Re: [PATCH v3 2/2] setup: don't fail if commondir reference is deleted.

[Eric Sunshine <sunshine@sunshineco.com>]

On Wed, Feb 20, 2019 at 11:17 AM Michal Suchanek <msuchanek@suse.de> wrote:
> Apparently it can happen that stat() claims there is a commondir file but when
> trying to open the file it is missing.

Under what circumstances?

> Another even rarer issue is that the file might be zero size because another
> process initializing a worktree opened the file but has not written is content
> yet.

Based upon the explanation thus far, I'm having trouble understanding
under what circumstances these race conditions can arise. Are you
trying to invoke Git commands in a particular worktree even as the
worktree itself is being created?

Without this information being spelled out clearly, it is going to be
difficult for someone in the future to reason about why the code is
the way it is following this change.

> When any of this happnes git aborts failing to perform perfectly valid
> command because unrelated worktree is not yet fully initialized.

s/happnes/happens/

> Rather than testing if the file exists before reading it handle ENOENT
> and ENOTDIR.

One more comment below...

> Signed-off-by: Michal Suchanek <msuchanek@suse.de>
> ---
> diff --git a/setup.c b/setup.c
> @@ -270,12 +270,20 @@ int get_common_dir_noenv(struct strbuf *sb, const char *gitdir)
>  {
>         strbuf_addf(&path, "%s/commondir", gitdir);
> -       if (file_exists(path.buf)) {
> -               if (strbuf_read_file(&data, path.buf, 0) <= 0)
> +       ret = strbuf_read_file(&data, path.buf, 0);
> +       if (ret <= 0) {
> +               /*
> +                * if file is missing or zero size (just being written)
> +                * assume default, bail otherwise
> +                */
> +               if (ret && errno != ENOENT && errno != ENOTDIR)
>                         die_errno(_("failed to read %s"), path.buf);

It's not clear from the explanation given in the commit message if the
new behavior is indeed sensible. The original intent of the code, as I
understand it, is to validate "commondir", to ensure that it is not
somehow corrupt (such as the user editing it and making it empty).
Following this change, that particular validation no longer takes
place. But, more importantly, what does it mean to fall back to
"default" for this particular worktree? I'm having trouble
understanding how the new behavior can be correct or desirable. (Am I
missing something obvious?)

> +               strbuf_addstr(sb, gitdir);
> +               ret = 0;
> +       } else {