1 files changed, 31 insertions, 0 deletions
diff --git a/ref/store/files/root_open_common.go b/ref/store/files/root_open_common.go
new file mode 100644
index 00000000..cac98cbc
--- /dev/null
+++ b/ref/store/files/root_open_common.go
@@ -0,0 +1,31 @@
+package files
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func openCommonRoot(gitRoot *os.Root) (*os.Root, error) {
+	content, err := gitRoot.ReadFile("commondir")
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return gitRoot.OpenRoot(".")
+		}
+
+		return nil, err
+	}
+
+	commonDir := strings.TrimSpace(string(content))
+	if commonDir == "" {
+		return nil, os.ErrNotExist
+	}
+
+	if filepath.IsAbs(commonDir) {
+		return os.OpenRoot(commonDir)
+	}
+
+	// This is okay because that's how Git defines it anyway.
+	return os.OpenRoot(filepath.Join(gitRoot.Name(), commonDir))
+}