting<bool> gcKeepDerivations{this, true, "keep-derivations",
-        "Whether the garbage collector should keep derivers of live paths.",
+    Setting<bool> gcKeepDerivations{
+        this, true, "keep-derivations",
+        R"(
+          If `true` (default), the garbage collector will keep the derivations
+          from which non-garbage store paths were built. If `false`, they will
+          be deleted unless explicitly registered as a root (or reachable from
+          other roots).
+
+          Keeping derivation around is useful for querying and traceability
+          (e.g., it allows you to ask with what dependencies or options a
+          store path was built), so by default this option is on. Turn it off
+          to save a bit of disk space (or a lot if `keep-outputs` is also
+          turned on).
+        )",
         {"gc-keep-derivations"}};
 
-    Setting<bool> autoOptimiseStore{this, false, "auto-optimise-store",
-        "Whether to automatically replace files with identical contents with hard links."};
+    Setting<bool> autoOptimiseStore{
+        this, false, "auto-optimise-store",
+        R"(
+          If set to `true`, Nix automatically detects files in the store
+          that have identical contents, and replaces them with hard links to
+          a single copy. This saves disk space. If set to `false` (the
+          default), you can still run `nix-store --optimise` to get rid of
+          duplicate files.
+        )"};
 
-    Setting<bool> envKeepDerivations{this, false, "keep-env-derivations",
-        "Whether to add derivations as a dependency of user environments "
-        "(to prevent them from being GCed).",
+    Setting<bool> envKeepDerivations{
+        this, false, "keep-env-derivations",
+        R"(
+          If `false` (default), derivations are not stored in Nix user
+          environments. That is, the derivations of any build-time-only
+          dependencies may be garbage-collected.
+
+          If `true`, when you add a Nix derivation to a user environment, the
+          path of the derivation is stored in the user environment. Thus, the
+          derivation will not be garbage-collected until the user environment
+          generation is deleted (`nix-env --delete-generations`). To prevent
+          build-time-only dependencies from being collected, you should also
+          turn on `keep-outputs`.
+
+          The difference between this option and `keep-derivations` is that
+          this one is “sticky”: it applies to any user environment created
+          while this option was enabled, while `keep-derivations` only applies
+          at the moment the garbage collector is run.
+        )",
         {"env-keep-derivations"}};
 
     /* Whether to lock the Nix client and worker to the same CPU. */
     bool lockCPU;
 
-    Setting<SandboxMode> sandboxMode{this,
+    Setting<SandboxMode> sandboxMode{
+        this,
         #if __linux__
           smEnabled
         #else
           smDisabled
         #endif
         , "sandbox",
-        "Whether to enable sandboxed builds. Can be \"true\", \"false\" or \"relaxed\".",
+        R"(
+          If set to `true`, builds will be performed in a *sandboxed
+          environment*, i.e., they’re isolated from the normal file system
+          hierarchy and will only see their dependencies in the Nix store,
+          the temporary build directory, private versions of `/proc`,
+          `/dev`, `/dev/shm` and `/dev/pts` (on Linux), and the paths
+          configured with the `sandbox-paths` option. This is useful to
+          prevent undeclared dependencies on files in directories such as
+          `/usr/bin`. In addition, on Linux, builds run in private PID,
+          mount, network, IPC and UTS namespaces to isolate them from other
+          processes in the system (except that fixed-output derivations do
+          not run in private network namespace to ensure they can access the
+          network).
+
+          Currently, sandboxing only work on Linux and macOS. The use of a
+          sandbox requires that Nix is run as root (so you should use the
+          “build users” feature to perform the actual builds under different
+          users than root).
+
+          If this option is set to `relaxed`, then fixed-output derivations
+          and derivations that have the `__noChroot` attribute set to `true`
+          do not run in sandboxes.
+
+          The default is `true` on Linux and `false` on all other platforms.
+        )",
         {"build-use-chroot", "build-use-sandbox"}};
 
-    Setting<PathSet> sandboxPaths{this, {}, "sandbox-paths",
-        "The paths to make available inside the build sandbox.",
+    Setting<PathSet> sandboxPaths{
+        this, {}, "sandbox-paths",
+        R"(
+          A list of paths bind-mounted into Nix sandbox environments. You can
+          use the syntax `target=source` to mount a path in a different
+          location in the sandbox; for instance, `/bin=/nix-bin` will mount
+          the path `/nix-bin` as `/bin` inside the sandbox. If *source* is
+          followed by `?`, then it is not an error if *source* does not exist;
+          for example, `/dev/nvidiactl?` specifies that `/dev/nvidiactl` will
+          only be mounted in the sandbox if it exists in the host filesystem.
+
+          Depending on how Nix was built, the default value for this option
+          may be empty or provide `/bin/sh` as a bind-mount of `bash`.
+        )",
         {"build-chroot-dirs", "build-sandbox-paths"}};
 
     Setting<bool> sandboxFallback{this, true, "sandbox-fallback",
         "Whether to disable sandboxing when the kernel doesn't allow it."};
 
-    Setting<PathSet> extraSandboxPaths{this, {}, "extra-sandbox-paths",
-        "Additional paths to make available inside the build sandbox.",
+    Setting<PathSet> extraSandboxPaths{
+        this, {}, "extra-sandbox-paths",
+        R"(
+          A list of additional paths appended to `sandbox-paths`. Useful if
+          you want to extend its default value.
+        )",
         {"build-extra-chroot-dirs", "build-extra-sandbox-paths"}};
 
-    Setting<size_t> buildRepeat{this, 0, "repeat",
-        "The number of times to repeat a build in order to verify determinism.",
+    Setting<size_t> buildRepeat{
+        this, 0, "repeat",
+        R"(
+          How many times to repeat builds to check whether they are
+          deterministic. The default value is 0. If the value is non-zero,
+          every build is repeated the specified number of times. If the
+          contents of any of the runs differs from the previous ones and
+          `enforce-determinism` is true, the build is rejected and the
+          resulting store paths are not registered as “valid” in Nix’s
+          database.
+        )",
         {"build-repeat"}};
 
 #if __linux__
-    Setting<std::string> sandboxShmSize{this, "50%", "sandbox-dev-shm-size",
-        "The size of /dev/shm in the build sandbox."};
+    Setting<std::string> sandboxShmSize{
+        this, "50%", "sandbox-dev-shm-size",
+        R"(
+          This option determines the maximum size of the `tmpfs` filesystem
+          mounted on `/dev/shm` in Linux sandboxes. For the format, see the
+          description of the `size` option of `tmpfs` in mount8. The default
+          is `50%`.
+        )"};
 
     Setting<Path> sandboxBuildDir{this, "/build", "sandbox-build-dir",
         "The build directory inside the sandbox."};
@@ -237,118 +456,411 @@ public:
         "Whether to log Darwin sandbox access violations to the system log."};
 #endif
 
-    Setting<bool> runDiffHook{this, false, "run-diff-hook",
-        "Whether to run the program specified by the diff-hook setting "
-        "repeated builds produce a different result. Typically used to "
-        "plug in diffoscope."};
+    Setting<bool> runDiffHook{
+        this, false, "run-diff-hook",
+        R"(
+          If true, enable the execution of the `diff-hook` program.
 
-    PathSetting diffHook{this, true, "", "diff-hook",
-        "A program that prints out the differences between the two paths "
-        "specified on its command line."};
+          When using the Nix daemon, `run-diff-hook` must be set in the
+          `nix.conf` configuration file, and cannot be passed at the command
+          line.
+        )"};
 
-    Setting<bool> enforceDeterminism{this, true, "enforce-determinism",
-        "Whether to fail if repeated builds produce different output."};
+    PathSetting diffHook{
+        this, true, "", "diff-hook",
+        R"(
+          Absolute path to an executable capable of diffing build
+          results. The hook is executed if `run-diff-hook` is true, and the
+          output of a build is known to not be the same. This program is not
+          executed to determine if two results are the same.
 
-    Setting<Strings> trustedPublicKeys{this,
+          The diff hook is executed by the same user and group who ran the
+          build. However, the diff hook does not have write access to the
+          store path just built.
+
+          The diff hook program receives three parameters:
+
+          1.  A path to the previous build's results
+
+          2.  A path to the current build's results
+
+          3.  The path to the build's derivation
+
+          4.  The path to the build's scratch directory. This directory will
+              exist only if the build was run with `--keep-failed`.
+
+          The stderr and stdout output from the diff hook will not be
+          displayed to the user. Instead, it will print to the nix-daemon's
+          log.
+
+          When using the Nix daemon, `diff-hook` must be set in the `nix.conf`
+          configuration file, and cannot be passed at the command line.
+        )"};
+
+    Setting<bool> enforceDeterminism{
+        this, true, "enforce-determinism",
+        "Whether to fail if repeated builds produce different output. See `repeat`."};
+
+    Setting<Strings> trustedPublicKeys{
+        this,
         {"cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="},
         "trusted-public-keys",
-        "Trusted public keys for secure substitution.",
+        R"(
+          A whitespace-separated list of public keys. When paths are copied
+          from another Nix store (such as a binary cache), they must be
+          signed with one of these keys. For example:
+          `cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+          hydra.nixos.org-1:CNHJZBh9K4tP3EKF6FkkgeVYsS3ohTl+oS0Qa8bezVs=`.
+        )",
         {"binary-cache-public-keys"}};
 
-    Setting<Strings> secretKeyFiles{this, {}, "secret-key-files",
-        "Secret keys with which to sign local builds."};
+    Setting<Strings> secretKeyFiles{
+        this, {}, "secret-key-files",
+        R"(
+          A whitespace-separated list of files containing secret (private)
+          keys. These are used to sign locally-built paths. They can be
+          generated using `nix-store --generate-binary-cache-key`. The
+          corresponding public key can be distributed to other users, who
+          can add it to `trusted-public-keys` in their `nix.conf`.
+        )"};
 
-    Setting<unsigned int> tarballTtl{this, 60 * 60, "tarball-ttl",
-        "How long downloaded files are considered up-to-date."};
+    Setting<unsigned int> tarballTtl{
+        this, 60 * 60, "tarball-ttl",
+        R"(
+          The number of seconds a downloaded tarball is considered fresh. If
+          the cached tarball is stale, Nix will check whether it is still up
+          to date using the ETag header. Nix will download a new version if
+          the ETag header is unsupported, or the cached ETag doesn't match.
 
-    Setting<bool> requireSigs{this, true, "require-sigs",
-        "Whether to check that any non-content-addressed path added to the "
-        "Nix store has a valid signature (that is, one signed using a key "
-        "listed in 'trusted-public-keys'."};
+          Setting the TTL to `0` forces Nix to always check if the tarball is
+          up to date.
 
-    Setting<StringSet> extraPlatforms{this,
+          Nix caches tarballs in `$XDG_CACHE_HOME/nix/tarballs`.
+
+          Files fetched via `NIX_PATH`, `fetchGit`, `fetchMercurial`,
+          `fetchTarball`, and `fetchurl` respect this TTL.
+        )"};
+
+    Setting<bool> requireSigs{
+        this, true, "require-sigs",
+        R"(
+          If set to `true` (the default), any non-content-addressed path added
+          or copied to the Nix store (e.g. when substituting from a binary
+          cache) must have a valid signature, that is, be signed using one of
+          the keys listed in `trusted-public-keys` or `secret-key-files`. Set
+          to `false` to disable signature checking.
+        )"};
+
+    Setting<StringSet> extraPlatforms{
+        this,
         std::string{SYSTEM} == "x86_64-linux" && !isWSL1() ? StringSet{"i686-linux"} : StringSet{},
         "extra-platforms",
-        "Additional platforms that can be built on the local system. "
-        "These may be supported natively (e.g. armv7 on some aarch64 CPUs "
-        "or using hacks like qemu-user."};
+        R"(
+          Platforms other than the native one which this machine is capable of
+          building for. This can be useful for supporting additional
+          architectures on compatible machines: i686-linux can be built on
+          x86\_64-linux machines (and the default for this setting reflects
+          this); armv7 is backwards-compatible with armv6 and armv5tel; some
+          aarch64 machines can also natively run 32-bit ARM code; and
+          qemu-user may be used to support non-native platforms (though this
+          may be slow and buggy). Most values for this are not enabled by
+          default because build systems will often misdetect the target
+          platform and generate incompatible code, so you may wish to
+          cross-check the results of using this option against proper
+          natively-built versions of your derivations.
+        )"};
 
-    Setting<StringSet> systemFeatures{this, getDefaultSystemFeatures(),
+    Setting<StringSet> systemFeatures{
+        this, getDefaultSystemFeatures(),
         "system-features",
-        "Optional features that this system implements (like \"kvm\")."};
+        R"(
+          A set of system “features” supported by this machine, e.g. `kvm`.
+          Derivations can express a dependency on such features through the
+          derivation attribute `requiredSystemFeatures`. For example, the
+          attribute
 
-    Setting<Strings> substituters{this,
+              requiredSystemFeatures = [ "kvm" ];
+
+          ensures that the derivation can only be built on a machine with the
+          `kvm` feature.
+
+          This setting by default includes `kvm` if `/dev/kvm` is accessible,
+          and the pseudo-features `nixos-test`, `benchmark` and `big-parallel`
+          that are used in Nixpkgs to route builds to specific machines.
+        )"};
+
+    Setting<Strings> substituters{
+        this,
         nixStore == "/nix/store" ? Strings{"https://cache.nixos.org/"} : Strings(),
         "substituters",
-        "The URIs of substituters (such as https://cache.nixos.org/).",
+        R"(
+          A list of URLs of substituters, separated by whitespace. The default
+          is `https://cache.nixos.org`.
+        )",
         {"binary-caches"}};
 
     // FIXME: provide a way to add to option values.
-    Setting<Strings> extraSubstituters{this, {}, "extra-substituters",
-        "Additional URIs of substituters.",
+    Setting<Strings> extraSubstituters{
+        this, {}, "extra-substituters",
+        R"(
+          Additional binary caches appended to those specified in
+          `substituters`. When used by unprivileged users, untrusted
+          substituters (i.e. those not listed in `trusted-substituters`) are
+          silently ignored.
+        )",
         {"extra-binary-caches"}};
 
-    Setting<StringSet> trustedSubstituters{this, {}, "trusted-substituters",
-        "Disabled substituters that may be enabled via the substituters option by untrusted users.",
+    Setting<StringSet> trustedSubstituters{
+        this, {}, "trusted-substituters",
+        R"(
+          A list of URLs of substituters, separated by whitespace. These are
+          not used by default, but can be enabled by users of the Nix daemon
+          by specifying `--option substituters urls` on the command
+          line. Unprivileged users are only allowed to pass a subset of the
+          URLs listed in `substituters` and `trusted-substituters`.
+        )",
         {"trusted-binary-caches"}};
 
-    Setting<Strings> trustedUsers{this, {"root"}, "trusted-users",
-        "Which users or groups are trusted to ask the daemon to do unsafe things."};
+    Setting<Strings> trustedUsers{
+        this, {"root"}, "trusted-users",
+        R"(
+          A list of names of users (separated by whitespace) that have
+          additional rights when connecting to the Nix daemon, such as the
+          ability to specify additional binary caches, or to import unsigned
+          NARs. You can also specify groups by prefixing them with `@`; for
+          instance, `@wheel` means all users in the `wheel` group. The default
+          is `root`.
 
-    Setting<unsigned int> ttlNegativeNarInfoCache{this, 3600, "narinfo-cache-negative-ttl",
-        "The TTL in seconds for negative lookups in the disk cache i.e binary cache lookups that "
-        "return an invalid path result"};
+          > **Warning**
+          > 
+          > Adding a user to `trusted-users` is essentially equivalent to
+          > giving that user root access to the system. For example, the user
+          > can set `sandbox-paths` and thereby obtain read access to
+          > directories that are otherwise inacessible to them.
+        )"};
 
-    Setting<unsigned int> ttlPositiveNarInfoCache{this, 30 * 24 * 3600, "narinfo-cache-positive-ttl",
-        "The TTL in seconds for positive lookups in the disk cache i.e binary cache lookups that "
-        "return a valid path result."};
+    Setting<unsigned int> ttlNegativeNarInfoCache{
+        this, 3600, "narinfo-cache-negative-ttl",
+        R"(
+          The TTL in seconds for negative lookups. If a store path is queried
+          from a substituter but was not found, there will be a negative
+          lookup cached in the local disk cache database for the specified
+          duration.
+        )"};
+
+    Setting<unsigned int> ttlPositiveNarInfoCache{
+        this, 30 * 24 * 3600, "narinfo-cache-positive-ttl",
+        R"(
+          The TTL in seconds for positive lookups. If a store path is queried
+          from a substituter, the result of the query will be cached in the
+          local disk cache database including some of the NAR metadata. The
+          default TTL is a month, setting a shorter TTL for positive lookups
+          can be useful for binary caches that have frequent garbage
+          collection, in which case having a more frequent cache invalidation
+          would prevent trying to pull the path again and failing with a hash
+          mismatch if the build isn't reproducible.
+        )"};
 
     /* ?Who we trust to use the daemon in safe ways */
-    Setting<Strings> allowedUsers{this, {"*"}, "allowed-users",
-        "Which users or groups are allowed to connect to the daemon."};
+    Setting<Strings> allowedUsers{
+        this, {"*"}, "allowed-users",
+        R"(
+          A list of names of users (separated by whitespace) that are allowed
+          to connect to the Nix daemon. As with the `trusted-users` option,
+          you can specify groups by prefixing them with `@`. Also, you can
+          allow all users by specifying `*`. The default is `*`.
+
+          Note that trusted users are always allowed to connect.
+        )"};
 
     Setting<bool> printMissing{this, true, "print-missing",
         "Whether to print what paths need to be built or downloaded."};
 
-    Setting<std::string> preBuildHook{this, "",
-        "pre-build-hook",
-        "A program to run just before a build to set derivation-specific build settings."};
+    Setting<std::string> preBuildHook{
+        this, "", "pre-build-hook",
+        R"(
+          If set, the path to a program that can set extra derivation-specific
+          settings for this system. This is used for settings that can't be
+          captured by the derivation model itself and are too variable between
+          different versions of the same system to be hard-coded into nix.
 
