Re: [PATCH v3 1/2] 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>
> ---

>  refs.h               |  8 ++++++++
>  refs/files-backend.c | 35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 43 insertions(+)
>
> diff --git a/refs.h b/refs.h
> index 2f3decb432cf..f11154cf5faf 100644
> --- a/refs.h
> +++ b/refs.h
> @@ -306,6 +306,14 @@ 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 is
> + * able to update the main working tree's HEAD wherever $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,
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 81f68f846b69..ec237efec35d 100644
> --- a/refs/files-backend.c
> +++ b/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;