Re: [PATCH 2/2] builtin/pack-objects.c: ensure pack validity from MIDX bitmap objects

[Taylor Blau <me@ttaylorr.com>]

On Fri, May 13, 2022 at 04:06:39PM -0700, Jonathan Tan wrote:
> (An alternative to the change in this patch may be to reset *found_pack
> to NULL when it is found that the pack is invalid, but I haven't
> investigated all the callers to see if they can tolerate *found_pack
> moving changing non-NULL to NULL, so the change in this patch is
> probably more practical.)

I haven't either, but I think that this points out a flaw in the patch I
originally posted.

Consider this:

  - `want_object_in_pack()` calls `want_found_object()` with a pack that
    has gone away and has zero open fds, and `want_found_object()`
    returns -1
  - `want_object_in_pack()` continues and calls
    `want_object_in_pack_one()` later on, with some pack that is the
    same as `*found_pack`
  - `want_object_in_pack_one()` then _doesn't_ call `is_pack_valid()`
    (since `p == *found_pack`), leaving us in the same situation as
    before.

I think that would be sufficient to hit this race even after this patch.

I'll take a look to see if `want_object_in_pack()` callers can handle
`*found_pack` being set back to NULL. They should be able to, but I want
to do a little more careful analysis to be sure.

Thanks for pointing this out, I am so glad for your review! :-)

Thanks,
Taylor