commit_ref(): if there is an empty dir in the way, delete it

Part of the bug revealed in the last commit is that resolve_ref_unsafe()
incorrectly returns EISDIR if it finds a directory in the place where it
is looking for a loose reference, even if the corresponding packed
reference exists. lock_ref_sha1_basic() notices the bogus EISDIR, and
use it as an indication that it should call remove_empty_directories()
and call resolve_ref_unsafe() again.

But resolve_ref_unsafe() shouldn't report EISDIR in this case. If we
would simply make that change, then remove_empty_directories() wouldn't
get called anymore, and the empty directory would get in the way when
commit_ref() calls commit_lock_file() to rename the lockfile into place.

So instead of relying on lock_ref_sha1_basic() to delete empty
directories, teach commit_ref(), just before calling commit_lock_file(),
to check whether a directory is in the way, and if so, try to delete it.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>

refs/files-backend.c

@@ -2457,6 +2457,30 @@ static int close_ref(struct ref_lock *lock)
 
 static int commit_ref(struct ref_lock *lock)
 {
+	char *path = get_locked_file_path(lock->lk);
+	struct stat st;
+
+	if (!lstat(path, &st) && S_ISDIR(st.st_mode)) {
+		/*
+		 * There is a directory at the path we want to rename
+		 * the lockfile to. Hopefully it is empty; try to
+		 * delete it.
+		 */
+		size_t len = strlen(path);
+		struct strbuf sb_path = STRBUF_INIT;
+
+		strbuf_attach(&sb_path, path, len, len);
+
+		/*
+		 * If this fails, commit_lock_file() will also fail
+		 * and will report the problem.
+		 */
+		remove_empty_directories(&sb_path);
+		strbuf_release(&sb_path);
+	} else {
+		free(path);
+	}
+
 	if (commit_lock_file(lock->lk))
 		return -1;
 	return 0;