-    Setting<std::string> postBuildHook{this, "", "post-build-hook",
-        "A program to run just after each successful build."};
+          The hook is passed the derivation path and, if sandboxes are
+          enabled, the sandbox directory. It can then modify the sandbox and
+          send a series of commands to modify various settings to stdout. The
+          currently recognized commands are:
 
-    Setting<std::string> netrcFile{this, fmt("%s/%s", nixConfDir, "netrc"), "netrc-file",
-        "Path to the netrc file used to obtain usernames/passwords for downloads."};
+            - `extra-sandbox-paths`  
+              Pass a list of files and directories to be included in the
+              sandbox for this build. One entry per line, terminated by an
+              empty line. Entries have the same format as `sandbox-paths`.
+        )"};
+
+    Setting<std::string> postBuildHook{
+        this, "", "post-build-hook",
+        R"(
+          Optional. The path to a program to execute after each build.
+
+          This option is only settable in the global `nix.conf`, or on the
+          command line by trusted users.
+
+          When using the nix-daemon, the daemon executes the hook as `root`.
+          If the nix-daemon is not involved, the hook runs as the user
+          executing the nix-build.
+
+            - The hook executes after an evaluation-time build.
+
+            - The hook does not execute on substituted paths.
+
+            - The hook's output always goes to the user's terminal.
+
+            - If the hook fails, the build succeeds but no further builds
+              execute.
+
+            - The hook executes synchronously, and blocks other builds from
+              progressing while it runs.
+
+          The program executes with no arguments. The program's environment
+          contains the following environment variables:
+
+            - `DRV_PATH`  
+              The derivation for the built paths.
+
+              Example:
+              `/nix/store/5nihn1a7pa8b25l9zafqaqibznlvvp3f-bash-4.4-p23.drv`
+
+            - `OUT_PATHS`  
+              Output paths of the built derivation, separated by a space
+              character.
+
+              Example:
+              `/nix/store/zf5lbh336mnzf1nlswdn11g4n2m8zh3g-bash-4.4-p23-dev
+              /nix/store/rjxwxwv1fpn9wa2x5ssk5phzwlcv4mna-bash-4.4-p23-doc
+              /nix/store/6bqvbzjkcp9695dq0dpl5y43nvy37pq1-bash-4.4-p23-info
+              /nix/store/r7fng3kk3vlpdlh2idnrbn37vh4imlj2-bash-4.4-p23-man
+              /nix/store/xfghy8ixrhz3kyy6p724iv3cxji088dx-bash-4.4-p23`.
+        )"};
+
+    Setting<std::string> netrcFile{
+        this, fmt("%s/%s", nixConfDir, "netrc"), "netrc-file",
+        R"(
+          If set to an absolute path to a `netrc` file, Nix will use the HTTP
+          authentication credentials in this file when trying to download from
+          a remote host through HTTP or HTTPS. Defaults to
+          `$NIX_CONF_DIR/netrc`.
+
+          The `netrc` file consists of a list of accounts in the following
+          format:
+
+              machine my-machine
+              login my-username
+              password my-password
+
+          For the exact syntax, see [the `curl`
+          documentation](https://ec.haxx.se/usingcurl-netrc.html).
+
+          > **Note**
+          > 
+          > This must be an absolute path, and `~` is not resolved. For
+          > example, `~/.netrc` won't resolve to your home directory's
+          > `.netrc`.
+        )"};
 
     /* Path to the SSL CA file used */
     Path caFile;
 
 #if __linux__
-    Setting<bool> filterSyscalls{this, true, "filter-syscalls",
-            "Whether to prevent certain dangerous system calls, such as "
-            "creation of setuid/setgid files or adding ACLs or extended "
-            "attributes. Only disable this if you're aware of the "
-            "security implications."};
+    Setting<bool> filterSyscalls{
+        this, true, "filter-syscalls",
+        R"(
+          Whether to prevent certain dangerous system calls, such as
+          creation of setuid/setgid files or adding ACLs or extended
+          attributes. Only disable this if you're aware of the
+          security implications.
+        )"};
 
-    Setting<bool> allowNewPrivileges{this, false, "allow-new-privileges",
-        "Whether builders can acquire new privileges by calling programs with "
-        "setuid/setgid bits or with file capabilities."};
+    Setting<bool> allowNewPrivileges{
+        this, false, "allow-new-privileges",
+        R"(
+          (Linux-specific.) By default, builders on Linux cannot acquire new
+          privileges by calling setuid/setgid programs or programs that have
+          file capabilities. For example, programs such as `sudo` or `ping`
+          will fail. (Note that in sandbox builds, no such programs are
+          available unless you bind-mount them into the sandbox via the
+          `sandbox-paths` option.) You can allow the use of such programs by
+          enabling this option. This is impure and usually undesirable, but
+          may be useful in certain scenarios (e.g. to spin up containers or
+          set up userspace network interfaces in tests).
+        )"};
 #endif
 
-    Setting<uint64_t> minFree{this, 0, "min-free",
-        "Automatically run the garbage collector when free disk space drops below the specified amount."};
+    Setting<Strings> hashedMirrors{
+        this, {}, "hashed-mirrors",
+        R"(
+          A list of web servers used by `builtins.fetchurl` to obtain files by
+          hash. The default is `http://tarballs.nixos.org/`. Given a hash type
+          *ht* and a base-16 hash *h*, Nix will try to download the file from
+          *hashed-mirror*/*ht*/*h*. This allows files to be downloaded even if
+          they have disappeared from their original URI. For example, given
+          the default mirror `http://tarballs.nixos.org/`, when building the
+          derivation
 
-    Setting<uint64_t> maxFree{this, std::numeric_limits<uint64_t>::max(), "max-free",
-        "Stop deleting garbage when free disk space is above the specified amount."};
+          ```nix
+          builtins.fetchurl {
+            url = "https://example.org/foo-1.2.3.tar.xz";
+            sha256 = "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae";
+          }
+          ```
+
+          Nix will attempt to download this file from
+          `http://tarballs.nixos.org/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae`
+          first. If it is not available there, if will try the original URI.
+        )"};
+
+    Setting<uint64_t> minFree{
+        this, 0, "min-free",
+        R"(
+          When free disk space in `/nix/store` drops below `min-free` during a
+          build, Nix performs a garbage-collection until `max-free` bytes are
+          available or there is no more garbage. A value of `0` (the default)
+          disables this feature.
+        )"};
+
+    Setting<uint64_t> maxFree{
+        this, std::numeric_limits<uint64_t>::max(), "max-free",
+        R"(
+          When a garbage collection is triggered by the `min-free` option, it
+          stops as soon as `max-free` bytes are available. The default is
+          infinity (i.e. delete all garbage).
+        )"};
 
     Setting<uint64_t> minFreeCheckInterval{this, 5, "min-free-check-interval",
         "Number of seconds between checking free disk space."};
 
-    Setting<Paths> pluginFiles{this, {}, "plugin-files",
-        "Plugins to dynamically load at nix initialization time."};
+    Setting<Paths> pluginFiles{
+        this, {}, "plugin-files",
+        R"(
+          A list of plugin files to be loaded by Nix. Each of these files will
+          be dlopened by Nix, allowing them to affect execution through static
+          initialization. In particular, these plugins may construct static
+          instances of RegisterPrimOp to add new primops or constants to the
+          expression language, RegisterStoreImplementation to add new store
+          implementations, RegisterCommand to add new subcommands to the `nix`
+          command, and RegisterSetting to add new nix config settings. See the
+          constructors for those types for more details.
+
+          Since these files are loaded into the same address space as Nix
+          itself, they must be DSOs compatible with the instance of Nix
+          running at the time (i.e. compiled against the same headers, not
+          linked to any incompatible libraries). They should not be linked to
+          any Nix libs directly, as those will be available already at load
+          time.
+
+          If an entry in the list is a directory, all files in the directory
+          are loaded as plugins (non-recursively).
+        )"};
 
     Setting<std::string> githubAccessToken{this, "", "github-access-token",
-        "GitHub access token to get access to GitHub data through the GitHub API for github:<..> flakes."};
+        "GitHub access token to get access to GitHub data through the GitHub API for `github:<..>` flakes."};
 
     Setting<Strings> experimentalFeatures{this, {}, "experimental-features",
         "Experimental Nix features to enable."};
@@ -368,6 +880,19 @@ public:
 
     Setting<std::string> flakeRegistry{this, "https://github.com/NixOS/flake-registry/raw/master/flake-registry.json", "flake-registry",
         "Path or URI of the global flake registry."};
+
+    Setting<bool> allowSymlinkedStore{
+        this, false, "allow-symlinked-store",
+        R"(
+          If set to `true`, Nix will stop complaining if the store directory
+          (typically /nix/store) contains symlink components.
+
+          This risks making some builds "impure" because builders sometimes
+          "canonicalise" paths by resolving all symlink components. Problems
+          occur if those builds are then deployed to machines where /nix/store
+          resolves to a different location from that of the build machine. You
+          can enable this setting if you are sure you're not going to do that.
+        )"};
 };
 
 

src/libstore/http-binary-cache-store.cc

@@ -2,12 +2,20 @@
 #include "filetransfer.hh"
 #include "globals.hh"
 #include "nar-info-disk-cache.hh"
+#include "callback.hh"
 
 namespace nix {
 
 MakeError(UploadToHTTP, Error);
 
-class HttpBinaryCacheStore : public BinaryCacheStore
+struct HttpBinaryCacheStoreConfig : virtual BinaryCacheStoreConfig
+{
+    using BinaryCacheStoreConfig::BinaryCacheStoreConfig;
+
+    const std::string name() override { return "Http Binary Cache Store"; }
+};
+
+class HttpBinaryCacheStore : public BinaryCacheStore, public HttpBinaryCacheStoreConfig
 {
 private:
 
@@ -24,9 +32,12 @@ private:
 public:
 
     HttpBinaryCacheStore(
-        const Params & params, const Path & _cacheUri)
-        : BinaryCacheStore(params)
-        , cacheUri(_cacheUri)
+        const std::string & scheme,
+        const Path & _cacheUri,
+        const Params & params)
+        : StoreConfig(params)
+        , BinaryCacheStore(params)
+        , cacheUri(scheme + "://" + _cacheUri)
     {
         if (cacheUri.back() == '/')
             cacheUri.pop_back();
@@ -55,6 +66,13 @@ public:
         }
     }
 
+    static std::set<std::string> uriSchemes()
+    {
+        static bool forceHttp = getEnv("_NIX_FORCE_HTTP") == "1";
+        auto ret = std::set<std::string>({"http", "https"});
+        if (forceHttp) ret.insert("file");
+        return ret;
+    }
 protected:
 
     void maybeDisable()
@@ -85,7 +103,7 @@ protected:
         checkEnabled();
 
         try {
-            FileTransferRequest request(cacheUri + "/" + path);
+            FileTransferRequest request(makeRequest(path));
             request.head = true;
             getFileTransfer()->download(request);
             return true;
@@ -103,7 +121,7 @@ protected:
         std::shared_ptr<std::basic_iostream<char>> istream,
         const std::string & mimeType) override
     {
-        auto req = FileTransferRequest(cacheUri + "/" + path);
+        auto req = makeRequest(path);
         req.data = std::make_shared<string>(StreamToSourceAdapter(istream).drain());
         req.mimeType = mimeType;
         try {
@@ -115,8 +133,11 @@ protected:
 
     FileTransferRequest makeRequest(const std::string & path)
     {
-        FileTransferRequest request(cacheUri + "/" + path);
-        return request;
+        return FileTransferRequest(
+            hasPrefix(path, "https://") || hasPrefix(path, "http://") || hasPrefix(path, "file://")
+            ? path
+            : cacheUri + "/" + path);
+
     }
 
     void getFile(const std::string & path, Sink & sink) override
@@ -159,18 +180,6 @@ protected:
 
 };
 
-static RegisterStoreImplementation regStore([](
-    const std::string & uri, const Store::Params & params)
-    -> std::shared_ptr<Store>
-{
-    static bool forceHttp = getEnv("_NIX_FORCE_HTTP") == "1";
-    if (std::string(uri, 0, 7) != "http://" &&
-        std::string(uri, 0, 8) != "https://" &&
-        (!forceHttp || std::string(uri, 0, 7) != "file://"))
-        return 0;
-    auto store = std::make_shared<HttpBinaryCacheStore>(params, uri);
-    store->init();
-    return store;
-});
+static RegisterStoreImplementation<HttpBinaryCacheStore, HttpBinaryCacheStoreConfig> regStore;
 
 }

src/libstore/legacy-ssh-store.cc

@@ -6,21 +6,28 @@
 #include "worker-protocol.hh"
 #include "ssh.hh"
 #include "derivations.hh"
+#include "callback.hh"
 
 namespace nix {
 
-static std::string uriScheme = "ssh://";
-
-struct LegacySSHStore : public Store
+struct LegacySSHStoreConfig : virtual StoreConfig
 {
-    const Setting<int> maxConnections{this, 1, "max-connections", "maximum number of concurrent SSH connections"};
-    const Setting<Path> sshKey{this, "", "ssh-key", "path to an SSH private key"};
-    const Setting<bool> compress{this, false, "compress", "whether to compress the connection"};
-    const Setting<Path> remoteProgram{this, "nix-store", "remote-program", "path to the nix-store executable on the remote system"};
-    const Setting<std::string> remoteStore{this, "", "remote-store", "URI of the store on the remote system"};
+    using StoreConfig::StoreConfig;
+    const Setting<int> maxConnections{(StoreConfig*) this, 1, "max-connections", "maximum number of concurrent SSH connections"};
+    const Setting<Path> sshKey{(StoreConfig*) this, "", "ssh-key", "path to an SSH private key"};
+    const Setting<bool> compress{(StoreConfig*) this, false, "compress", "whether to compress the connection"};
+    const Setting<Path> remoteProgram{(StoreConfig*) this, "nix-store", "remote-program", "path to the nix-store executable on the remote system"};
+    const Setting<std::string> remoteStore{(StoreConfig*) this, "", "remote-store", "URI of the store on the remote system"};
 
+    const std::string name() override { return "Legacy SSH Store"; }
+};
+
+struct LegacySSHStore : public Store, public virtual LegacySSHStoreConfig
+{
     // Hack for getting remote build log output.
-    const Setting<int> logFD{this, -1, "log-fd", "file descriptor to which SSH's stderr is connected"};
+    // Intentionally not in `LegacySSHStoreConfig` so that it doesn't appear in
+    // the documentation
+    const Setting<int> logFD{(StoreConfig*) this, -1, "log-fd", "file descriptor to which SSH's stderr is connected"};
 
     struct Connection
     {
@@ -37,8 +44,11 @@ struct LegacySSHStore : public Store
 
     SSHMaster master;
 
-    LegacySSHStore(const string & host, const Params & params)
-        : Store(params)
+    static std::set<std::string> uriSchemes() { return {"ssh"}; }
+
+    LegacySSHStore(const string & scheme, const string & host, const Params & params)
+        : StoreConfig(params)
+        , Store(params)
         , host(host)
         , connections(make_ref<Pool<Connection>>(
             std::max(1, (int) maxConnections),
@@ -84,7 +94,7 @@ struct LegacySSHStore : public Store
 
     string getUri() override
     {
-        return uriScheme + host;
+        return *uriSchemes().begin() + "://" + host;
     }
 
     void queryPathInfoUncached(const StorePath & path,
@@ -93,6 +103,9 @@ struct LegacySSHStore : public Store
         try {
             auto conn(connections->get());
 
