ref locking: allow 'foo' when 'foo/bar' used to exist but not anymore.

It is normal to have .git/refs/heads/foo directory which is
empty after the last branch whose name starts with foo/ is
removed.  Make sure we notice this case and allow creation of
branch foo by removing the empty directory.

Signed-off-by: Junio C Hamano <junkio@cox.net>

refs.c

@@ -473,6 +473,59 @@ static struct ref_lock *verify_lock(struct ref_lock *lock,
 	return lock;
 }
 
+static int remove_empty_dir_recursive(char *path, int len)
+{
+	DIR *dir = opendir(path);
+	struct dirent *e;
+	int ret = 0;
+
+	if (!dir)
+		return -1;
+	if (path[len-1] != '/')
+		path[len++] = '/';
+	while ((e = readdir(dir)) != NULL) {
+		struct stat st;
+		int namlen;
+		if ((e->d_name[0] == '.') &&
+		    ((e->d_name[1] == 0) ||
+		     ((e->d_name[1] == '.') && e->d_name[2] == 0)))
+			continue; /* "." and ".." */
+
+		namlen = strlen(e->d_name);
+		if ((len + namlen < PATH_MAX) &&
+		    strcpy(path + len, e->d_name) &&
+		    !lstat(path, &st) &&
+		    S_ISDIR(st.st_mode) &&
+		    remove_empty_dir_recursive(path, len + namlen))
+			continue; /* happy */
+
+		/* path too long, stat fails, or non-directory still exists */
+		ret = -1;
+		break;
+	}
+	closedir(dir);
+	if (!ret) {
+		path[len] = 0;
+		ret = rmdir(path);
+	}
+	return ret;
+}
+
+static int remove_empty_directories(char *file)
+{
+	/* we want to create a file but there is a directory there;
+	 * if that is an empty directory (or a directory that contains
+	 * only empty directories), remove them.
+	 */
+	char path[PATH_MAX];
+	int len = strlen(file);
+
+	if (len >= PATH_MAX) /* path too long ;-) */
+		return -1;
+	strcpy(path, file);
+	return remove_empty_dir_recursive(path, len);
+}
+
 static struct ref_lock *lock_ref_sha1_basic(const char *ref, const unsigned char *old_sha1)
 {
 	char *ref_file;
@@ -485,6 +538,17 @@ static struct ref_lock *lock_ref_sha1_basic(const char *ref, const unsigned char
 	lock->lock_fd = -1;
 
 	ref = resolve_ref(ref, lock->old_sha1, mustexist, NULL);
+	if (!ref && errno == EISDIR) {
+		/* we are trying to lock foo but we used to
+		 * have foo/bar which now does not exist;
+		 * it is normal for the empty directory 'foo'
+		 * to remain.
+		 */
+		ref_file = git_path("%s", orig_ref);
+		if (remove_empty_directories(ref_file))
+			die("there are still refs under '%s'", orig_ref);
+		ref = resolve_ref(orig_ref, lock->old_sha1, mustexist, NULL);
+	}
 	if (!ref) {
 		int last_errno = errno;
 		error("unable to resolve reference %s: %s",