Re: [PATCH 1/2] worktree repair: refactor and reduce .git file reads

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 8, 2026, 20:55 UTC
Message-ID: <xmqqh5ivyks1.fsf@gitster.g>
In-Reply-To: <dc7ebb427bedc7318ebbf84c05ecd02063408353.1789269613.git.gitgitgadget@gmail.com>

"Yoichi NAKAYAMA via GitGitGadget" <gitgitgadget@gmail.com> writes:

Show 7 quoted lines

> +static const char *get_worktree_id(const char *dotgit_contents)
> +{
> +	const char *slash = find_last_dir_sep(dotgit_contents);
> +	if (!slash)
> +		return "";
> +	return slash + 1;
> +}

This returns a pointer into dotgit_contents; it is the last component of a pathname, similar to what basename(3) gives us.

> @@ -798,30 +806,20 @@ static int is_main_worktree_path(struct repository *repo, const char *path)

The unified diff is a bit hard to follow, so let's see if we can compare preimage and postimage more easily.

Show 27 quoted lines

>  static ssize_t infer_backlink(struct repository *repo,
> -			      const char *gitfile,
>  			      struct strbuf *inferred)
>  {
> -	struct strbuf actual = STRBUF_INIT;
>  	const char *id;
>  
> -	if (strbuf_read_file(&actual, gitfile, 0) < 0)
> -		goto error;
> -	if (!starts_with(actual.buf, "gitdir:"))
> -		goto error;
> -	if (!(id = find_last_dir_sep(actual.buf)))
> -		goto error;
> -	strbuf_trim(&actual);
> -	id++; /* advance past '/' to point at <id> */
>  	if (!*id)
>  		goto error;
>  	repo_common_path_replace(repo, inferred, "worktrees/%s", id);
>  	if (!is_directory(inferred->buf))
>  		goto error;
>  
> -	strbuf_release(&actual);
>  	return inferred->len;
>  error:
> -	strbuf_release(&actual);
>  	strbuf_reset(inferred); /* clear invalid path */
>  	return -1;

We used to receive the filename of ".git", read it and made sure we have "gitdir:" prefix, and find the last component, but then trimmed the actual buffer. Which means a few things.

 - If the contents of the gitfile were "gitdir:foo/bar/baz \n", our
   id pointer found the slash after "foo/bar", trimmed the buffer to
   have "gitdir:foo/bar/baz", and then incremented id, which now
   points at "baz".

 - If the contents of the gitfile were "gitdir: foo/bar/   \n", then
   after triming, the buffer would have "gitdir: foo/bar/" and id
   would be pointing at the NUL at the end, which would have lead us
   to error.

Now let's look at the new code.

Show 20 quoted lines

> @@ -798,30 +806,20 @@ static int is_main_worktree_path(struct repository *repo, const char *path)
>   * Returns -1 on failure and strbuf.len on success.
>   */
>  static ssize_t infer_backlink(struct repository *repo,
> +			      const char *dotgit_contents,
>  			      struct strbuf *inferred)
>  {
>  	const char *id;
>  
> +	id = get_worktree_id(dotgit_contents);
>  	if (!*id)
>  		goto error;
>  	repo_common_path_replace(repo, inferred, "worktrees/%s", id);
>  	if (!is_directory(inferred->buf))
>  		goto error;
>  
>  	return inferred->len;
>  error:
>  	strbuf_reset(inferred); /* clear invalid path */
>  	return -1;

The caller is expected to give us the contents of gitfile read by setup.c:read_gitfile_raw(), which reads the file in full, validates that the file begins with "gitdir: " (notice the trailing space), removes arbitrary run of CR or LF from the end, and then returns the string after skipping "gitdir: " prefix (8 bytes).

In the normal case, read_gitfile_raw() would see "gitdir: foo/bar/baz\n" in the file and returns "foo/bar/baz" to our caller. In fishy cases we examined for the preimage above:

 - If the contents of the gitfile were "gitdir:foo/bar/baz \n", our
   caller would have received an error from read_gitfile_raw() and
   wouldn't have called us.

 - If the contents of the gitfile were "gitdir: foo/bar/   \n", our
   caller would have given us "foo/bar/   ".

get_worktree_id() will give us "baz" in the normal case, and " " in the last case. We fail to error out in the latter with "*id" check, but is_directory() check will catch us, as the inferred directory is "worktrees/ " in that bad case.

So there are certain differences in error cases, but they behave the same in the most basic cases.

Now, this is the caller in the preimage (i.e., what we used to do).

Show 7 quoted lines

> @@ -856,51 +855,49 @@ void repair_worktree_at_path(struct repository *repo,
>  		goto done;
>  	}
>  
> -	infer_backlink(repo, dotgit.buf, &inferred_backlink);
> -	strbuf_realpath_forgiving(&inferred_backlink, inferred_backlink.buf, 0);
> -	dotgit_contents = xstrdup_or_null(read_gitfile_gently(dotgit.buf, &err));

We used to have infer_backlink() read the .git file to compute "worktree/$id", then again called read_gitfile_gently() to read it again.