+            /* No longer support missing NAR hash */
+            assert(GET_PROTOCOL_MINOR(conn->remoteVersion) >= 4);
+
             debug("querying remote host '%s' for info on '%s'", host, printStorePath(path));
 
             conn->to << cmdQueryPathInfos << PathSet{printStorePath(path)};
@@ -100,8 +113,10 @@ struct LegacySSHStore : public Store
 
             auto p = readString(conn->from);
             if (p.empty()) return callback(nullptr);
-            auto info = std::make_shared<ValidPathInfo>(parseStorePath(p));
-            assert(path == info->path);
+            auto path2 = parseStorePath(p);
+            assert(path == path2);
+            /* Hash will be set below. FIXME construct ValidPathInfo at end. */
+            auto info = std::make_shared<ValidPathInfo>(path, Hash::dummy);
 
             auto deriver = readString(conn->from);
             if (deriver != "")
@@ -110,12 +125,14 @@ struct LegacySSHStore : public Store
             readLongLong(conn->from); // download size
             info->narSize = readLongLong(conn->from);
 
-            if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 4) {
+            {
                 auto s = readString(conn->from);
-                info->narHash = s.empty() ? std::optional<Hash>{} : Hash{s};
-                info->ca = parseContentAddressOpt(readString(conn->from));
-                info->sigs = readStrings<StringSet>(conn->from);
+                if (s == "")
+                    throw Error("NAR hash is now mandatory");
+                info->narHash = Hash::parseAnyPrefixed(s);
             }
+            info->ca = parseContentAddressOpt(readString(conn->from));
+            info->sigs = readStrings<StringSet>(conn->from);
 
             auto s = readString(conn->from);
             assert(s == "");
@@ -137,7 +154,7 @@ struct LegacySSHStore : public Store
                 << cmdAddToStoreNar
                 << printStorePath(info.path)
                 << (info.deriver ? printStorePath(*info.deriver) : "")
-                << info.narHash->to_string(Base16, false);
+                << info.narHash.to_string(Base16, false);
             writeStorePaths(*this, conn->to, info.referencesPossiblyToSelf());
             conn->to
                 << info.registrationTime
@@ -201,6 +218,24 @@ struct LegacySSHStore : public Store
         const StorePathSet & references, RepairFlag repair) override
     { unsupported("addTextToStore"); }
 
+private:
+
+    void putBuildSettings(Connection & conn)
+    {
+        conn.to
+            << settings.maxSilentTime
+            << settings.buildTimeout;
+        if (GET_PROTOCOL_MINOR(conn.remoteVersion) >= 2)
+            conn.to
+                << settings.maxLogSize;
+        if (GET_PROTOCOL_MINOR(conn.remoteVersion) >= 3)
+            conn.to
+                << settings.buildRepeat
+                << settings.enforceDeterminism;
+    }
+
+public:
+
     BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
         BuildMode buildMode) override
     {
@@ -210,16 +245,8 @@ struct LegacySSHStore : public Store
             << cmdBuildDerivation
             << printStorePath(drvPath);
         writeDerivation(conn->to, *this, drv);
-        conn->to
-            << settings.maxSilentTime
-            << settings.buildTimeout;
-        if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 2)
-            conn->to
-                << settings.maxLogSize;
-        if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 3)
-            conn->to
-                << settings.buildRepeat
-                << settings.enforceDeterminism;
+
+        putBuildSettings(*conn);
 
         conn->to.flush();
 
@@ -233,6 +260,29 @@ struct LegacySSHStore : public Store
         return status;
     }
 
+    void buildPaths(const std::vector<StorePathWithOutputs> & drvPaths, BuildMode buildMode) override
+    {
+        auto conn(connections->get());
+
+        conn->to << cmdBuildPaths;
+        Strings ss;
+        for (auto & p : drvPaths)
+            ss.push_back(p.to_string(*this));
+        conn->to << ss;
+
+        putBuildSettings(*conn);
+
+        conn->to.flush();
+
+        BuildResult result;
+        result.status = (BuildResult::Status) readInt(conn->from);
+
+        if (!result.success()) {
+            conn->from >> result.errorMsg;
+            throw Error(result.status, result.errorMsg);
+        }
+    }
+
     void ensurePath(const StorePath & path) override
     { unsupported("ensurePath"); }
 
@@ -284,12 +334,6 @@ struct LegacySSHStore : public Store
     }
 };
 
-static RegisterStoreImplementation regStore([](
-    const std::string & uri, const Store::Params & params)
-    -> std::shared_ptr<Store>
-{
-    if (std::string(uri, 0, uriScheme.size()) != uriScheme) return 0;
-    return std::make_shared<LegacySSHStore>(std::string(uri, uriScheme.size()), params);
-});
+static RegisterStoreImplementation<LegacySSHStore, LegacySSHStoreConfig> regStore;
 
 }

src/libstore/local-binary-cache-store.cc

@@ -4,7 +4,14 @@
 
 namespace nix {
 
-class LocalBinaryCacheStore : public BinaryCacheStore
+struct LocalBinaryCacheStoreConfig : virtual BinaryCacheStoreConfig
+{
+    using BinaryCacheStoreConfig::BinaryCacheStoreConfig;
+
+    const std::string name() override { return "Local Binary Cache Store"; }
+};
+
+class LocalBinaryCacheStore : public BinaryCacheStore, public virtual LocalBinaryCacheStoreConfig
 {
 private:
 
@@ -13,8 +20,11 @@ private:
 public:
 
     LocalBinaryCacheStore(
-        const Params & params, const Path & binaryCacheDir)
-        : BinaryCacheStore(params)
+        const std::string scheme,
+        const Path & binaryCacheDir,
+        const Params & params)
+        : StoreConfig(params)
+        , BinaryCacheStore(params)
         , binaryCacheDir(binaryCacheDir)
     {
     }
@@ -26,6 +36,8 @@ public:
         return "file://" + binaryCacheDir;
     }
 
+    static std::set<std::string> uriSchemes();
+
 protected:
 
     bool fileExists(const std::string & path) override;
@@ -85,16 +97,14 @@ bool LocalBinaryCacheStore::fileExists(const std::string & path)
     return pathExists(binaryCacheDir + "/" + path);
 }
 
-static RegisterStoreImplementation regStore([](
-    const std::string & uri, const Store::Params & params)
-    -> std::shared_ptr<Store>
+std::set<std::string> LocalBinaryCacheStore::uriSchemes()
 {
-    if (getEnv("_NIX_FORCE_HTTP_BINARY_CACHE_STORE") == "1" ||
-        std::string(uri, 0, 7) != "file://")
-        return 0;
-    auto store = std::make_shared<LocalBinaryCacheStore>(params, std::string(uri, 7));
-    store->init();
-    return store;
-});
+    if (getEnv("_NIX_FORCE_HTTP_BINARY_CACHE_STORE") == "1")
+        return {};
+    else
+        return {"file"};
+}
+
+static RegisterStoreImplementation<LocalBinaryCacheStore, LocalBinaryCacheStoreConfig> regStore;
 
 }

src/libstore/local-store.cc

@@ -6,6 +6,7 @@
 #include "derivations.hh"
 #include "nar-info.hh"
 #include "references.hh"
+#include "callback.hh"
 
 #include <iostream>
 #include <algorithm>
@@ -42,7 +43,8 @@ namespace nix {
 
 
 LocalStore::LocalStore(const Params & params)
-    : Store(params)
+    : StoreConfig(params)
+    , Store(params)
     , LocalFSStore(params)
     , realStoreDir_{this, false, rootDir != "" ? rootDir + "/nix/store" : storeDir, "real",
         "physical path to the Nix store"}
@@ -108,12 +110,11 @@ LocalStore::LocalStore(const Params & params)
     }
 
     /* Ensure that the store and its parents are not symlinks. */
-    if (getEnv("NIX_IGNORE_SYMLINK_STORE") != "1") {
+    if (!settings.allowSymlinkedStore) {
         Path path = realStoreDir;
         struct stat st;
         while (path != "/") {
-            if (lstat(path.c_str(), &st))
-                throw SysError("getting status of '%1%'", path);
+            st = lstat(path);
             if (S_ISLNK(st.st_mode))
                 throw Error(
                         "the path '%1%' is a symlink; "
@@ -417,10 +418,7 @@ static void canonicaliseTimestampAndPermissions(const Path & path, const struct
 
 void canonicaliseTimestampAndPermissions(const Path & path)
 {
-    struct stat st;
-    if (lstat(path.c_str(), &st))
-        throw SysError("getting attributes of path '%1%'", path);
-    canonicaliseTimestampAndPermissions(path, st);
+    canonicaliseTimestampAndPermissions(path, lstat(path));
 }
 
 
@@ -438,9 +436,7 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
     }
 #endif
 
-    struct stat st;
-    if (lstat(path.c_str(), &st))
-        throw SysError("getting attributes of path '%1%'", path);
+    auto st = lstat(path);
 
     /* Really make sure that the path is of a supported type. */
     if (!(S_ISREG(st.st_mode) || S_ISDIR(st.st_mode) || S_ISLNK(st.st_mode)))
@@ -476,8 +472,7 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
        ensure that we don't fail on hard links within the same build
        (i.e. "touch $out/foo; ln $out/foo $out/bar"). */
     if (fromUid != (uid_t) -1 && st.st_uid != fromUid) {
-        assert(!S_ISDIR(st.st_mode));
-        if (inodesSeen.find(Inode(st.st_dev, st.st_ino)) == inodesSeen.end())
+        if (S_ISDIR(st.st_mode) || !inodesSeen.count(Inode(st.st_dev, st.st_ino)))
             throw BuildError("invalid ownership on file '%1%'", path);
         mode_t mode = st.st_mode & ~S_IFMT;
         assert(S_ISLNK(st.st_mode) || (st.st_uid == geteuid() && (mode == 0444 || mode == 0555) && st.st_mtime == mtimeStore));
@@ -520,9 +515,7 @@ void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & ino
 
     /* On platforms that don't have lchown(), the top-level path can't
        be a symlink, since we can't change its ownership. */
-    struct stat st;
-    if (lstat(path.c_str(), &st))
-        throw SysError("getting attributes of path '%1%'", path);
+    auto st = lstat(path);
 
     if (st.st_uid != geteuid()) {
         assert(S_ISLNK(st.st_mode));
@@ -544,11 +537,8 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
     std::string drvName(drvPath.name());
     drvName = string(drvName, 0, drvName.size() - drvExtension.size());
 
-    auto check = [&](const StorePath & expected, const StorePath & actual, const std::string & varName)
+    auto envHasRightPath = [&](const StorePath & actual, const std::string & varName)
     {
-        if (actual != expected)
-            throw Error("derivation '%s' has incorrect output '%s', should be '%s'",
-                printStorePath(drvPath), printStorePath(actual), printStorePath(expected));
         auto j = drv.env.find(varName);
         if (j == drv.env.end() || parseStorePath(j->second) != actual)
             throw Error("derivation '%s' has incorrect environment variable '%s', should be '%s'",
@@ -556,20 +546,57 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
     };
 
 
-    if (drv.isFixedOutput()) {
-        DerivationOutputs::const_iterator out = drv.outputs.find("out");
-        if (out == drv.outputs.end())
-            throw Error("derivation '%s' does not have an output named 'out'", printStorePath(drvPath));
-    }
+    // Don't need the answer, but do this anyways to assert is proper
+    // combination. The code below is more general and naturally allows
+    // combinations that are currently prohibited.
+    drv.type();
 
-    else {
-        Hash h = hashDerivationModulo(*this, drv, true);
-        for (auto & i : drv.outputs)
-            check(makeOutputPath(i.first, h, drvName), i.second.path(*this, drv.name), i.first);
+    std::optional<Hash> h;
+    for (auto & i : drv.outputs) {
+        std::visit(overloaded {
+            [&](DerivationOutputInputAddressed doia) {
+                if (!h) {
+                    // somewhat expensive so we do lazily
+                    auto temp = hashDerivationModulo(*this, drv, true);
+                    h = std::get<Hash>(temp);
+                }
+                StorePath recomputed = makeOutputPath(i.first, *h, drvName);
+                if (doia.path != recomputed)
+                    throw Error("derivation '%s' has incorrect output '%s', should be '%s'",
+                        printStorePath(drvPath), printStorePath(doia.path), printStorePath(recomputed));
+                envHasRightPath(doia.path, i.first);
+            },
+            [&](DerivationOutputCAFixed dof) {
+                StorePath path = makeFixedOutputPath(dof.hash.method, dof.hash.hash, drvName);
+                envHasRightPath(path, i.first);
+            },
+            [&](DerivationOutputCAFloating _) {
+                /* Nothing to check */
+            },
+        }, i.second.output);
     }
 }
 
 
+void LocalStore::linkDeriverToPath(const StorePath & deriver, const string & outputName, const StorePath & output)
+{
+    auto state(_state.lock());
+    return linkDeriverToPath(*state, queryValidPathId(*state, deriver), outputName, output);
+}
+
+void LocalStore::linkDeriverToPath(State & state, uint64_t deriver, const string & outputName, const StorePath & output)
+{
+    retrySQLite<void>([&]() {
+        state.stmtAddDerivationOutput.use()
+            (deriver)
+            (outputName)
+            (printStorePath(output))
+            .exec();
+    });
+
+}
+
+
 uint64_t LocalStore::addValidPath(State & state,
     const ValidPathInfo & info, bool checkOutputs)
 {
@@ -579,7 +606,7 @@ uint64_t LocalStore::addValidPath(State & state,
 
     state.stmtRegisterValidPath.use()
         (printStorePath(info.path))
-        (info.narHash->to_string(Base16, true))
+        (info.narHash.to_string(Base16, true))
         (info.registrationTime == 0 ? time(0) : info.registrationTime)
         (info.deriver ? printStorePath(*info.deriver) : "", (bool) info.deriver)
         (info.narSize, info.narSize != 0)
@@ -603,12 +630,11 @@ uint64_t LocalStore::addValidPath(State & state,
            registration above is undone. */
         if (checkOutputs) checkDerivationOutputs(info.path, drv);
 
-        for (auto & i : drv.outputs) {
-            state.stmtAddDerivationOutput.use()
-                (id)
-                (i.first)
-                (printStorePath(i.second.path(*this, drv.name)))
-                .exec();
+        for (auto & i : drv.outputsAndOptPaths(*this)) {
+            /* Floating CA derivations have indeterminate output paths until
+               they are built, so don't register anything in that case */
+            if (i.second.second)
+                linkDeriverToPath(state, id, i.first, *i.second.second);
         }
     }
 
@@ -626,25 +652,28 @@ void LocalStore::queryPathInfoUncached(const StorePath & path,
     Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept
 {
     try {
-        auto info = std::make_shared<ValidPathInfo>(path);
-
         callback(retrySQLite<std::shared_ptr<ValidPathInfo>>([&]() {
             auto state(_state.lock());
 
             /* Get the path info. */
-            auto useQueryPathInfo(state->stmtQueryPathInfo.use()(printStorePath(info->path)));
+            auto useQueryPathInfo(state->stmtQueryPathInfo.use()(printStorePath(path)));
 
             if (!useQueryPathInfo.next())
                 return std::shared_ptr<ValidPathInfo>();
 
-            info->id = useQueryPathInfo.getInt(0);
+            auto id = useQueryPathInfo.getInt(0);
 
+            auto narHash = Hash::dummy;
             try {
-                info->narHash = Hash(useQueryPathInfo.getStr(1));
+                narHash = Hash::parseAnyPrefixed(useQueryPathInfo.getStr(1));
             } catch (BadHash & e) {
-                throw Error("in valid-path entry for '%s': %s", printStorePath(path), e.what());
+                throw Error("invalid-path entry for '%s': %s", printStorePath(path), e.what());
             }
 
+            auto info = std::make_shared<ValidPathInfo>(path, narHash);
+
+            info->id = id;
+
             info->registrationTime = useQueryPathInfo.getInt(2);
 
             auto s = (const char *) sqlite3_column_text(state->stmtQueryPathInfo, 3);
@@ -681,7 +710,7 @@ void LocalStore::updatePathInfo(State & state, const ValidPathInfo & info)
 {
     state.stmtUpdatePathInfo.use()
         (info.narSize, info.narSize != 0)
-        (info.narHash->to_string(Base16, true))
+        (info.narHash.to_string(Base16, true))
         (info.ultimate ? 1 : 0, info.ultimate)
         (concatStringsSep(" ", info.sigs), !info.sigs.empty())
         (renderContentAddress(info.ca), (bool) info.ca)
@@ -769,17 +798,21 @@ StorePathSet LocalStore::queryValidDerivers(const StorePath & path)
 }
 
