Merge pull request #14050 from NixOS/fix-fetch-to-store-caching

Fix fetchToStore caching

src/libutil/include/nix/util/source-accessor.hh

@@ -121,7 +121,7 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
         std::string typeString();
     };
 
-    Stat lstat(const CanonPath & path);
+    virtual Stat lstat(const CanonPath & path);
 
     virtual std::optional<Stat> maybeLstat(const CanonPath & path) = 0;
 
@@ -180,6 +180,27 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
      */
     std::optional<std::string> fingerprint;
 
+    /**
+     * Return the fingerprint for `path`. This is usually the
+     * fingerprint of the current accessor, but for composite
+     * accessors (like `MountedSourceAccessor`), we want to return the
+     * fingerprint of the "inner" accessor if the current one lacks a
+     * fingerprint.
+     *
+     * So this method is intended to return the most-outer accessor
+     * that has a fingerprint for `path`. It also returns the path that `path`
+     * corresponds to in that accessor.
+     *
+     * For example: in a `MountedSourceAccessor` that has
+     * `/nix/store/foo` mounted,
+     * `getFingerprint("/nix/store/foo/bar")` will return the path
+     * `/bar` and the fingerprint of the `/nix/store/foo` accessor.
+     */
+    virtual std::pair<CanonPath, std::optional<std::string>> getFingerprint(const CanonPath & path)
+    {
+        return {path, fingerprint};
+    }
+
     /**
      * Return the maximum last-modified time of the files in this
      * tree, if available.

src/libutil/mounted-source-accessor.cc

@@ -27,6 +27,12 @@ struct MountedSourceAccessorImpl : MountedSourceAccessor
         return accessor->readFile(subpath);
     }
 
+    Stat lstat(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->lstat(subpath);
+    }
+
     std::optional<Stat> maybeLstat(const CanonPath & path) override
     {
         auto [accessor, subpath] = resolve(path);
@@ -85,6 +91,14 @@ struct MountedSourceAccessorImpl : MountedSourceAccessor
         else
             return nullptr;
     }
+
+    std::pair<CanonPath, std::optional<std::string>> getFingerprint(const CanonPath & path) override
+    {
+        if (fingerprint)
+            return {path, fingerprint};
+        auto [accessor, subpath] = resolve(path);
+        return accessor->getFingerprint(subpath);
+    }
 };
 
 ref<MountedSourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts)

src/libutil/union-source-accessor.cc

@@ -72,6 +72,18 @@ struct UnionSourceAccessor : SourceAccessor
         }
         return std::nullopt;
     }
+
+    std::pair<CanonPath, std::optional<std::string>> getFingerprint(const CanonPath & path) override
+    {
+        if (fingerprint)
+            return {path, fingerprint};
+        for (auto & accessor : accessors) {
+            auto [subpath, fingerprint] = accessor->getFingerprint(path);
+            if (fingerprint)
+                return {subpath, fingerprint};
+        }
+        return {path, std::nullopt};
+    }
 };
 
 ref<SourceAccessor> makeUnionSourceAccessor(std::vector<ref<SourceAccessor>> && accessors)