fetchers/git: make relative path absolute for local repo

src/libfetchers/git.cc

@@ -426,7 +426,16 @@ struct GitInputScheme : InputScheme
         auto url = parseURL(getStrAttr(input.attrs, "url"));
         bool isBareRepository = url.scheme == "file" && !pathExists(url.path + "/.git");
         repoInfo.isLocal = url.scheme == "file" && !forceHttp && !isBareRepository;
-        repoInfo.url = repoInfo.isLocal ? url.path : url.to_string();
+        //
+        // FIXME: here we turn a possibly relative path into an absolute path.
+        // This allows relative git flake inputs to be resolved against the
+        // **current working directory** (as in POSIX), which tends to work out
+        // ok in the context of flakes, but is the wrong behavior,
+        // as it should resolve against the flake.nix base directory instead.
+        //
+        // See: https://discourse.nixos.org/t/57783 and #9708
+        //
+        repoInfo.url = repoInfo.isLocal ? std::filesystem::absolute(url.path).string() : url.to_string();
 
         // If this is a local directory and no ref or revision is
         // given, then allow the use of an unclean working tree.

tests/functional/flakes/flake-in-submodule.sh

@@ -76,3 +76,21 @@ git -C "$rootRepo" commit -m "Add flake.nix"
 
 storePath=$(nix flake metadata --json "$rootRepo?submodules=1" | jq -r .path)
 [[ -e "$storePath/submodule" ]]
+
+# The root repo may use the submodule repo as an input
+# through the relative path. This may change in the future;
+# see: https://discourse.nixos.org/t/57783 and #9708.
+cat > "$rootRepo"/flake.nix <<EOF
+{
+    inputs.subRepo.url = "git+file:./submodule";
+    outputs = { ... }: { };
+}
+EOF
+git -C "$rootRepo" add flake.nix
+git -C "$rootRepo" commit -m "Add subRepo input"
+(
+  cd "$rootRepo"
+  # The submodule must be locked to the relative path,
+  # _not_ the absolute path:
+  [[ $(nix flake metadata --json | jq -r .locks.nodes.subRepo.locked.url) = "file:./submodule" ]]
+)

tests/functional/flakes/flakes.sh

@@ -106,6 +106,15 @@ nix build -o "$TEST_ROOT/result" "git+file://$flake1Dir#default"
 nix build -o "$TEST_ROOT/result" "$flake1Dir?ref=HEAD#default"
 nix build -o "$TEST_ROOT/result" "git+file://$flake1Dir?ref=HEAD#default"
 
+# Check that relative paths are allowed for git flakes.
+# This may change in the future once git submodule support is refined.
+# See: https://discourse.nixos.org/t/57783 and #9708.
+(
+  # This `cd` should not be required and is indicative of aforementioned bug.
+  cd "$flake1Dir/.."
+  nix build -o "$TEST_ROOT/result" "git+file:./$(basename "$flake1Dir")"
+)
+
 # Check that store symlinks inside a flake are not interpreted as flakes.
 nix build -o "$flake1Dir/result" "git+file://$flake1Dir"
 nix path-info "$flake1Dir/result"