 
-OutputPathMap LocalStore::queryDerivationOutputMap(const StorePath & path)
+std::map<std::string, std::optional<StorePath>> LocalStore::queryPartialDerivationOutputMap(const StorePath & path)
 {
-    return retrySQLite<OutputPathMap>([&]() {
+    std::map<std::string, std::optional<StorePath>> outputs;
+    BasicDerivation drv = readDerivation(path);
+    for (auto & [outName, _] : drv.outputs) {
+        outputs.insert_or_assign(outName, std::nullopt);
+    }
+    return retrySQLite<std::map<std::string, std::optional<StorePath>>>([&]() {
         auto state(_state.lock());
 
         auto useQueryDerivationOutputs(state->stmtQueryDerivationOutputs.use()
             (queryValidPathId(*state, path)));
 
-        OutputPathMap outputs;
         while (useQueryDerivationOutputs.next())
-            outputs.emplace(
+            outputs.insert_or_assign(
                 useQueryDerivationOutputs.getStr(0),
                 parseStorePath(useQueryDerivationOutputs.getStr(1))
             );
@@ -909,7 +942,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
         StorePathSet paths;
 
         for (auto & i : infos) {
-            assert(i.narHash && i.narHash->type == htSHA256);
+            assert(i.narHash.type == htSHA256);
             if (isValidPath_(*state, i.path))
                 updatePathInfo(*state, i);
             else
@@ -973,9 +1006,6 @@ const PublicKeys & LocalStore::getPublicKeys()
 void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
     RepairFlag repair, CheckSigsFlag checkSigs)
 {
-    if (!info.narHash)
-        throw Error("cannot add path '%s' because it lacks a hash", printStorePath(info.path));
-
     if (requireSigs && checkSigs && !info.checkSignatures(*this, getPublicKeys()))
         throw Error("cannot add path '%s' because it lacks a valid signature", printStorePath(info.path));
 
@@ -1009,11 +1039,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
             else
                 hashSink = std::make_unique<HashModuloSink>(htSHA256, std::string(info.path.hashPart()));
 
-            LambdaSource wrapperSource([&](unsigned char * data, size_t len) -> size_t {
-                size_t n = source.read(data, len);
-                (*hashSink)(data, n);
-                return n;
-            });
+            TeeSource wrapperSource { source, *hashSink };
 
             restorePath(realPath, wrapperSource);
 
@@ -1021,7 +1047,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 
             if (hashResult.first != info.narHash)
                 throw Error("hash mismatch importing path '%s';\n  wanted: %s\n  got:    %s",
-                    printStorePath(info.path), info.narHash->to_string(Base32, true), hashResult.first.to_string(Base32, true));
+                    printStorePath(info.path), info.narHash.to_string(Base32, true), hashResult.first.to_string(Base32, true));
 
             if (hashResult.second != info.narSize)
                 throw Error("size mismatch importing path '%s';\n  wanted: %s\n  got:   %s",
@@ -1143,8 +1169,7 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, const string & name,
 
             optimisePath(realPath);
 
-            ValidPathInfo info(dstPath);
-            info.narHash = narHash.first;
+            ValidPathInfo info { dstPath, narHash.first };
             info.narSize = narHash.second;
             info.ca = FixedOutputHash { .method = method, .hash = hash };
             registerValidPath(info);
@@ -1187,8 +1212,7 @@ StorePath LocalStore::addTextToStore(const string & name, const string & s,
 
             optimisePath(realPath);
 
-            ValidPathInfo info(dstPath);
-            info.narHash = narHash;
+            ValidPathInfo info { dstPath, narHash };
             info.narSize = sink.s->size();
             info.references = references;
             info.ca = TextHash { .hash = hash };
@@ -1303,9 +1327,9 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
 
                 std::unique_ptr<AbstractHashSink> hashSink;
                 if (!info->ca || !info->hasSelfReference)
-                    hashSink = std::make_unique<HashSink>(info->narHash->type);
+                    hashSink = std::make_unique<HashSink>(info->narHash.type);
                 else
-                    hashSink = std::make_unique<HashModuloSink>(info->narHash->type, std::string(info->path.hashPart()));
+                    hashSink = std::make_unique<HashModuloSink>(info->narHash.type, std::string(info->path.hashPart()));
 
                 dumpPath(Store::toRealPath(i), *hashSink);
                 auto current = hashSink->finish();
@@ -1314,7 +1338,7 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
                     logError({
                         .name = "Invalid hash - path modified",
                         .hint = hintfmt("path '%s' was modified! expected hash '%s', got '%s'",
-                        printStorePath(i), info->narHash->to_string(Base32, true), current.first.to_string(Base32, true))
+                        printStorePath(i), info->narHash.to_string(Base32, true), current.first.to_string(Base32, true))
                     });
                     if (repair) repairPath(i); else errors = true;
                 } else {
@@ -1425,7 +1449,7 @@ static void makeMutable(const Path & path)
 {
     checkInterrupt();
 
-    struct stat st = lstat(path);
+    auto st = lstat(path);
 
     if (!S_ISDIR(st.st_mode) && !S_ISREG(st.st_mode)) return;
 

src/libstore/local-store.hh

@@ -23,9 +23,6 @@ namespace nix {
 const int nixSchemaVersion = 10;
 
 
-struct Derivation;
-
-
 struct OptimiseStats
 {
     unsigned long filesLinked = 0;
@@ -33,8 +30,19 @@ struct OptimiseStats
     uint64_t blocksFreed = 0;
 };
 
+struct LocalStoreConfig : virtual LocalFSStoreConfig
+{
+    using LocalFSStoreConfig::LocalFSStoreConfig;
 
-class LocalStore : public LocalFSStore
+    Setting<bool> requireSigs{(StoreConfig*) this,
+        settings.requireSigs,
+        "require-sigs", "whether store paths should have a trusted signature on import"};
+
+    const std::string name() override { return "Local Store"; }
+};
+
+
+class LocalStore : public LocalFSStore, public virtual LocalStoreConfig
 {
 private:
 
@@ -98,10 +106,6 @@ public:
 
 private:
 
-    Setting<bool> requireSigs{(Store*) this,
-        settings.requireSigs,
-        "require-sigs", "whether store paths should have a trusted signature on import"};
-
     const PublicKeys & getPublicKeys();
 
 public:
@@ -133,7 +137,7 @@ public:
 
     StorePathSet queryValidDerivers(const StorePath & path) override;
 
-    OutputPathMap queryDerivationOutputMap(const StorePath & path) override;
+    std::map<std::string, std::optional<StorePath>> queryPartialDerivationOutputMap(const StorePath & path) override;
 
     std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override;
 
@@ -282,6 +286,11 @@ private:
        specified by the ‘secret-key-files’ option. */
     void signPathInfo(ValidPathInfo & info);
 
+    /* Register the store path 'output' as the output named 'outputName' of
+       derivation 'deriver'. */
+    void linkDeriverToPath(const StorePath & deriver, const string & outputName, const StorePath & output);
+    void linkDeriverToPath(State & state, uint64_t deriver, const string & outputName, const StorePath & output);
+
     Path getRealStoreDir() override { return realStoreDir; }
 
     void createUser(const std::string & userName, uid_t userId) override;

src/libstore/machines.cc

@@ -1,6 +1,7 @@
 #include "machines.hh"
 #include "util.hh"
 #include "globals.hh"
+#include "store-api.hh"
 
 #include <algorithm>
 
@@ -48,6 +49,29 @@ bool Machine::mandatoryMet(const std::set<string> & features) const {
         });
 }
 
+ref<Store> Machine::openStore() const {
+    Store::Params storeParams;
+    if (hasPrefix(storeUri, "ssh://")) {
+        storeParams["max-connections"] = "1";
+        storeParams["log-fd"] = "4";
+        if (sshKey != "")
+            storeParams["ssh-key"] = sshKey;
+    }
+    {
+        auto & fs = storeParams["system-features"];
+        auto append = [&](auto feats) {
+            for (auto & f : feats) {
+                if (fs.size() > 0) fs += ' ';
+                fs += f;
+            }
+        };
+        append(supportedFeatures);
+        append(mandatoryFeatures);
+    }
+
+    return nix::openStore(storeUri, storeParams);
+}
+
 void parseMachines(const std::string & s, Machines & machines)
 {
     for (auto line : tokenizeString<std::vector<string>>(s, "\n;")) {

src/libstore/machines.hh

@@ -4,6 +4,8 @@
 
 namespace nix {
 
+class Store;
+
 struct Machine {
 
     const string storeUri;
@@ -28,6 +30,8 @@ struct Machine {
         decltype(supportedFeatures) supportedFeatures,
         decltype(mandatoryFeatures) mandatoryFeatures,
         decltype(sshPublicHostKey) sshPublicHostKey);
+
+    ref<Store> openStore() const;
 };
 
 typedef std::vector<Machine> Machines;

src/libstore/misc.cc

@@ -4,7 +4,8 @@
 #include "local-store.hh"
 #include "store-api.hh"
 #include "thread-pool.hh"
-
+#include "topo-sort.hh"
+#include "callback.hh"
 
 namespace nix {
 
@@ -111,7 +112,7 @@ std::optional<ContentAddress> getDerivationCA(const BasicDerivation & drv)
 {
     auto out = drv.outputs.find("out");
     if (out != drv.outputs.end()) {
-        if (auto v = std::get_if<DerivationOutputFixed>(&out->second.output))
+        if (auto v = std::get_if<DerivationOutputCAFixed>(&out->second.output))
             return v->hash;
     }
     return std::nullopt;
@@ -201,17 +202,24 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
                 return;
             }
 
+            PathSet invalid;
+            /* true for regular derivations, and CA derivations for which we
+               have a trust mapping for all wanted outputs. */
+            auto knownOutputPaths = true;
+            for (auto & [outputName, pathOpt] : queryPartialDerivationOutputMap(path.path)) {
+                if (!pathOpt) {
+                    knownOutputPaths = false;
+                    break;
+                }
+                if (wantOutput(outputName, path.outputs) && !isValidPath(*pathOpt))
+                    invalid.insert(printStorePath(*pathOpt));
+            }
+            if (knownOutputPaths && invalid.empty()) return;
+
             auto drv = make_ref<Derivation>(derivationFromPath(path.path));
             ParsedDerivation parsedDrv(StorePath(path.path), *drv);
 
-            PathSet invalid;
-            for (auto & j : drv->outputs)
-                if (wantOutput(j.first, path.outputs)
-                    && !isValidPath(j.second.path(*this, drv->name)))
-                    invalid.insert(printStorePath(j.second.path(*this, drv->name)));
-            if (invalid.empty()) return;
-
-            if (settings.useSubstitutes && parsedDrv.substitutesAllowed()) {
+            if (knownOutputPaths && settings.useSubstitutes && parsedDrv.substitutesAllowed()) {
                 auto drvState = make_ref<Sync<DrvState>>(DrvState(invalid.size()));
                 for (auto & output : invalid)
                     pool.enqueue(std::bind(checkOutput, printStorePath(path.path), drv, output, drvState));
@@ -255,41 +263,21 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
 
 StorePaths Store::topoSortPaths(const StorePathSet & paths)
 {
-    StorePaths sorted;
-    StorePathSet visited, parents;
-
-    std::function<void(const StorePath & path, const StorePath * parent)> dfsVisit;
-
-    dfsVisit = [&](const StorePath & path, const StorePath * parent) {
-        if (parents.count(path))
-            throw BuildError("cycle detected in the references of '%s' from '%s'",
-                printStorePath(path), printStorePath(*parent));
-
-        if (!visited.insert(path).second) return;
-        parents.insert(path);
-
-        StorePathSet references;
-        try {
-            references = queryPathInfo(path)->references;
-        } catch (InvalidPath &) {
-        }
-
-        for (auto & i : references)
-            /* Don't traverse into paths that don't exist.  That can
-               happen due to substitutes for non-existent paths. */
-            if (paths.count(i))
-                dfsVisit(i, &path);
-
-        sorted.push_back(path);
-        parents.erase(path);
-    };
-
-    for (auto & i : paths)
-        dfsVisit(i, nullptr);
-
-    std::reverse(sorted.begin(), sorted.end());
-
-    return sorted;
+    return topoSort(paths,
+        {[&](const StorePath & path) {
+            StorePathSet references;
+            try {
+                references = queryPathInfo(path)->references;
+            } catch (InvalidPath &) {
+            }
+            return references;
+        }},
+        {[&](const StorePath & path, const StorePath & parent) {
+            return BuildError(
+                "cycle detected in the references of '%s' from '%s'",
+                printStorePath(path),
+                printStorePath(parent));
+        }});
 }
 
 

src/libstore/names.cc

@@ -1,10 +1,18 @@
 #include "names.hh"
 #include "util.hh"
 
+#include <regex>
+
 
 namespace nix {
 
 
+struct Regex
+{
+    std::regex regex;
+};
+
+
 DrvName::DrvName()
 {
     name = "";
@@ -30,11 +38,18 @@ DrvName::DrvName(std::string_view s) : hits(0)
 }
 
 
+DrvName::~DrvName()
+{ }
+
+
 bool DrvName::matches(DrvName & n)
 {
     if (name != "*") {
-        if (!regex) regex = std::unique_ptr<std::regex>(new std::regex(name, std::regex::extended));
-        if (!std::regex_match(n.name, *regex)) return false;
+        if (!regex) {
+            regex = std::make_unique<Regex>();
+            regex->regex = std::regex(name, std::regex::extended);
+        }
+        if (!std::regex_match(n.name, regex->regex)) return false;
     }
     if (version != "" && version != n.version) return false;
     return true;
@@ -99,7 +114,7 @@ DrvNames drvNamesFromArgs(const Strings & opArgs)
 {
     DrvNames result;
     for (auto & i : opArgs)
-        result.push_back(DrvName(i));
+        result.emplace_back(i);
     return result;
 }
 

src/libstore/names.hh

@@ -3,10 +3,11 @@
 #include <memory>
 
 #include "types.hh"
-#include <regex>
 
 namespace nix {
 
+struct Regex;
+
 struct DrvName
 {
     string fullName;
@@ -16,10 +17,12 @@ struct DrvName
 
     DrvName();
     DrvName(std::string_view s);
+    ~DrvName();
+
     bool matches(DrvName & n);
 
 private:
-    std::unique_ptr<std::regex> regex;
+    std::unique_ptr<Regex> regex;
 };
 
 typedef list<DrvName> DrvNames;

src/libstore/nar-accessor.cc

@@ -49,7 +49,8 @@ struct NarAccessor : public FSAccessor
             : acc(acc), source(source)
         { }
 
-        void createMember(const Path & path, NarMember member) {
+        void createMember(const Path & path, NarMember member)
+        {
             size_t level = std::count(path.begin(), path.end(), '/');
             while (parents.size() > level) parents.pop();
 

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

@@ -189,13 +189,14 @@ public:
                 return {oInvalid, 0};
 
             auto namePart = queryNAR.getStr(1);
-            auto narInfo = make_ref<NarInfo>(StorePath(hashPart + "-" + namePart));
+            auto narInfo = make_ref<NarInfo>(
+                StorePath(hashPart + "-" + namePart),
+                Hash::parseAnyPrefixed(queryNAR.getStr(6)));
             narInfo->url = queryNAR.getStr(2);
             narInfo->compression = queryNAR.getStr(3);
             if (!queryNAR.isNull(4))
-                narInfo->fileHash = Hash(queryNAR.getStr(4));
+                narInfo->fileHash = Hash::parseAnyPrefixed(queryNAR.getStr(4));
             narInfo->fileSize = queryNAR.getInt(5);
-            narInfo->narHash = Hash(queryNAR.getStr(6));
             narInfo->narSize = queryNAR.getInt(7);
             for (auto & r : tokenizeString<Strings>(queryNAR.getStr(8), " "))
                 narInfo->insertReferencePossiblyToSelf(StorePath(r));
@@ -232,7 +233,7 @@ public:
                     (narInfo ? narInfo->compression : "", narInfo != 0)
                     (narInfo && narInfo->fileHash ? narInfo->fileHash->to_string(Base32, true) : "", narInfo && narInfo->fileHash)
                     (narInfo ? narInfo->fileSize : 0, narInfo != 0 && narInfo->fileSize)
-                    (info->narHash->to_string(Base32, true))
+                    (info->narHash.to_string(Base32, true))
                     (info->narSize)
                     (concatStringsSep(" ", info->shortRefs()))
                     (info->deriver ? std::string(info->deriver->to_string()) : "", (bool) info->deriver)

src/libstore/nar-info.cc

@@ -1,10 +1,11 @@
 #include "globals.hh"
 #include "nar-info.hh"
+#include "store-api.hh"
 
 namespace nix {
 
 NarInfo::NarInfo(const Store & store, const std::string & s, const std::string & whence)
-    : ValidPathInfo(StorePath(StorePath::dummy)) // FIXME: hack
+    : ValidPathInfo(StorePath(StorePath::dummy), Hash(Hash::dummy)) // FIXME: hack
 {
     auto corrupt = [&]() {
         return Error("NAR info file '%1%' is corrupt", whence);
@@ -12,13 +13,14 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
 
     auto parseHashField = [&](const string & s) {
         try {
-            return Hash(s);
+            return Hash::parseAnyPrefixed(s);
         } catch (BadHash &) {
             throw corrupt();
         }
     };
 
     bool havePath = false;
+    bool haveNarHash = false;
 
     size_t pos = 0;
     while (pos < s.size()) {
@@ -46,8 +48,10 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         else if (name == "FileSize") {
             if (!string2Int(value, fileSize)) throw corrupt();
         }
-        else if (name == "NarHash")
+        else if (name == "NarHash") {
             narHash = parseHashField(value);
+            haveNarHash = true;
+        }
         else if (name == "NarSize") {
             if (!string2Int(value, narSize)) throw corrupt();
         }
@@ -76,7 +80,7 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
 
     if (compression == "") compression = "bzip2";
 
