Mount inputs on storeFS to restore fetchToStore() caching

fetchToStore() caching was broken because it uses the fingerprint of
the accessor, but now that the accessor (typically storeFS) is a
composite (like MountedSourceAccessor or AllowListSourceAccessor),
there was no fingerprint anymore. So fetchToStore now uses the new
getFingerprint() method to get the specific fingerprint for the
subpath.

src/libexpr/eval.cc

@@ -227,24 +227,17 @@ EvalState::EvalState(
           {CanonPath(store->storeDir), store->getFSAccessor(settings.pureEval)},
       }))
     , rootFS([&] {
-        auto accessor = [&]() -> decltype(rootFS) {
+        /* In pure eval mode, we provide a filesystem that only
-            /* In pure eval mode, we provide a filesystem that only
+           contains the Nix store.
-               contains the Nix store. */
-            if (settings.pureEval)
-                return storeFS;
 
-            /* If we have a chroot store and pure eval is not enabled,
+           Otherwise, use a union accessor to make the augmented store
-               use a union accessor to make the chroot store available
+           available at its logical location while still having the
-               at its logical location while still having the underlying
+           underlying directory available. This is necessary for
-               directory available. This is necessary for instance if
+           instance if we're evaluating a file from the physical
-               we're evaluating a file from the physical /nix/store
+           /nix/store while using a chroot store, and also for lazy
-               while using a chroot store. */
+           mounted fetchTree. */
-            auto realStoreDir = dirOf(store->toRealPath(StorePath::dummy));
+        auto accessor = settings.pureEval ? storeFS.cast<SourceAccessor>()
-            if (store->storeDir != realStoreDir)
+                                          : makeUnionSourceAccessor({getFSSourceAccessor(), storeFS});
-                return makeUnionSourceAccessor({getFSSourceAccessor(), storeFS});
-
-            return getFSSourceAccessor();
-        }();
 
         /* Apply access control if needed. */
         if (settings.restrictEval || settings.pureEval)

src/libexpr/include/nix/expr/eval.hh

@@ -42,6 +42,7 @@ class Store;
 namespace fetchers {
 struct Settings;
 struct InputCache;
+struct Input;
 } // namespace fetchers
 struct EvalSettings;
 class EvalState;
@@ -514,6 +515,11 @@ public:
 
     void checkURI(const std::string & uri);
 
+    /**
+     * Mount an input on the Nix store.
+     */
+    StorePath mountInput(fetchers::Input & input, const fetchers::Input & originalInput, ref<SourceAccessor> accessor);
+
     /**
      * Parse a Nix expression from the specified file.
      */

src/libexpr/paths.cc

@@ -1,5 +1,7 @@
 #include "nix/store/store-api.hh"
 #include "nix/expr/eval.hh"
+#include "nix/util/mounted-source-accessor.hh"
+#include "nix/fetchers/fetch-to-store.hh"
 
 namespace nix {
 
@@ -18,4 +20,27 @@ SourcePath EvalState::storePath(const StorePath & path)
     return {rootFS, CanonPath{store->printStorePath(path)}};
 }
 
+StorePath
+EvalState::mountInput(fetchers::Input & input, const fetchers::Input & originalInput, ref<SourceAccessor> accessor)
+{
+    auto storePath = fetchToStore(fetchSettings, *store, accessor, FetchMode::Copy, input.getName());
+
+    allowPath(storePath); // FIXME: should just whitelist the entire virtual store
+
+    storeFS->mount(CanonPath(store->printStorePath(storePath)), accessor);
+
+    auto narHash = store->queryPathInfo(storePath)->narHash;
+    input.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
+
+    if (originalInput.getNarHash() && narHash != *originalInput.getNarHash())
+        throw Error(
+            (unsigned int) 102,
+            "NAR hash mismatch in input '%s', expected '%s' but got '%s'",
+            originalInput.to_string(),
+            narHash.to_string(HashFormat::SRI, true),
+            originalInput.getNarHash()->to_string(HashFormat::SRI, true));
+
+    return storePath;
+}
+
 } // namespace nix

src/libexpr/primops/fetchTree.cc

@@ -10,6 +10,7 @@
 #include "nix/util/url.hh"
 #include "nix/expr/value-to-json.hh"
 #include "nix/fetchers/fetch-to-store.hh"
+#include "nix/fetchers/input-cache.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -218,11 +219,11 @@ static void fetchTree(
             throw Error("input '%s' is not allowed to use the '__final' attribute", input.to_string());
     }
 
-    auto [storePath, input2] = input.fetchToStore(state.store);
+    auto cachedInput = state.inputCache->getAccessor(state.store, input, fetchers::UseRegistries::No);
 
