diff --git a/cache.h b/cache.h
index c6b7770f60746b..b780794d6c7870 100644
--- a/cache.h
+++ b/cache.h
@@ -567,6 +567,7 @@ extern NORETURN void unable_to_lock_index_die(const char *path, int err);
 extern int hold_lock_file_for_update(struct lock_file *, const char *path, int);
 extern int hold_lock_file_for_append(struct lock_file *, const char *path, int);
 extern int commit_lock_file(struct lock_file *);
+extern int reopen_lock_file(struct lock_file *);
 extern void update_index_if_able(struct index_state *, struct lock_file *);
 
 extern int hold_locked_index(struct lock_file *, int);
diff --git a/lockfile.c b/lockfile.c
index b7066143494bcf..9c12ec5ca6da8a 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -228,6 +228,16 @@ int close_lock_file(struct lock_file *lk)
 	return close(fd);
 }
 
+int reopen_lock_file(struct lock_file *lk)
+{
+	if (0 <= lk->fd)
+		die(_("BUG: reopen a lockfile that is still open"));
+	if (!lk->filename[0])
+		die(_("BUG: reopen a lockfile that has been committed"));
+	lk->fd = open(lk->filename, O_WRONLY);
+	return lk->fd;
+}
+
 int commit_lock_file(struct lock_file *lk)
 {
 	char result_file[PATH_MAX];