-    if (!havePath || url.empty() || narSize == 0 || !narHash) throw corrupt();
+    if (!havePath || !haveNarHash || url.empty() || narSize == 0) throw corrupt();
 }
 
 std::string NarInfo::to_string(const Store & store) const
@@ -89,8 +93,8 @@ std::string NarInfo::to_string(const Store & store) const
     assert(fileHash && fileHash->type == htSHA256);
     res += "FileHash: " + fileHash->to_string(Base32, true) + "\n";
     res += "FileSize: " + std::to_string(fileSize) + "\n";
-    assert(narHash && narHash->type == htSHA256);
-    res += "NarHash: " + narHash->to_string(Base32, true) + "\n";
+    assert(narHash.type == htSHA256);
+    res += "NarHash: " + narHash.to_string(Base32, true) + "\n";
     res += "NarSize: " + std::to_string(narSize) + "\n";
 
     res += "References: " + concatStringsSep(" ", shortRefs()) + "\n";

src/libstore/nar-info.hh

@@ -2,10 +2,12 @@
 
 #include "types.hh"
 #include "hash.hh"
-#include "store-api.hh"
+#include "path-info.hh"
 
 namespace nix {
 
+class Store;
+
 struct NarInfo : ValidPathInfo
 {
     std::string url;
@@ -15,7 +17,7 @@ struct NarInfo : ValidPathInfo
     std::string system;
 
     NarInfo() = delete;
-    NarInfo(StorePath && path) : ValidPathInfo(std::move(path)) { }
+    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/optimise-store.cc

@@ -17,9 +17,7 @@ namespace nix {
 
 static void makeWritable(const Path & path)
 {
-    struct stat st;
-    if (lstat(path.c_str(), &st))
-        throw SysError("getting attributes of path '%1%'", path);
+    auto st = lstat(path);
     if (chmod(path.c_str(), st.st_mode | S_IWUSR) == -1)
         throw SysError("changing writability of '%1%'", path);
 }
@@ -94,9 +92,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
 {
     checkInterrupt();
 
-    struct stat st;
-    if (lstat(path.c_str(), &st))
-        throw SysError("getting attributes of path '%1%'", path);
+    auto st = lstat(path);
 
 #if __APPLE__
     /* HFS/macOS has some undocumented security feature disabling hardlinking for
@@ -187,9 +183,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
 
     /* Yes!  We've seen a file with the same contents.  Replace the
        current file with a hard link to that file. */
-    struct stat stLink;
-    if (lstat(linkPath.c_str(), &stLink))
-        throw SysError("getting attributes of path '%1%'", linkPath);
+    auto stLink = lstat(linkPath);
 
     if (st.st_ino == stLink.st_ino) {
         debug(format("'%1%' is already linked to '%2%'") % path % linkPath);

src/libstore/parsed-derivations.cc

@@ -94,7 +94,7 @@ StringSet ParsedDerivation::getRequiredSystemFeatures() const
     return res;
 }
 
-bool ParsedDerivation::canBuildLocally() const
+bool ParsedDerivation::canBuildLocally(Store & localStore) const
 {
     if (drv.platform != settings.thisSystem.get()
         && !settings.extraPlatforms.get().count(drv.platform)
@@ -102,14 +102,14 @@ bool ParsedDerivation::canBuildLocally() const
         return false;
 
     for (auto & feature : getRequiredSystemFeatures())
-        if (!settings.systemFeatures.get().count(feature)) return false;
+        if (!localStore.systemFeatures.get().count(feature)) return false;
 
     return true;
 }
 
-bool ParsedDerivation::willBuildLocally() const
+bool ParsedDerivation::willBuildLocally(Store & localStore) const
 {
-    return getBoolAttr("preferLocalBuild") && canBuildLocally();
+    return getBoolAttr("preferLocalBuild") && canBuildLocally(localStore);
 }
 
 bool ParsedDerivation::substitutesAllowed() const
@@ -117,9 +117,4 @@ bool ParsedDerivation::substitutesAllowed() const
     return getBoolAttr("allowSubstitutes", true);
 }
 
-bool ParsedDerivation::contentAddressed() const
-{
-    return getBoolAttr("__contentAddressed", false);
-}
-
 }

src/libstore/parsed-derivations.hh

@@ -29,13 +29,11 @@ public:
 
     StringSet getRequiredSystemFeatures() const;
 
-    bool canBuildLocally() const;
+    bool canBuildLocally(Store & localStore) const;
 
-    bool willBuildLocally() const;
+    bool willBuildLocally(Store & localStore) const;
 
     bool substitutesAllowed() const;
-
-    bool contentAddressed() const;
 };
 
 }

src/libstore/path-info.hh

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "crypto.hh"
 #include "path.hh"
 #include "hash.hh"
 #include "content-address.hh"
@@ -68,7 +69,7 @@ struct ValidPathInfo : PathReferences<StorePath>
     StorePath path;
     std::optional<StorePath> deriver;
     // TODO document this
-    std::optional<Hash> narHash;
+    Hash narHash;
     time_t registrationTime = 0;
     uint64_t narSize = 0; // 0 = unknown
     uint64_t id; // internal use only
@@ -139,8 +140,8 @@ struct ValidPathInfo : PathReferences<StorePath>
 
     ValidPathInfo(const ValidPathInfo & other) = default;
 
-    ValidPathInfo(StorePath && path) : path(std::move(path)) { };
-    ValidPathInfo(const StorePath & path) : path(path) { };
+    ValidPathInfo(StorePath && path, Hash narHash) : path(std::move(path)), narHash(narHash) { };
+    ValidPathInfo(const StorePath & path, Hash narHash) : path(path), narHash(narHash) { };
 
     virtual ~ValidPathInfo() { }
 };

src/libstore/profiles.cc

@@ -39,13 +39,10 @@ std::pair<Generations, std::optional<GenerationNumber>> findGenerations(Path pro
     for (auto & i : readDirectory(profileDir)) {
         if (auto n = parseName(profileName, i.name)) {
             auto path = profileDir + "/" + i.name;
-            struct stat st;
-            if (lstat(path.c_str(), &st) != 0)
-                throw SysError("statting '%1%'", path);
             gens.push_back({
                 .number = *n,
                 .path = path,
-                .creationTime = st.st_mtime
+                .creationTime = lstat(path).st_mtime
             });
         }
     }
@@ -72,7 +69,7 @@ static void makeName(const Path & profile, GenerationNumber num,
 }
 
 
-Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath)
+Path createGeneration(ref<LocalFSStore> store, Path profile, StorePath outPath)
 {
     /* The new generation number should be higher than old the
        previous ones. */
@@ -82,7 +79,7 @@ Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath)
     if (gens.size() > 0) {
         Generation last = gens.back();
 
-        if (readLink(last.path) == outPath) {
+        if (readLink(last.path) == store->printStorePath(outPath)) {
             /* We only create a new generation symlink if it differs
                from the last one.
 
@@ -105,7 +102,7 @@ Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath)
        user environment etc. we've just built. */
     Path generation;
     makeName(profile, num + 1, generation);
-    store->addPermRoot(store->parseStorePath(outPath), generation, false, true);
+    store->addPermRoot(outPath, generation);
 
     return generation;
 }

src/libstore/profiles.hh

@@ -8,6 +8,8 @@
 
 namespace nix {
 
+class StorePath;
+
 
 typedef unsigned int GenerationNumber;
 
@@ -28,7 +30,7 @@ std::pair<Generations, std::optional<GenerationNumber>> findGenerations(Path pro
 
 class LocalFSStore;
 
-Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath);
+Path createGeneration(ref<LocalFSStore> store, Path profile, StorePath outPath);
 
 void deleteGeneration(const Path & profile, GenerationNumber gen);
 

src/libstore/references.cc

@@ -79,9 +79,17 @@ void RefScanSink::operator () (const unsigned char * data, size_t len)
 std::pair<PathSet, HashResult> scanForReferences(const string & path,
     const PathSet & refs)
 {
-    RefScanSink refsSink;
     HashSink hashSink { htSHA256 };
-    TeeSink sink { refsSink, hashSink };
+    auto found = scanForReferences(hashSink, path, refs);
+    auto hash = hashSink.finish();
+    return std::pair<PathSet, HashResult>(found, hash);
+}
+
+PathSet scanForReferences(Sink & toTee,
+    const string & path, const PathSet & refs)
+{
+    RefScanSink refsSink;
+    TeeSink sink { refsSink, toTee };
     std::map<string, Path> backMap;
 
     /* For efficiency (and a higher hit rate), just search for the
@@ -111,9 +119,7 @@ std::pair<PathSet, HashResult> scanForReferences(const string & path,
         found.insert(j->second);
     }
 
-    auto hash = hashSink.finish();
-
-    return std::pair<PathSet, HashResult>(found, hash);
+    return found;
 }
 
 

src/libstore/references.hh

@@ -7,6 +7,8 @@ namespace nix {
 
 std::pair<PathSet, HashResult> scanForReferences(const Path & path, const PathSet & refs);
 
+PathSet scanForReferences(Sink & toTee, const Path & path, const PathSet & refs);
+
 struct RewritingSink : Sink
 {
     std::string from, to, prev;

src/libstore/remote-store.cc

@@ -1,5 +1,6 @@
 #include "serialise.hh"
 #include "util.hh"
+#include "remote-fs-accessor.hh"
 #include "remote-store.hh"
 #include "worker-protocol.hh"
 #include "archive.hh"
@@ -9,6 +10,7 @@
 #include "pool.hh"
 #include "finally.hh"
 #include "logging.hh"
+#include "callback.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -31,7 +33,6 @@ template<> StorePathSet readStorePaths(const Store & store, Source & from)
     return paths;
 }
 
-
 void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths)
 {
     out << paths.size();
@@ -39,12 +40,16 @@ void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths
         out << store.printStorePath(i);
 }
 
+
 StorePathCAMap readStorePathCAMap(const Store & store, Source & from)
 {
     StorePathCAMap paths;
     auto count = readNum<size_t>(from);
-    while (count--)
-        paths.insert_or_assign(store.parseStorePath(readString(from)), parseContentAddressOpt(readString(from)));
+    while (count--) {
+        auto path = store.parseStorePath(readString(from));
+        auto ca = parseContentAddressOpt(readString(from));
+        paths.insert_or_assign(path, ca);
+    }
     return paths;
 }
 
@@ -57,36 +62,52 @@ void writeStorePathCAMap(const Store & store, Sink & out, const StorePathCAMap &
     }
 }
 
-std::map<string, StorePath> readOutputPathMap(const Store & store, Source & from)
+
+namespace worker_proto {
+
+StorePath read(const Store & store, Source & from, Phantom<StorePath> _)
 {
-    std::map<string, StorePath> pathMap;
-    auto rawInput = readStrings<Strings>(from);
-    if (rawInput.size() % 2)
-        throw Error("got an odd number of elements from the daemon when trying to read a output path map");
-    auto curInput = rawInput.begin();
-    while (curInput != rawInput.end()) {
-        auto thisKey = *curInput++;
-        auto thisValue = *curInput++;
-        pathMap.emplace(thisKey, store.parseStorePath(thisValue));
-    }
-    return pathMap;
+    return store.parseStorePath(readString(from));
 }
 
-void writeOutputPathMap(const Store & store, Sink & out, const std::map<string, StorePath> & pathMap)
+void write(const Store & store, Sink & out, const StorePath & storePath)
 {
-    out << 2*pathMap.size();
-    for (auto & i : pathMap) {
-        out << i.first;
-        out << store.printStorePath(i.second);
-    }
+    out << store.printStorePath(storePath);
 }
 
+
+template<>
+std::optional<StorePath> read(const Store & store, Source & from, Phantom<std::optional<StorePath>> _)
+{
+    auto s = readString(from);
+    return s == "" ? std::optional<StorePath> {} : store.parseStorePath(s);
+}
+
+template<>
+void write(const Store & store, Sink & out, const std::optional<StorePath> & storePathOpt)
+{
+    out << (storePathOpt ? store.printStorePath(*storePathOpt) : "");
+}
+
+}
+
+
 /* TODO: Separate these store impls into different files, give them better names */
 RemoteStore::RemoteStore(const Params & params)
     : Store(params)
+    , RemoteStoreConfig(params)
     , connections(make_ref<Pool<Connection>>(
             std::max(1, (int) maxConnections),
-            [this]() { return openConnectionWrapper(); },
+            [this]() {
+                auto conn = openConnectionWrapper();
+                try {
+                    initConnection(*conn);
+                } catch (...) {
+                    failed = true;
+                    throw;
+                }
+                return conn;
+            },
             [this](const ref<Connection> & r) {
                 return
                     r->to.good()
@@ -113,19 +134,21 @@ ref<RemoteStore::Connection> RemoteStore::openConnectionWrapper()
 
 
 UDSRemoteStore::UDSRemoteStore(const Params & params)
-    : Store(params)
+    : StoreConfig(params)
+    , Store(params)
     , LocalFSStore(params)
     , RemoteStore(params)
 {
 }
 
 
-UDSRemoteStore::UDSRemoteStore(std::string socket_path, const Params & params)
-    : Store(params)
-    , LocalFSStore(params)
-    , RemoteStore(params)
-    , path(socket_path)
+UDSRemoteStore::UDSRemoteStore(
+        const std::string scheme,
+        std::string socket_path,
+        const Params & params)
+    : UDSRemoteStore(params)
 {
+    path.emplace(socket_path);
 }
 
 
@@ -169,8 +192,6 @@ ref<RemoteStore::Connection> UDSRemoteStore::openConnection()
 
     conn->startTime = std::chrono::steady_clock::now();
 
-    initConnection(*conn);
-
     return conn;
 }
 
@@ -278,14 +299,16 @@ struct ConnectionHandle
 
     RemoteStore::Connection * operator -> () { return &*handle; }
 
-    void processStderr(Sink * sink = 0, Source * source = 0)
+    void processStderr(Sink * sink = 0, Source * source = 0, bool flush = true)
     {
-        auto ex = handle->processStderr(sink, source);
+        auto ex = handle->processStderr(sink, source, flush);
         if (ex) {
             daemonException = true;
             std::rethrow_exception(ex);
         }
     }
+
+    void withFramedSink(std::function<void(Sink & sink)> fun);
 };
 
 
@@ -400,11 +423,28 @@ void RemoteStore::querySubstitutablePathInfos(const StorePathCAMap & pathsMap, S
 }
 
 
+ref<const ValidPathInfo> RemoteStore::readValidPathInfo(ConnectionHandle & conn, const StorePath & path)
+{
+    auto deriver = readString(conn->from);
+    auto narHash = Hash::parseAny(readString(conn->from), htSHA256);
+    auto info = make_ref<ValidPathInfo>(path, narHash);
+    if (deriver != "") info->deriver = parseStorePath(deriver);
+    info->setReferencesPossiblyToSelf(readStorePaths<StorePathSet>(*this, conn->from));
+    conn->from >> info->registrationTime >> info->narSize;
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 16) {
+        conn->from >> info->ultimate;
+        info->sigs = readStrings<StringSet>(conn->from);
+        info->ca = parseContentAddressOpt(readString(conn->from));
+    }
+    return info;
+}
+
+
 void RemoteStore::queryPathInfoUncached(const StorePath & path,
     Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept
 {
     try {
-        std::shared_ptr<ValidPathInfo> info;
+        std::shared_ptr<const ValidPathInfo> info;
         {
             auto conn(getConnection());
             conn->to << wopQueryPathInfo << printStorePath(path);
@@ -420,17 +460,7 @@ void RemoteStore::queryPathInfoUncached(const StorePath & path,
                 bool valid; conn->from >> valid;
                 if (!valid) throw InvalidPath("path '%s' is not valid", printStorePath(path));
             }
-            info = std::make_shared<ValidPathInfo>(StorePath(path));
-            auto deriver = readString(conn->from);
-            if (deriver != "") info->deriver = parseStorePath(deriver);
-            info->narHash = Hash(readString(conn->from), htSHA256);
-            info->setReferencesPossiblyToSelf(readStorePaths<StorePathSet>(*this, conn->from));
-            conn->from >> info->registrationTime >> info->narSize;
-            if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 16) {
-                conn->from >> info->ultimate;
-                info->sigs = readStrings<StringSet>(conn->from);
-                info->ca = parseContentAddressOpt(readString(conn->from));
-            }
+            info = readValidPathInfo(conn, path);
         }
         callback(std::move(info));
     } catch (...) { callback.rethrow(); }
@@ -469,12 +499,28 @@ StorePathSet RemoteStore::queryDerivationOutputs(const StorePath & path)
 }
 