-    state.allowPath(storePath);
+    auto storePath = state.mountInput(cachedInput.lockedInput, input, cachedInput.accessor);
 
-    emitTreeAttrs(state, storePath, input2, v, params.emptyRevFallback, false);
+    emitTreeAttrs(state, storePath, cachedInput.lockedInput, v, params.emptyRevFallback, false);
 }
 
 static void prim_fetchTree(EvalState & state, const PosIdx pos, Value ** args, Value & v)

src/libfetchers/fetch-to-store.cc

@@ -27,14 +27,19 @@ StorePath fetchToStore(
 
     std::optional<fetchers::Cache::Key> cacheKey;
 
-    if (!filter && path.accessor->fingerprint) {
+    auto [subpath, fingerprint] = filter ? std::pair<CanonPath, std::optional<std::string>>{path.path, std::nullopt}
-        cacheKey = makeFetchToStoreCacheKey(std::string{name}, *path.accessor->fingerprint, method, path.path.abs());
+                                         : path.accessor->getFingerprint(path.path);
+
+    if (fingerprint) {
+        cacheKey = makeFetchToStoreCacheKey(std::string{name}, *fingerprint, method, subpath.abs());
         if (auto res = settings.getCache()->lookupStorePath(*cacheKey, store)) {
             debug("store path cache hit for '%s'", path);
             return res->storePath;
         }
-    } else
+    } else {
+        // FIXME: could still provide in-memory caching keyed on `SourcePath`.
         debug("source path '%s' is uncacheable", path);
+    }
 
     Activity act(
         *logger,

src/libfetchers/fetchers.cc

@@ -356,8 +356,10 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
 
     auto [accessor, result] = scheme->getAccessor(store, *this);
 
-    assert(!accessor->fingerprint);
+    if (!accessor->fingerprint)
-    accessor->fingerprint = result.getFingerprint(store);
+        accessor->fingerprint = result.getFingerprint(store);
+    else
+        result.cachedFingerprint = accessor->fingerprint;
 
     return {accessor, std::move(result)};
 }

src/libflake/flake.cc

@@ -24,21 +24,6 @@ using namespace flake;
 
 namespace flake {
 
-static StorePath copyInputToStore(
-    EvalState & state, fetchers::Input & input, const fetchers::Input & originalInput, ref<SourceAccessor> accessor)
-{
-    auto storePath = fetchToStore(*input.settings, *state.store, accessor, FetchMode::Copy, input.getName());
-
-    state.allowPath(storePath);
-
-    auto narHash = state.store->queryPathInfo(storePath)->narHash;
-    input.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
-
-    assert(!originalInput.getNarHash() || storePath == originalInput.computeStorePath(*state.store));
-
-    return storePath;
-}
-
 static void forceTrivialValue(EvalState & state, Value & value, const PosIdx pos)
 {
     if (value.isThunk() && value.isTrivial())
@@ -360,11 +345,14 @@ static Flake getFlake(
         lockedRef = FlakeRef(std::move(cachedInput2.lockedInput), newLockedRef.subdir);
     }
 
-    // Copy the tree to the store.
-    auto storePath = copyInputToStore(state, lockedRef.input, originalRef.input, cachedInput.accessor);
-
     // Re-parse flake.nix from the store.
-    return readFlake(state, originalRef, resolvedRef, lockedRef, state.storePath(storePath), lockRootAttrPath);
+    return readFlake(
+        state,
+        originalRef,
+        resolvedRef,
+        lockedRef,
+        state.storePath(state.mountInput(lockedRef.input, originalRef.input, cachedInput.accessor)),
+        lockRootAttrPath);
 }
 
 Flake getFlake(EvalState & state, const FlakeRef & originalRef, fetchers::UseRegistries useRegistries)
@@ -721,11 +709,10 @@ lockFlake(const Settings & settings, EvalState & state, const FlakeRef & topRef,
 
                                     auto lockedRef = FlakeRef(std::move(cachedInput.lockedInput), input.ref->subdir);
 
-                                    // FIXME: allow input to be lazy.
+                                    return {
-                                    auto storePath = copyInputToStore(
+                                        state.storePath(
-                                        state, lockedRef.input, input.ref->input, cachedInput.accessor);
+                                            state.mountInput(lockedRef.input, input.ref->input, cachedInput.accessor)),
-
+                                        lockedRef};
-                                    return {state.storePath(storePath), lockedRef};
                                 }
                             }();
 

tests/functional/lang/eval-fail-hashfile-missing.err.exp

@@ -10,4 +10,4 @@ error:
 
        … while calling the 'hashFile' builtin
 
-       error: opening file '/pwd/lang/this-file-is-definitely-not-there-7392097': No such file or directory
+       error: path '/pwd/lang/this-file-is-definitely-not-there-7392097' does not exist