Don't fetch from store to store

If the source path for fetchToStore is a direct store path, (i.e.
/nix/store/hash-name. things like /nix/store/hash-name/file.sh does not
count), return it directly instead of copying it.

Related: NixOS#10627

src/libfetchers/fetch-to-store.cc

@@ -15,6 +15,14 @@ StorePath fetchToStore(
 {
     // FIXME: add an optimisation for the case where the accessor is
     // a `PosixSourceAccessor` pointing to a store path.
+    if (store.isInStore(path.path.abs()) && !filter) {
+        auto storePath = store.toStorePath(path.path.abs());
+        debug("source path '%s' is potentially already in the store: '%s', '%s'",
+            path, store.printStorePath(storePath.first), storePath.second);
+        if (storePath.second == "") {
+            return storePath.first;
+        }
+    }
 
     std::optional<fetchers::Cache::Key> cacheKey;
 

tests/functional/flakes/flakes.sh

@@ -246,11 +246,6 @@ nix build -o "$TEST_ROOT/result" --expr "(builtins.getFlake \"git+file://$flake1
 # Regression test for baseNameOf on the root of the flake.
 [[ $(nix eval --raw flake1#baseName) =~ ^[a-z0-9]+-source$ ]]
 
-# Test that the root of a tree returns a path named /nix/store/<hash1>-<hash2>-source.
-# This behavior is *not* desired, but has existed for a while.
-# Issue #10627 what to do about it.
-[[ $(nix eval --raw flake1#root) =~ ^.*/[a-z0-9]+-[a-z0-9]+-source$ ]]
-
 # Building a flake with an unlocked dependency should fail in pure mode.
 (! nix build -o "$TEST_ROOT/result" flake2#bar --no-registries)
 (! nix build -o "$TEST_ROOT/result" flake2#bar --no-use-registries)