 
-OutputPathMap RemoteStore::queryDerivationOutputMap(const StorePath & path)
+std::map<std::string, std::optional<StorePath>> RemoteStore::queryPartialDerivationOutputMap(const StorePath & path)
 {
-    auto conn(getConnection());
-    conn->to << wopQueryDerivationOutputMap << printStorePath(path);
-    conn.processStderr();
-    return readOutputPathMap(*this, conn->from);
+    if (GET_PROTOCOL_MINOR(getProtocol()) >= 0x16) {
+        auto conn(getConnection());
+        conn->to << wopQueryDerivationOutputMap << printStorePath(path);
+        conn.processStderr();
+        return worker_proto::read(*this, conn->from, Phantom<std::map<std::string, std::optional<StorePath>>> {});
+    } else {
+        // Fallback for old daemon versions.
+        // For floating-CA derivations (and their co-dependencies) this is an
+        // under-approximation as it only returns the paths that can be inferred
+        // from the derivation itself (and not the ones that are known because
+        // the have been built), but as old stores don't handle floating-CA
+        // derivations this shouldn't matter
+        auto derivation = readDerivation(path);
+        auto outputsWithOptPaths = derivation.outputsAndOptPaths(*this);
+        std::map<std::string, std::optional<StorePath>> ret;
+        for (auto & [outputName, outputAndPath] : outputsWithOptPaths) {
+            ret.emplace(outputName, outputAndPath.second);
+        }
+        return ret;
+    }
 
 }
 
@@ -489,6 +535,93 @@ std::optional<StorePath> RemoteStore::queryPathFromHashPart(const std::string &
 }
 
 
+ref<const ValidPathInfo> RemoteStore::addCAToStore(
+    Source & dump,
+    const string & name,
+    ContentAddressMethod caMethod,
+    const StorePathSet & references,
+    RepairFlag repair)
+{
+    std::optional<ConnectionHandle> conn_(getConnection());
+    auto & conn = *conn_;
+
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 25) {
+
+        conn->to
+            << wopAddToStore
+            << name
+            << renderContentAddressMethod(caMethod);
+        writeStorePaths(*this, conn->to, references);
+        conn->to << repair;
+
+        conn.withFramedSink([&](Sink & sink) {
+            dump.drainInto(sink);
+        });
+
+        auto path = parseStorePath(readString(conn->from));
+        return readValidPathInfo(conn, path);
+    }
+    else {
+        if (repair) throw Error("repairing is not supported when building through the Nix daemon protocol < 1.25");
+
+        std::visit(overloaded {
+            [&](TextHashMethod thm) -> void {
+                std::string s = dump.drain();
+                conn->to << wopAddTextToStore << name << s;
+                writeStorePaths(*this, conn->to, references);
+                conn.processStderr();
+            },
+            [&](FixedOutputHashMethod fohm) -> void {
+                conn->to
+                    << wopAddToStore
+                    << name
+                    << ((fohm.hashType == htSHA256 && fohm.fileIngestionMethod == FileIngestionMethod::Recursive) ? 0 : 1) /* backwards compatibility hack */
+                    << (fohm.fileIngestionMethod == FileIngestionMethod::Recursive ? 1 : 0)
+                    << printHashType(fohm.hashType);
+
+                try {
+                    conn->to.written = 0;
+                    conn->to.warn = true;
+                    connections->incCapacity();
+                    {
+                        Finally cleanup([&]() { connections->decCapacity(); });
+                        if (fohm.fileIngestionMethod == FileIngestionMethod::Recursive) {
+                            dump.drainInto(conn->to);
+                        } else {
+                            std::string contents = dump.drain();
+                            dumpString(contents, conn->to);
+                        }
+                    }
+                    conn->to.warn = false;
+                    conn.processStderr();
+                } catch (SysError & e) {
+                    /* Daemon closed while we were sending the path. Probably OOM
+                      or I/O error. */
+                    if (e.errNo == EPIPE)
+                        try {
+                            conn.processStderr();
+                        } catch (EndOfFile & e) { }
+                    throw;
+                }
+
+            }
+        }, caMethod);
+        auto path = parseStorePath(readString(conn->from));
+        // Release our connection to prevent a deadlock in queryPathInfo().
+        conn_.reset();
+        return queryPathInfo(path);
+    }
+}
+
+
+StorePath RemoteStore::addToStoreFromDump(Source & dump, const string & name,
+        FileIngestionMethod method, HashType hashType, RepairFlag repair)
+{
+    StorePathSet references;
+    return addCAToStore(dump, name, FixedOutputHashMethod{ .fileIngestionMethod = method, .hashType = hashType }, references, repair)->path;
+}
+
+
 void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
     RepairFlag repair, CheckSigsFlag checkSigs)
 {
@@ -522,83 +655,16 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
         conn->to << wopAddToStoreNar
                  << printStorePath(info.path)
                  << (info.deriver ? printStorePath(*info.deriver) : "")
-                 << info.narHash->to_string(Base16, false);
+                 << info.narHash.to_string(Base16, false);
         writeStorePaths(*this, conn->to, info.referencesPossiblyToSelf());
         conn->to << info.registrationTime << info.narSize
                  << info.ultimate << info.sigs << renderContentAddress(info.ca)
                  << repair << !checkSigs;
 
         if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 23) {
-
-            std::exception_ptr ex;
-
-            struct FramedSink : BufferedSink
-            {
-                ConnectionHandle & conn;
-                std::exception_ptr & ex;
-
-                FramedSink(ConnectionHandle & conn, std::exception_ptr & ex) : conn(conn), ex(ex)
-                { }
-
-                ~FramedSink()
-                {
-                    try {
-                        conn->to << 0;
-                        conn->to.flush();
-                    } catch (...) {
-                        ignoreException();
-                    }
-                }
-
-                void write(const unsigned char * data, size_t len) override
-                {
-                    /* Don't send more data if the remote has
-                       encountered an error. */
-                    if (ex) {
-                        auto ex2 = ex;
-                        ex = nullptr;
-                        std::rethrow_exception(ex2);
-                    }
-                    conn->to << len;
-                    conn->to(data, len);
-                };
-            };
-
-            /* Handle log messages / exceptions from the remote on a
-               separate thread. */
-            std::thread stderrThread([&]()
-            {
-                try {
-                    conn.processStderr();
-                } catch (...) {
-                    ex = std::current_exception();
-                }
-            });
-
-            Finally joinStderrThread([&]()
-            {
-                if (stderrThread.joinable()) {
-                    stderrThread.join();
-                    if (ex) {
-                        try {
-                            std::rethrow_exception(ex);
-                        } catch (...) {
-                            ignoreException();
-                        }
-                    }
-                }
-            });
-
-            {
-                FramedSink sink(conn, ex);
+            conn.withFramedSink([&](Sink & sink) {
                 copyNAR(source, sink);
-                sink.flush();
-            }
-
-            stderrThread.join();
-            if (ex)
-                std::rethrow_exception(ex);
-
+            });
         } else if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21) {
             conn.processStderr(0, &source);
         } else {
@@ -609,57 +675,11 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
 }
 
 
-StorePath RemoteStore::addToStore(const string & name, const Path & _srcPath,
-    FileIngestionMethod method, HashType hashAlgo, PathFilter & filter, RepairFlag repair)
-{
-    if (repair) throw Error("repairing is not supported when building through the Nix daemon");
-
-    auto conn(getConnection());
-
-    Path srcPath(absPath(_srcPath));
-
-    conn->to
-        << wopAddToStore
-        << name
-        << ((hashAlgo == htSHA256 && method == FileIngestionMethod::Recursive) ? 0 : 1) /* backwards compatibility hack */
-        << (method == FileIngestionMethod::Recursive ? 1 : 0)
-        << printHashType(hashAlgo);
-
-    try {
-        conn->to.written = 0;
-        conn->to.warn = true;
-        connections->incCapacity();
-        {
-            Finally cleanup([&]() { connections->decCapacity(); });
-            dumpPath(srcPath, conn->to, filter);
-        }
-        conn->to.warn = false;
-        conn.processStderr();
-    } catch (SysError & e) {
-        /* Daemon closed while we were sending the path. Probably OOM
-           or I/O error. */
-        if (e.errNo == EPIPE)
-            try {
-                conn.processStderr();
-            } catch (EndOfFile & e) { }
-        throw;
-    }
-
-    return parseStorePath(readString(conn->from));
-}
-
-
 StorePath RemoteStore::addTextToStore(const string & name, const string & s,
     const StorePathSet & references, RepairFlag repair)
 {
-    if (repair) throw Error("repairing is not supported when building through the Nix daemon");
-
-    auto conn(getConnection());
-    conn->to << wopAddTextToStore << name << s;
-    writeStorePaths(*this, conn->to, references);
-
-    conn.processStderr();
-    return parseStorePath(readString(conn->from));
+    StringSource source(s);
+    return addCAToStore(source, name, TextHashMethod{}, references, repair)->path;
 }
 
 
@@ -861,6 +881,18 @@ RemoteStore::Connection::~Connection()
     }
 }
 
+void RemoteStore::narFromPath(const StorePath & path, Sink & sink)
+{
+    auto conn(connections->get());
+    conn->to << wopNarFromPath << printStorePath(path);
+    conn->processStderr();
+    copyNAR(conn->from, sink);
+}
+
+ref<FSAccessor> RemoteStore::getFSAccessor()
+{
+    return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()));
+}
 
 static Logger::Fields readFields(Source & from)
 {
@@ -879,9 +911,10 @@ static Logger::Fields readFields(Source & from)
 }
 
 
-std::exception_ptr RemoteStore::Connection::processStderr(Sink * sink, Source * source)
+std::exception_ptr RemoteStore::Connection::processStderr(Sink * sink, Source * source, bool flush)
 {
-    to.flush();
+    if (flush)
+        to.flush();
 
     while (true) {
 
@@ -942,14 +975,49 @@ std::exception_ptr RemoteStore::Connection::processStderr(Sink * sink, Source *
     return nullptr;
 }
 
-static std::string uriScheme = "unix://";
-
-static RegisterStoreImplementation regStore([](
-    const std::string & uri, const Store::Params & params)
-    -> std::shared_ptr<Store>
+void ConnectionHandle::withFramedSink(std::function<void(Sink &sink)> fun)
 {
-    if (std::string(uri, 0, uriScheme.size()) != uriScheme) return 0;
-    return std::make_shared<UDSRemoteStore>(std::string(uri, uriScheme.size()), params);
-});
+    (*this)->to.flush();
+
+    std::exception_ptr ex;
+
+    /* Handle log messages / exceptions from the remote on a
+        separate thread. */
+    std::thread stderrThread([&]()
+    {
+        try {
+            processStderr(nullptr, nullptr, false);
+        } catch (...) {
+            ex = std::current_exception();
+        }
+    });
+
+    Finally joinStderrThread([&]()
+    {
+        if (stderrThread.joinable()) {
+            stderrThread.join();
+            if (ex) {
+                try {
+                    std::rethrow_exception(ex);
+                } catch (...) {
+                    ignoreException();
+                }
+            }
+        }
+    });
+
+    {
+        FramedSink sink((*this)->to, ex);
+        fun(sink);
+        sink.flush();
+    }
+
+    stderrThread.join();
+    if (ex)
+        std::rethrow_exception(ex);
+
+}
+
+static RegisterStoreImplementation<UDSRemoteStore, UDSRemoteStoreConfig> regStore;
 
 }

src/libstore/remote-store.hh

@@ -16,19 +16,23 @@ struct FdSource;
 template<typename T> class Pool;
 struct ConnectionHandle;
 
+struct RemoteStoreConfig : virtual StoreConfig
+{
+    using StoreConfig::StoreConfig;
+
+    const Setting<int> maxConnections{(StoreConfig*) this, 1,
+            "max-connections", "maximum number of concurrent connections to the Nix daemon"};
+
+    const Setting<unsigned int> maxConnectionAge{(StoreConfig*) this, std::numeric_limits<unsigned int>::max(),
+            "max-connection-age", "number of seconds to reuse a connection"};
+};
 
 /* FIXME: RemoteStore is a misnomer - should be something like
    DaemonStore. */
-class RemoteStore : public virtual Store
+class RemoteStore : public virtual Store, public virtual RemoteStoreConfig
 {
 public:
 
-    const Setting<int> maxConnections{(Store*) this, 1,
-            "max-connections", "maximum number of concurrent connections to the Nix daemon"};
-
-    const Setting<unsigned int> maxConnectionAge{(Store*) this, std::numeric_limits<unsigned int>::max(),
-            "max-connection-age", "number of seconds to reuse a connection"};
-
     virtual bool sameMachine() = 0;
 
     RemoteStore(const Params & params);
@@ -51,7 +55,7 @@ public:
 
     StorePathSet queryDerivationOutputs(const StorePath & path) override;
 
-    OutputPathMap queryDerivationOutputMap(const StorePath & path) override;
+    std::map<std::string, std::optional<StorePath>> queryPartialDerivationOutputMap(const StorePath & path) override;
     std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override;
 
     StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
@@ -59,13 +63,21 @@ public:
     void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) override;
 
+    /* Add a content-addressable store path. `dump` will be drained. */
+    ref<const ValidPathInfo> addCAToStore(
+        Source & dump,
+        const string & name,
+        ContentAddressMethod caMethod,
+        const StorePathSet & references,
+        RepairFlag repair);
+
+    /* Add a content-addressable store path. Does not support references. `dump` will be drained. */
+    StorePath addToStoreFromDump(Source & dump, const string & name,
+        FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair) override;
+
     void addToStore(const ValidPathInfo & info, Source & nar,
         RepairFlag repair, CheckSigsFlag checkSigs) override;
 
-    StorePath addToStore(const string & name, const Path & srcPath,
-        FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256,
-        PathFilter & filter = defaultPathFilter, RepairFlag repair = NoRepair) override;
-
     StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair) override;
 
@@ -102,8 +114,6 @@ public:
 
     void flushBadConnections();
 
-protected:
-
     struct Connection
     {
         AutoCloseFD fd;
@@ -114,11 +124,13 @@ protected:
 
         virtual ~Connection();
 
-        std::exception_ptr processStderr(Sink * sink = 0, Source * source = 0);
+        std::exception_ptr processStderr(Sink * sink = 0, Source * source = 0, bool flush = true);
     };
 
     ref<Connection> openConnectionWrapper();
 
+protected:
+
     virtual ref<Connection> openConnection() = 0;
 
     void initConnection(Connection & conn);
@@ -131,24 +143,56 @@ protected:
 
     friend struct ConnectionHandle;
 
+    virtual ref<FSAccessor> getFSAccessor() override;
+
+    virtual void narFromPath(const StorePath & path, Sink & sink) override;
+
+    ref<const ValidPathInfo> readValidPathInfo(ConnectionHandle & conn, const StorePath & path);
+
 private:
 
     std::atomic_bool failed{false};
 
 };
 
-class UDSRemoteStore : public LocalFSStore, public RemoteStore
+struct UDSRemoteStoreConfig : virtual LocalFSStoreConfig, virtual RemoteStoreConfig
+{
+    UDSRemoteStoreConfig(const Store::Params & params)
+        : StoreConfig(params)
+        , LocalFSStoreConfig(params)
+        , RemoteStoreConfig(params)
+    {
+    }
+
+    UDSRemoteStoreConfig()
+        : UDSRemoteStoreConfig(Store::Params({}))
+    {
+    }
+
+    const std::string name() override { return "Local Daemon Store"; }
+};
+
+class UDSRemoteStore : public LocalFSStore, public RemoteStore, public virtual UDSRemoteStoreConfig
 {
 public:
 
     UDSRemoteStore(const Params & params);
-    UDSRemoteStore(std::string path, const Params & params);
+    UDSRemoteStore(const std::string scheme, std::string path, const Params & params);
 
     std::string getUri() override;
 
+    static std::set<std::string> uriSchemes()
+    { return {"unix"}; }
+
     bool sameMachine() override
     { return true; }
 
+    ref<FSAccessor> getFSAccessor() override
+    { return LocalFSStore::getFSAccessor(); }
+
+    void narFromPath(const StorePath & path, Sink & sink) override
+    { LocalFSStore::narFromPath(path, sink); }
+
 private:
 
     ref<RemoteStore::Connection> openConnection() override;

src/libstore/s3-binary-cache-store.cc

@@ -172,20 +172,26 @@ S3Helper::FileTransferResult S3Helper::getObject(
     return res;
 }
 
-struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
+struct S3BinaryCacheStoreConfig : virtual BinaryCacheStoreConfig
 {
-    const Setting<std::string> profile{this, "", "profile", "The name of the AWS configuration profile to use."};
-    const Setting<std::string> region{this, Aws::Region::US_EAST_1, "region", {"aws-region"}};
-    const Setting<std::string> scheme{this, "", "scheme", "The scheme to use for S3 requests, https by default."};
-    const Setting<std::string> endpoint{this, "", "endpoint", "An optional override of the endpoint to use when talking to S3."};
-    const Setting<std::string> narinfoCompression{this, "", "narinfo-compression", "compression method for .narinfo files"};
-    const Setting<std::string> lsCompression{this, "", "ls-compression", "compression method for .ls files"};
-    const Setting<std::string> logCompression{this, "", "log-compression", "compression method for log/* files"};
+    using BinaryCacheStoreConfig::BinaryCacheStoreConfig;
+    const Setting<std::string> profile{(StoreConfig*) this, "", "profile", "The name of the AWS configuration profile to use."};
+    const Setting<std::string> region{(StoreConfig*) this, Aws::Region::US_EAST_1, "region", {"aws-region"}};
+    const Setting<std::string> scheme{(StoreConfig*) this, "", "scheme", "The scheme to use for S3 requests, https by default."};
+    const Setting<std::string> endpoint{(StoreConfig*) this, "", "endpoint", "An optional override of the endpoint to use when talking to S3."};
+    const Setting<std::string> narinfoCompression{(StoreConfig*) this, "", "narinfo-compression", "compression method for .narinfo files"};
+    const Setting<std::string> lsCompression{(StoreConfig*) this, "", "ls-compression", "compression method for .ls files"};
+    const Setting<std::string> logCompression{(StoreConfig*) this, "", "log-compression", "compression method for log/* files"};
     const Setting<bool> multipartUpload{
-        this, false, "multipart-upload", "whether to use multi-part uploads"};
+        (StoreConfig*) this, false, "multipart-upload", "whether to use multi-part uploads"};
     const Setting<uint64_t> bufferSize{
-        this, 5 * 1024 * 1024, "buffer-size", "size (in bytes) of each part in multi-part uploads"};
+        (StoreConfig*) this, 5 * 1024 * 1024, "buffer-size", "size (in bytes) of each part in multi-part uploads"};
 
+    const std::string name() override { return "S3 Binary Cache Store"; }
+};
+
+struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore, virtual S3BinaryCacheStoreConfig
+{
     std::string bucketName;
 
     Stats stats;
@@ -193,8 +199,11 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
     S3Helper s3Helper;
 
     S3BinaryCacheStoreImpl(
-        const Params & params, const std::string & bucketName)
-        : S3BinaryCacheStore(params)
+        const std::string & scheme,
+        const std::string & bucketName,
+        const Params & params)
+        : StoreConfig(params)
+        , S3BinaryCacheStore(params)
         , bucketName(bucketName)
         , s3Helper(profile, region, scheme, endpoint)
     {
@@ -426,17 +435,11 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
         return paths;
     }
 
+    static std::set<std::string> uriSchemes() { return {"s3"}; }
+
 };
 
-static RegisterStoreImplementation regStore([](
-    const std::string & uri, const Store::Params & params)
-    -> std::shared_ptr<Store>
-{
-    if (std::string(uri, 0, 5) != "s3://") return 0;
-    auto store = std::make_shared<S3BinaryCacheStoreImpl>(params, std::string(uri, 5));
-    store->init();
-    return store;
-});
+static RegisterStoreImplementation<S3BinaryCacheStoreImpl, S3BinaryCacheStoreConfig> regStore;
 
 }
 

src/libstore/ssh-store.cc

@@ -8,19 +8,25 @@
 
 namespace nix {
 
-static std::string uriScheme = "ssh-ng://";
+struct SSHStoreConfig : virtual RemoteStoreConfig
+{
+    using RemoteStoreConfig::RemoteStoreConfig;
 
-class SSHStore : public RemoteStore
+    const Setting<Path> sshKey{(StoreConfig*) this, "", "ssh-key", "path to an SSH private key"};
+    const Setting<bool> compress{(StoreConfig*) this, false, "compress", "whether to compress the connection"};
+    const Setting<Path> remoteProgram{(StoreConfig*) this, "nix-daemon", "remote-program", "path to the nix-daemon executable on the remote system"};
+    const Setting<std::string> remoteStore{(StoreConfig*) this, "", "remote-store", "URI of the store on the remote system"};
+
+    const std::string name() override { return "SSH Store"; }
+};
+
+class SSHStore : public virtual RemoteStore, public virtual SSHStoreConfig
 {
 public:
 
-    const Setting<Path> sshKey{(Store*) this, "", "ssh-key", "path to an SSH private key"};
-    const Setting<bool> compress{(Store*) this, false, "compress", "whether to compress the connection"};
-    const Setting<Path> remoteProgram{(Store*) this, "nix-daemon", "remote-program", "path to the nix-daemon executable on the remote system"};
-    const Setting<std::string> remoteStore{(Store*) this, "", "remote-store", "URI of the store on the remote system"};
-
-    SSHStore(const std::string & host, const Params & params)
-        : Store(params)
+    SSHStore(const std::string & scheme, const std::string & host, const Params & params)
+        : StoreConfig(params)
+        , Store(params)
         , RemoteStore(params)
         , host(host)
         , master(
@@ -32,18 +38,16 @@ public:
     {
     }
 
+    static std::set<std::string> uriSchemes() { return {"ssh-ng"}; }
+
     std::string getUri() override
     {
-        return uriScheme + host;
+        return *uriSchemes().begin() + "://" + host;
     }
 
     bool sameMachine() override
     { return false; }
 
-    void narFromPath(const StorePath & path, Sink & sink) override;
-
-    ref<FSAccessor> getFSAccessor() override;
-
 private:
 
     struct Connection : RemoteStore::Connection
@@ -68,19 +72,6 @@ private:
     };
 };
 
-void SSHStore::narFromPath(const StorePath & path, Sink & sink)
-{
-    auto conn(connections->get());
-    conn->to << wopNarFromPath << printStorePath(path);
-    conn->processStderr();
-    copyNAR(conn->from, sink);
-}
-
-ref<FSAccessor> SSHStore::getFSAccessor()
-{
-    return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()));
-}
-
 ref<RemoteStore::Connection> SSHStore::openConnection()
 {
     auto conn = make_ref<Connection>();
@@ -89,16 +80,9 @@ ref<RemoteStore::Connection> SSHStore::openConnection()
         + (remoteStore.get() == "" ? "" : " --store " + shellEscape(remoteStore.get())));
     conn->to = FdSink(conn->sshConn->in.get());
     conn->from = FdSource(conn->sshConn->out.get());
-    initConnection(*conn);
     return conn;
 }
 
-static RegisterStoreImplementation regStore([](
-    const std::string & uri, const Store::Params & params)
-    -> std::shared_ptr<Store>
-{
-    if (std::string(uri, 0, uriScheme.size()) != uriScheme) return 0;
-    return std::make_shared<SSHStore>(std::string(uri, uriScheme.size()), params);
-});
+static RegisterStoreImplementation<SSHStore, SSHStoreConfig> regStore;
 
 }

src/libstore/store-api.cc

@@ -1,16 +1,14 @@
 #include "crypto.hh"
+#include "fs-accessor.hh"
 #include "globals.hh"
 #include "store-api.hh"
 #include "util.hh"
 #include "nar-info-disk-cache.hh"
 #include "thread-pool.hh"
 #include "json.hh"
-#include "derivations.hh"
 #include "url.hh"
 #include "archive.hh"
-
-#include <future>
-
+#include "callback.hh"
 
 namespace nix {
 
@@ -140,21 +138,28 @@ StorePathWithOutputs Store::followLinksToStorePathWithOutputs(std::string_view p
 */
 
 
-StorePath Store::makeStorePath(const string & type,
-    const Hash & hash, std::string_view name) const
+StorePath Store::makeStorePath(std::string_view type,
+    std::string_view hash, std::string_view name) const
 {
     /* e.g., "source:sha256:1abc...:/nix/store:foo.tar.gz" */
-    string s = type + ":" + hash.to_string(Base16, true) + ":" + storeDir + ":" + std::string(name);
+    string s = std::string { type } + ":" + std::string { hash }
+        + ":" + storeDir + ":" + std::string { name };
     auto h = compressHash(hashString(htSHA256, s), 20);
     return StorePath(h, name);
 }
 
 
-StorePath Store::makeOutputPath(const string & id,
+StorePath Store::makeStorePath(std::string_view type,
     const Hash & hash, std::string_view name) const
 {
-    return makeStorePath("output:" + id, hash,
-        std::string(name) + (id == "out" ? "" : "-" + id));
+    return makeStorePath(type, hash.to_string(Base16, true), name);
+}
+
+
+StorePath Store::makeOutputPath(std::string_view id,
+    const Hash & hash, std::string_view name) const
+{
+    return makeStorePath("output:" + std::string { id }, hash, outputPathName(name, id));
 }
 
 
@@ -193,10 +198,6 @@ StorePath Store::makeFixedOutputPath(
     }
 }
 
-// FIXME Put this somewhere?
-template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
-template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
-
 StorePath Store::makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
     const StorePathSet & references, bool hasSelfReference) const
 {
@@ -324,8 +325,10 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
     if (expectedCAHash && expectedCAHash != hash)
         throw Error("hash mismatch for '%s'", srcPath);
 
-    ValidPathInfo info(makeFixedOutputPath(method, hash, name));
-    info.narHash = narHash;
+    ValidPathInfo info {
+        makeFixedOutputPath(method, hash, name),
+        narHash,
+    };
     info.narSize = narSize;
     info.ca = FixedOutputHash { .method = method, .hash = hash };
 
@@ -341,7 +344,7 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
 
 
 Store::Store(const Params & params)
-    : Config(params)
+    : StoreConfig(params)
     , state({(size_t) pathInfoCacheSize})
 {
 }
@@ -361,6 +364,17 @@ bool Store::PathInfoCacheValue::isKnownNow()
     return std::chrono::steady_clock::now() < time_point + ttl;
 }
 
+OutputPathMap Store::queryDerivationOutputMap(const StorePath & path) {
+    auto resp = queryPartialDerivationOutputMap(path);
+    OutputPathMap result;
+    for (auto & [outName, optOutPath] : resp) {
+        if (!optOutPath)
+            throw Error("output '%s' has no store path mapped to it", outName);
+        result.insert_or_assign(outName, *optOutPath);
+    }
+    return result;
+}
+
 StorePathSet Store::queryDerivationOutputs(const StorePath & path)
 {
     auto outputMap = this->queryDerivationOutputMap(path);
@@ -569,7 +583,7 @@ string Store::makeValidityRegistration(const StorePathSet & paths,
         auto info = queryPathInfo(i);
 
         if (showHash) {
-            s += info->narHash->to_string(Base16, false) + "\n";
+            s += info->narHash.to_string(Base16, false) + "\n";
             s += (format("%1%\n") % info->narSize).str();
         }
 
@@ -601,7 +615,7 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & store
             auto info = queryPathInfo(storePath);
 
             jsonPath
-                .attr("narHash", info->narHash->to_string(hashBase, true))
+                .attr("narHash", info->narHash.to_string(hashBase, true))
                 .attr("narSize", info->narSize);
 
             {
@@ -729,20 +743,6 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
         info = info2;
     }
 
-    if (!info->narHash) {
-        StringSink sink;
-        srcStore->narFromPath({storePath}, sink);
-        auto info2 = make_ref<ValidPathInfo>(*info);
-        info2->narHash = hashString(htSHA256, *sink.s);
-        if (!info->narSize) info2->narSize = sink.s->size();
-        if (info->ultimate) info2->ultimate = false;
-        info = info2;
-
-        StringSource source(*sink.s);
-        dstStore->addToStore(*info, source, repair, checkSigs);
-        return;
-    }
-
     if (info->ultimate) {
         auto info2 = make_ref<ValidPathInfo>(*info);
         info2->ultimate = false;
@@ -750,12 +750,12 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
     }
 
     auto source = sinkToSource([&](Sink & sink) {
-        LambdaSink wrapperSink([&](const unsigned char * data, size_t len) {
-            sink(data, len);
+        LambdaSink progressSink([&](const unsigned char * data, size_t len) {
             total += len;
             act.progress(total, info->narSize);
         });
-        srcStore->narFromPath(storePath, wrapperSink);
+        TeeSink tee { sink, progressSink };
+        srcStore->narFromPath(storePath, tee);
     }, [&]() {
            throw EndOfFile("NAR for '%s' fetched from '%s' is incomplete", srcStore->printStorePath(storePath), srcStore->getUri());
     });
@@ -867,19 +867,22 @@ void copyClosure(ref<Store> srcStore, ref<Store> dstStore,
 }
 
 
-std::optional<ValidPathInfo> decodeValidPathInfo(const Store & store, std::istream & str, bool hashGiven)
+std::optional<ValidPathInfo> decodeValidPathInfo(const Store & store, std::istream & str, std::optional<HashResult> hashGiven)
 {
     std::string path;
     getline(str, path);
     if (str.eof()) { return {}; }
-    ValidPathInfo info(store.parseStorePath(path));
-    if (hashGiven) {
+    if (!hashGiven) {
         string s;
         getline(str, s);
-        info.narHash = Hash(s, htSHA256);
+        auto narHash = Hash::parseAny(s, htSHA256);
         getline(str, s);
-        if (!string2Int(s, info.narSize)) throw Error("number expected");
+        uint64_t narSize;
+        if (!string2Int(s, narSize)) throw Error("number expected");
+        hashGiven = { narHash, narSize };
     }
+    ValidPathInfo info(store.parseStorePath(path), hashGiven->first);
+    info.narSize = hashGiven->second;
     std::string deriver;
     getline(str, deriver);
     if (deriver != "") info.deriver = store.parseStorePath(deriver);
@@ -928,12 +931,12 @@ void ValidPathInfo::setReferencesPossiblyToSelf(StorePathSet && refs)
 
 std::string ValidPathInfo::fingerprint(const Store & store) const
 {
-    if (narSize == 0 || !narHash)
-        throw Error("cannot calculate fingerprint of path '%s' because its size/hash is not known",
+    if (narSize == 0)
+        throw Error("cannot calculate fingerprint of path '%s' because its size is not known",
             store.printStorePath(path));
     return
         "1;" + store.printStorePath(path) + ";"
-        + narHash->to_string(Base32, true) + ";"
+        + narHash.to_string(Base32, true) + ";"
         + std::to_string(narSize) + ";"
         + concatStringsSep(",", store.printStorePathSet(referencesPossiblyToSelf()));
 }
@@ -994,6 +997,25 @@ Strings ValidPathInfo::shortRefs() const
 }
 
 
+Derivation Store::derivationFromPath(const StorePath & drvPath)
+{
+    ensurePath(drvPath);
+    return readDerivation(drvPath);
+}
+
+
+Derivation Store::readDerivation(const StorePath & drvPath)
+{
+    auto accessor = getFSAccessor();
+    try {
+        return parseDerivation(*this,
+            accessor->readFile(printStorePath(drvPath)),
+            Derivation::nameFromPath(drvPath));
+    } catch (FormatError & e) {
+        throw Error("error parsing derivation '%s': %s", printStorePath(drvPath), e.msg());
+    }
+}
+
 }
 
 
@@ -1003,9 +1025,6 @@ Strings ValidPathInfo::shortRefs() const
 
 namespace nix {
 
-
-RegisterStoreImplementation::Implementations * RegisterStoreImplementation::implementations = 0;
-
 /* Split URI into protocol+hierarchy part and its parameter set. */
 std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri_)
 {
@@ -1019,24 +1038,6 @@ std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri_
     return {uri, params};
 }
 
-ref<Store> openStore(const std::string & uri_,
-    const Store::Params & extraParams)
-{
-    auto [uri, uriParams] = splitUriAndParams(uri_);
-    auto params = extraParams;
-    params.insert(uriParams.begin(), uriParams.end());
-
-    for (auto fun : *RegisterStoreImplementation::implementations) {
-        auto store = fun(uri, params);
-        if (store) {
-            store->warnUnknownSettings();
-            return ref<Store>(store);
-        }
-    }
-
-    throw Error("don't know how to open Nix store '%s'", uri);
-}
-
 static bool isNonUriPath(const std::string & spec) {
     return
         // is not a URL
@@ -1046,44 +1047,62 @@ static bool isNonUriPath(const std::string & spec) {
         && spec.find("/") != std::string::npos;
 }
 
