Re: [PATCH v7 4/8] worktree: simplify find_shared_symref() memory ownership model

[Eric Sunshine <sunshine@sunshineco.com>]

On Wed, Dec 1, 2021 at 5:16 PM Anders Kaseorg <andersk@mit.edu> wrote:
> Storing the worktrees list in a static variable meant that
> find_shared_symref() had to rebuild the list on each call (which is
> inefficient when the call site is in a loop), and also that each call
> invalidated the pointer returned by the previous call (which is
> confusing).
>
> Instead, make it the caller’s responsibility to pass in the worktrees
> list and manage its lifetime.
>
> Signed-off-by: Anders Kaseorg <andersk@mit.edu>
> ---
> diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
> @@ -1486,12 +1486,17 @@ static const char *update(struct command *cmd, struct shallow_info *si)
> -       const struct worktree *worktree = is_bare_repository() ? NULL : find_shared_symref("HEAD", name);
> +       struct worktree **worktrees = get_worktrees();
> +       const struct worktree *worktree =
> +               is_bare_repository() ?
> +                       NULL :
> +                       find_shared_symref(worktrees, "HEAD", name);

As far as I can see, this code only cares whether find_shared_symref()
returned a result; it doesn't actually consult the returned worktree
at all, thus it semantically considers `worktree` as a boolean, not as
a `struct worktree`. I see in [7/8] that you do add an access to the
`worktree.is_bare` field, but that also is used purely in a boolean
fashion. If my understanding is correct, then it seems as if it would
be cleaner and make for a much less noisy patch if you did this
instead:

    struct worktree **worktrees = get_worktrees();
    int has_worktree = 0;
    if (!is_bare_repository()) {
        struct worktree *w = find_shared_symref(worktrees, ...);
        has_worktree = !!w;
    }
    free_worktrees(worktrees);
    ...
    if (has_worktree) {
        ...
    }

and in patch [7/8] augment that to:

    int is_worktree_bare = 0;
    if (!is_bare_repository()) {
        struct worktree *w = find_shared_symref(worktrees, ...);
        if (w) {
            has_worktree = 1;
            is_worktree_bare = w->is_bare;
        }
    }
    free_worktrees(worktrees);

which would then allow you to...

>         if (!starts_with(name, "refs/") || check_refname_format(name + 5, 0)) {
>                 rp_error("refusing to create funny ref '%s' remotely", name);
> -               return "funny refname";
> +               ret = "funny refname";
> +               goto out;
>         }

... drop this change and all the other similar changes to this file
since `worktrees` gets cleaned up as soon as `has_worktree` and
`is_worktree_bare` have been determined, so there's no need to
introduce an `out` label just to clean up `worktrees` at the end of
the function.