refdb_fs: load packed-refs only when necessarypacked-refs-race

When checking if a ref exists, we load the packed-refs file and then
return true if the ref is found either loose or in the packed-refs
cache.

This is subject to the same race described in the previous commit, where
a simultaneous pack-refs may migrate a ref from loose to packed and
cause us to see neither. We can fix it by making sure we load
packed-refs only after checking the loose store.

As a bonus, this is a minor optimization: we'll avoid even looking at
the packed-refs file if we see a loose version.

1 files changed, 8 insertions, 5 deletions

diff --git a/src/refdb_fs.c b/src/refdb_fs.c
index 3ac7e8347..d8447eea6 100644
--- a/src/refdb_fs.c
+++ b/src/refdb_fs.c
@@ -333,14 +333,17 @@ static int refdb_fs_backend__exists(
 
 	assert(backend);
 
-	if ((error = packed_reload(backend)) < 0 ||
-		(error = git_buf_joinpath(&ref_path, backend->gitpath, ref_name)) < 0)
+	if ((error = git_buf_joinpath(&ref_path, backend->gitpath, ref_name)) < 0)
 		return error;
+	*exists = git_path_isfile(ref_path.ptr);
+	git_buf_dispose(&ref_path);
 
-	*exists = git_path_isfile(ref_path.ptr) ||
-		(git_sortedcache_lookup(backend->refcache, ref_name) != NULL);
+	if (!*exists) {
+		if ((error = packed_reload(backend)) < 0)
+		    return error;
+		*exists = git_sortedcache_lookup(backend->refcache, ref_name) != NULL;
+	}
 
-	git_buf_dispose(&ref_path);
 	return 0;
 }