-StoreType getStoreType(const std::string & uri, const std::string & stateDir)
+std::shared_ptr<Store> openFromNonUri(const std::string & uri, const Store::Params & params)
 {
-    if (uri == "daemon") {
-        return tDaemon;
-    } else if (uri == "local" || isNonUriPath(uri)) {
-        return tLocal;
-    } else if (uri == "" || uri == "auto") {
+    if (uri == "" || uri == "auto") {
+        auto stateDir = get(params, "state").value_or(settings.nixStateDir);
         if (access(stateDir.c_str(), R_OK | W_OK) == 0)
-            return tLocal;
+            return std::make_shared<LocalStore>(params);
         else if (pathExists(settings.nixDaemonSocketFile))
-            return tDaemon;
+            return std::make_shared<UDSRemoteStore>(params);
         else
-            return tLocal;
+            return std::make_shared<LocalStore>(params);
+    } else if (uri == "daemon") {
+        return std::make_shared<UDSRemoteStore>(params);
+    } else if (uri == "local") {
+        return std::make_shared<LocalStore>(params);
+    } else if (isNonUriPath(uri)) {
+        Store::Params params2 = params;
+        params2["root"] = absPath(uri);
+        return std::make_shared<LocalStore>(params2);
     } else {
-        return tOther;
+        return nullptr;
     }
 }
 
-
-static RegisterStoreImplementation regStore([](
-    const std::string & uri, const Store::Params & params)
-    -> std::shared_ptr<Store>
+ref<Store> openStore(const std::string & uri_,
+    const Store::Params & extraParams)
 {
-    switch (getStoreType(uri, get(params, "state").value_or(settings.nixStateDir))) {
-        case tDaemon:
-            return std::shared_ptr<Store>(std::make_shared<UDSRemoteStore>(params));
-        case tLocal: {
-            Store::Params params2 = params;
-            if (isNonUriPath(uri)) {
-                params2["root"] = absPath(uri);
-            }
-            return std::shared_ptr<Store>(std::make_shared<LocalStore>(params2));
-        }
-        default:
-            return nullptr;
-    }
-});
+    auto params = extraParams;
+    try {
+        auto parsedUri = parseURL(uri_);
+        params.insert(parsedUri.query.begin(), parsedUri.query.end());
 
+        auto baseURI = parsedUri.authority.value_or("") + parsedUri.path;
+
+        for (auto implem : *Implementations::registered) {
+            if (implem.uriSchemes.count(parsedUri.scheme)) {
+                auto store = implem.create(parsedUri.scheme, baseURI, params);
+                if (store) {
+                    store->init();
+                    store->warnUnknownSettings();
+                    return ref<Store>(store);
+                }
+            }
+        }
+    }
+    catch (BadURL &) {
+        auto [uri, uriParams] = splitUriAndParams(uri_);
+        params.insert(uriParams.begin(), uriParams.end());
+
+        if (auto store = openFromNonUri(uri, params)) {
+            store->warnUnknownSettings();
+            return ref<Store>(store);
+        }
+    }
+
+    throw Error("don't know how to open Nix store '%s'", uri_);
+}
 
 std::list<ref<Store>> getDefaultSubstituters()
 {
@@ -1117,5 +1136,6 @@ std::list<ref<Store>> getDefaultSubstituters()
     return stores;
 }
 
+std::vector<StoreFactory> * Implementations::registered = 0;
 
 }

src/libstore/store-api.hh

@@ -4,7 +4,6 @@
 #include "hash.hh"
 #include "content-address.hh"
 #include "serialise.hh"
-#include "crypto.hh"
 #include "lru-cache.hh"
 #include "sync.hh"
 #include "globals.hh"
@@ -25,6 +24,31 @@
 
 namespace nix {
 
+/**
+ * About the class hierarchy of the store implementations:
+ *
+ * Each store type `Foo` consists of two classes:
+ *
+ * 1. A class `FooConfig : virtual StoreConfig` that contains the configuration
+ *   for the store
+ *
+ *   It should only contain members of type `const Setting<T>` (or subclasses
+ *   of it) and inherit the constructors of `StoreConfig`
+ *   (`using StoreConfig::StoreConfig`).
+ *
+ * 2. A class `Foo : virtual Store, virtual FooConfig` that contains the
+ *   implementation of the store.
+ *
+ *   This class is expected to have a constructor `Foo(const Params & params)`
+ *   that calls `StoreConfig(params)` (otherwise you're gonna encounter an
+ *   `assertion failure` when trying to instantiate it).
+ *
+ * You can then register the new store using:
+ *
+ * ```
+ * cpp static RegisterStoreImplementation<Foo, FooConfig> regStore;
+ * ```
+ */
 
 MakeError(SubstError, Error);
 MakeError(BuildError, Error); // denotes a permanent build failure
@@ -34,6 +58,7 @@ MakeError(SubstituteGone, Error);
 MakeError(SubstituterDisabled, Error);
 MakeError(BadStorePath, Error);
 
+MakeError(InvalidStoreURI, Error);
 
 class FSAccessor;
 class NarInfoDiskCache;
@@ -145,12 +170,31 @@ struct BuildResult
     }
 };
 
-
-class Store : public std::enable_shared_from_this<Store>, public Config
+struct StoreConfig : public Config
 {
-public:
+    using Config::Config;
 
-    typedef std::map<std::string, std::string> Params;
+    /**
+     * When constructing a store implementation, we pass in a map `params` of
+     * parameters that's supposed to initialize the associated config.
+     * To do that, we must use the `StoreConfig(StringMap & params)`
+     * constructor, so we'd like to `delete` its default constructor to enforce
+     * it.
+     *
+     * However, actually deleting it means that all the subclasses of
+     * `StoreConfig` will have their default constructor deleted (because it's
+     * supposed to call the deleted default constructor of `StoreConfig`). But
+     * because we're always using virtual inheritance, the constructors of
+     * child classes will never implicitely call this one, so deleting it will
+     * be more painful than anything else.
+     *
+     * So we `assert(false)` here to ensure at runtime that the right
+     * constructor is always called without having to redefine a custom
+     * constructor for each `*Config` class.
+     */
+    StoreConfig() { assert(false); }
+
+    virtual const std::string name() = 0;
 
     const PathSetting storeDir_{this, false, settings.nixStore,
         "store", "path to the Nix store"};
@@ -164,6 +208,18 @@ public:
 
     Setting<bool> wantMassQuery{this, false, "want-mass-query", "whether this substituter can be queried efficiently for path validity"};
 
+    Setting<StringSet> systemFeatures{this, settings.systemFeatures,
+        "system-features",
+        "Optional features that the system this store builds on implements (like \"kvm\")."};
+
+};
+
+class Store : public std::enable_shared_from_this<Store>, public virtual StoreConfig
+{
+public:
+
+    typedef std::map<std::string, std::string> Params;
+
 protected:
 
     struct PathInfoCacheValue {
@@ -197,6 +253,11 @@ protected:
     Store(const Params & params);
 
 public:
+    /**
+     * Perform any necessary effectful operation to make the store up and
+     * running
+     */
+    virtual void init() {};
 
     virtual ~Store() { }
 
@@ -244,10 +305,12 @@ public:
     StorePathWithOutputs followLinksToStorePathWithOutputs(std::string_view path) const;
 
     /* Constructs a unique store path name. */
-    StorePath makeStorePath(const string & type,
+    StorePath makeStorePath(std::string_view type,
+        std::string_view hash, std::string_view name) const;
+    StorePath makeStorePath(std::string_view type,
         const Hash & hash, std::string_view name) const;
 
-    StorePath makeOutputPath(const string & id,
+    StorePath makeOutputPath(std::string_view id,
         const Hash & hash, std::string_view name) const;
 
     StorePath makeFixedOutputPath(FileIngestionMethod method,
@@ -340,9 +403,15 @@ public:
     /* Query the outputs of the derivation denoted by `path'. */
     virtual StorePathSet queryDerivationOutputs(const StorePath & path);
 
-    /* Query the mapping outputName=>outputPath for the given derivation */
-    virtual OutputPathMap queryDerivationOutputMap(const StorePath & path)
-    { unsupported("queryDerivationOutputMap"); }
+    /* Query the mapping outputName => outputPath for the given derivation. All
+       outputs are mentioned so ones mising the mapping are mapped to
+       `std::nullopt`.  */
+    virtual std::map<std::string, std::optional<StorePath>> queryPartialDerivationOutputMap(const StorePath & path)
+    { unsupported("queryPartialDerivationOutputMap"); }
+
+    /* Query the mapping outputName=>outputPath for the given derivation.
+       Assume every output has a mapping and throw an exception otherwise. */
+    OutputPathMap queryDerivationOutputMap(const StorePath & path);
 
     /* Query the full store path given the hash part of a valid store
        path, or empty if the path doesn't exist. */
@@ -380,7 +449,8 @@ public:
     /* Like addToStore(), but the contents of the path are contained
        in `dump', which is either a NAR serialisation (if recursive ==
        true) or simply the contents of a regular file (if recursive ==
-       false). */
+       false).
+       `dump` may be drained */
     // FIXME: remove?
     virtual StorePath addToStoreFromDump(Source & dump, const string & name,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair)
@@ -615,22 +685,25 @@ protected:
 
 };
 
-
-class LocalFSStore : public virtual Store
+struct LocalFSStoreConfig : virtual StoreConfig
 {
-public:
-
-    // FIXME: the (Store*) cast works around a bug in gcc that causes
+    using StoreConfig::StoreConfig;
+    // FIXME: the (StoreConfig*) cast works around a bug in gcc that causes
     // it to omit the call to the Setting constructor. Clang works fine
     // either way.
-    const PathSetting rootDir{(Store*) this, true, "",
+    const PathSetting rootDir{(StoreConfig*) this, true, "",
         "root", "directory prefixed to all other paths"};
-    const PathSetting stateDir{(Store*) this, false,
+    const PathSetting stateDir{(StoreConfig*) this, false,
         rootDir != "" ? rootDir + "/nix/var/nix" : settings.nixStateDir,
         "state", "directory where Nix will store state"};
-    const PathSetting logDir{(Store*) this, false,
+    const PathSetting logDir{(StoreConfig*) this, false,
         rootDir != "" ? rootDir + "/nix/var/log/nix" : settings.nixLogDir,
         "log", "directory where Nix will store state"};
+};
+
+class LocalFSStore : public virtual Store, public virtual LocalFSStoreConfig
+{
+public:
 
     const static string drvsLogDir;
 
@@ -640,8 +713,7 @@ public:
     ref<FSAccessor> getFSAccessor() override;
 
     /* Register a permanent GC root. */
-    Path addPermRoot(const StorePath & storePath,
-        const Path & gcRoot, bool indirect, bool allowOutsideRootsDir = false);
+    Path addPermRoot(const StorePath & storePath, const Path & gcRoot);
 
     virtual Path getRealStoreDir() { return storeDir; }
 
@@ -720,37 +792,47 @@ ref<Store> openStore(const std::string & uri = settings.storeUri.get(),
     const Store::Params & extraParams = Store::Params());
 
 
-enum StoreType {
-    tDaemon,
-    tLocal,
-    tOther
-};
-
-
-StoreType getStoreType(const std::string & uri = settings.storeUri.get(),
-    const std::string & stateDir = settings.nixStateDir);
-
 /* Return the default substituter stores, defined by the
    ‘substituters’ option and various legacy options. */
 std::list<ref<Store>> getDefaultSubstituters();
 
-
-/* Store implementation registration. */
-typedef std::function<std::shared_ptr<Store>(
-    const std::string & uri, const Store::Params & params)> OpenStore;
-
-struct RegisterStoreImplementation
+struct StoreFactory
 {
-    typedef std::vector<OpenStore> Implementations;
-    static Implementations * implementations;
+    std::set<std::string> uriSchemes;
+    std::function<std::shared_ptr<Store> (const std::string & scheme, const std::string & uri, const Store::Params & params)> create;
+    std::function<std::shared_ptr<StoreConfig> ()> getConfig;
+};
+struct Implementations
+{
+    static std::vector<StoreFactory> * registered;
 
-    RegisterStoreImplementation(OpenStore fun)
+    template<typename T, typename TConfig>
+    static void add()
     {
-        if (!implementations) implementations = new Implementations;
-        implementations->push_back(fun);
+        if (!registered) registered = new std::vector<StoreFactory>();
+        StoreFactory factory{
+            .uriSchemes = T::uriSchemes(),
+            .create =
+                ([](const std::string & scheme, const std::string & uri, const Store::Params & params)
+                 -> std::shared_ptr<Store>
+                 { return std::make_shared<T>(scheme, uri, params); }),
+            .getConfig =
+                ([]()
+                 -> std::shared_ptr<StoreConfig>
+                 { return std::make_shared<TConfig>(StringMap({})); })
+        };
+        registered->push_back(factory);
     }
 };
 
+template<typename T, typename TConfig>
+struct RegisterStoreImplementation
+{
+    RegisterStoreImplementation()
+    {
+        Implementations::add<T, TConfig>();
+    }
+};
 
 
 /* Display a set of paths in human-readable form (i.e., between quotes
@@ -761,7 +843,7 @@ string showPaths(const PathSet & paths);
 std::optional<ValidPathInfo> decodeValidPathInfo(
     const Store & store,
     std::istream & str,
-    bool hashGiven = false);
+    std::optional<HashResult> hashGiven = std::nullopt);
 
 /* Split URI into protocol+hierarchy part and its parameter set. */
 std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri);

src/libstore/worker-protocol.hh

@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x117
+#define PROTOCOL_VERSION 0x119
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -18,7 +18,7 @@ typedef enum {
     wopQueryReferences = 5, // obsolete
     wopQueryReferrers = 6,
     wopAddToStore = 7,
-    wopAddTextToStore = 8,
+    wopAddTextToStore = 8, // obsolete since 1.25, Nix 3.0. Use wopAddToStore
     wopBuildPaths = 9,
     wopEnsurePath = 10,
     wopAddTempRoot = 11,
@@ -70,10 +70,84 @@ template<class T> T readStorePaths(const Store & store, Source & from);
 
 void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths);
 
+/* To guide overloading */
+template<typename T>
+struct Phantom {};
+
+
+namespace worker_proto {
+/* FIXME maybe move more stuff inside here */
+
+StorePath read(const Store & store, Source & from, Phantom<StorePath> _);
+void write(const Store & store, Sink & out, const StorePath & storePath);
+
+template<typename T>
+std::map<std::string, T> read(const Store & store, Source & from, Phantom<std::map<std::string, T>> _);
+template<typename T>
+void write(const Store & store, Sink & out, const std::map<string, T> & resMap);
+template<typename T>
+std::optional<T> read(const Store & store, Source & from, Phantom<std::optional<T>> _);
+template<typename T>
+void write(const Store & store, Sink & out, const std::optional<T> & optVal);
+
+/* Specialization which uses and empty string for the empty case, taking
+   advantage of the fact StorePaths always serialize to a non-empty string.
+   This is done primarily for backwards compatability, so that StorePath <=
+   std::optional<StorePath>, where <= is the compatability partial order.
+ */
+template<>
+void write(const Store & store, Sink & out, const std::optional<StorePath> & optVal);
+
+template<typename T>
+std::map<std::string, T> read(const Store & store, Source & from, Phantom<std::map<std::string, T>> _)
+{
+    std::map<string, T> resMap;
+    auto size = (size_t)readInt(from);
+    while (size--) {
+        auto thisKey = readString(from);
+        resMap.insert_or_assign(std::move(thisKey), nix::worker_proto::read(store, from, Phantom<T> {}));
+    }
+    return resMap;
+}
+
+template<typename T>
+void write(const Store & store, Sink & out, const std::map<string, T> & resMap)
+{
+    out << resMap.size();
+    for (auto & i : resMap) {
+        out << i.first;
+        nix::worker_proto::write(store, out, i.second);
+    }
+}
+
+template<typename T>
+std::optional<T> read(const Store & store, Source & from, Phantom<std::optional<T>> _)
+{
+    auto tag = readNum<uint8_t>(from);
+    switch (tag) {
+    case 0:
+        return std::nullopt;
+    case 1:
+        return nix::worker_proto::read(store, from, Phantom<T> {});
+    default:
+        throw Error("got an invalid tag bit for std::optional: %#04x", tag);
+    }
+}
+
+template<typename T>
+void write(const Store & store, Sink & out, const std::optional<T> & optVal)
+{
+    out << (optVal ? 1 : 0);
+    if (optVal)
+        nix::worker_proto::write(store, out, *optVal);
+}
+
+
+}
+
+
 StorePathCAMap readStorePathCAMap(const Store & store, Source & from);
 
 void writeStorePathCAMap(const Store & store, Sink & out, const StorePathCAMap & paths);
 
-void writeOutputPathMap(const Store & store, Sink & out, const OutputPathMap & paths);
-
 }