> -	if (dotgit_contents) {
> -		strbuf_addstr(&backlink, dotgit_contents);

This is the happy path.  We successfully read from .git and use it.

> -	} else if (err == READ_GITFILE_ERR_NOT_A_FILE ||
> -			err == READ_GITFILE_ERR_IS_A_DIR) {
>  		fn(1, dotgit.buf, _("unable to locate repository; .git is not a file"), cb_data);
>  		goto done;

This is inherited badness, but overly long lines like this one needs to be fixed.

> -	} else if (err == READ_GITFILE_ERR_NOT_A_REPO) {

The _gently() did read something, but that does not point at a git directory.

Show 9 quoted lines

> -		if (inferred_backlink.len) {
> -			/*
> -			 * Worktree's .git file does not point at a repository
> -			 * but we found a .git/worktrees/<id> in this
> -			 * repository with the same <id> as recorded in the
> -			 * worktree's .git file so make the worktree point at
> -			 * the discovered .git/worktrees/<id>.
> -			 */
> -			strbuf_swap(&backlink, &inferred_backlink);

If we had the "worktree/$id" thing, we use it.

Show 8 quoted lines

> -		} else {
> -			fn(1, dotgit.buf, _("unable to locate repository; .git file does not reference a repository"), cb_data);
> -			goto done;
> -		}
> -	} else {
>  		fn(1, dotgit.buf, _("unable to locate repository; .git file broken"), cb_data);
>  		goto done;
>  	}

These lines to show error messages should also be folded to avoid overly long lines.

So, what does the updated code in the postimage do?

Show 5 quoted lines

> @@ -856,51 +855,49 @@ void repair_worktree_at_path(struct repository *repo,
>  		goto done;
>  	}
>  
> +	err = read_gitfile_raw(&contents, dotgit.buf);

We use read_gitfile_raw() just once.

Show 8 quoted lines

> +	if (err == READ_GITFILE_ERR_NOT_A_FILE ||
> +	    err == READ_GITFILE_ERR_IS_A_DIR) {
>  		fn(1, dotgit.buf, _("unable to locate repository; .git is not a file"), cb_data);
>  		goto done;
> +	} else if (err) {
>  		fn(1, dotgit.buf, _("unable to locate repository; .git file broken"), cb_data);
>  		goto done;
>  	}

The original code handled the happy case that read_gitfile_gently() successfully returned first. Underlying read_gitfile_raw() would not have given any of these errors when read_gitfile_gently() succeeded, so handling the error cases first would not affect the behaviour of the code in these cases. Again, these overlong lines are annoying.

Now the simplest error cases are behind us. How would we do in the happy case?

> +	dotgit_contents = contents.buf;
> +	infer_backlink(repo, dotgit_contents, &inferred_backlink);
> +	strbuf_realpath_forgiving(&inferred_backlink, inferred_backlink.buf, 0);

We reuse what we already read with read_gitfile_raw(), which prepared "worktrees/$id", and do the same realpath_forgiving() the original used to do a bit earlier.

> +	if (is_absolute_path(dotgit_contents)) {
> +		strbuf_addstr(&backlink, dotgit_contents);

I am not sure which part of the original this logic corresponds to. If the result from read_gitfile_raw() is an absolute path, even if it later turns out not to be is_git_directory(), the inferred backlink is not given a chance to act as a fallback. The original made a call to read_gitfile_gently() which checked is_git_directory() to give us an error, and that is how it allowed inferred backlink to substitute for a bad contents stored in .git file. Now we do not allow that fallback if .git file has an absolute path?

Ah, outside the context of this patch, before we barf for "unable to locate repository" when we complain backlink.buf is not naming a git directory, there is the fallback logic, and in order to reach there, we have "if (!is_git_directory(backlink.buf) && !inferred_backlink.len)" there. OK, so this may be doing the same thing as the original, but it is rather hard to follow and convince readers that this is a no-op conversion.

Show 5 quoted lines

> +	} else {
> +		strbuf_addbuf(&backlink, &dotgit);
> +		strbuf_strip_suffix(&backlink, ".git");
> +		strbuf_addstr(&backlink, dotgit_contents);
> +		strbuf_realpath_forgiving(&backlink, backlink.buf, 0);

This converts dotgit_contents relative to the computed backlink, which needs to be done here because read_gitfile_gently() used to do that for us, which we no longer use.

Show 6 quoted lines

> +	}
> +
> +	if (!is_git_directory(backlink.buf) && !inferred_backlink.len) {
> +		fn(1, dotgit.buf, _("unable to locate repository; .git file does not reference a repository"), cb_data);
> +		goto done;
> +	}

I'll stop here.