refs: add a new function set_worktree_head_symref

Add a new function set_worktree_head_symref, to update HEAD symref for
the specified worktree.

To update HEAD of a linked working tree,
create_symref("worktrees/$work_tree/HEAD", "refs/heads/$branch", msg)
could be used. However when it comes to updating HEAD of the main
working tree, it is unusable because it uses $GIT_DIR for
worktree-specific symrefs (HEAD).

The new function takes git_dir (real directory) as an argument, and
updates HEAD of the working tree. This function will be used when
renaming a branch.

Signed-off-by: Kazuki Yamaguchi <k@rhe.jp>
Acked-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -306,6 +306,15 @@ extern int rename_ref(const char *oldref, const char *newref, const char *logmsg
 
 extern int create_symref(const char *refname, const char *target, const char *logmsg);
 
+/*
+ * Update HEAD of the specified gitdir.
+ * Similar to create_symref("relative-git-dir/HEAD", target, NULL), but
+ * this can update the main working tree's HEAD regardless of where
+ * $GIT_DIR points to.
+ * Return 0 if successful, non-zero otherwise.
+ * */
+extern int set_worktree_head_symref(const char *gitdir, const char *target);
+
 enum action_on_err {
 	UPDATE_REFS_MSG_ON_ERR,
 	UPDATE_REFS_DIE_ON_ERR,

refs/files-backend.c

@@ -2894,6 +2894,41 @@ int create_symref(const char *refname, const char *target, const char *logmsg)
 	return ret;
 }
 
+int set_worktree_head_symref(const char *gitdir, const char *target)
+{
+	static struct lock_file head_lock;
+	struct ref_lock *lock;
+	struct strbuf err = STRBUF_INIT;
+	struct strbuf head_path = STRBUF_INIT;
+	const char *head_rel;
+	int ret;
+
+	strbuf_addf(&head_path, "%s/HEAD", absolute_path(gitdir));
+	if (hold_lock_file_for_update(&head_lock, head_path.buf,
+				      LOCK_NO_DEREF) < 0) {
+		error("%s", err.buf);
+		strbuf_release(&err);
+		strbuf_release(&head_path);
+		return -1;
+	}
+
+	/* head_rel will be "HEAD" for the main tree, "worktrees/wt/HEAD" for
+	   linked trees */
+	head_rel = remove_leading_path(head_path.buf,
+				       absolute_path(get_git_common_dir()));
+	/* to make use of create_symref_locked(), initialize ref_lock */
+	lock = xcalloc(1, sizeof(struct ref_lock));
+	lock->lk = &head_lock;
+	lock->ref_name = xstrdup(head_rel);
+	lock->orig_ref_name = xstrdup(head_rel);
+
+	ret = create_symref_locked(lock, head_rel, target, NULL);
+
+	unlock_ref(lock); /* will free lock */
+	strbuf_release(&head_path);
+	return ret;
+}
+
 int reflog_exists(const char *refname)
 {
 	struct stat st;