Subject: Re: Race condition between repack and loose-objects maintenance task

From: Junio C Hamano <[email-masked]>
Date: Thu, 30 Jun 2022 13:12:20 -0700

Taylor Blau <[email-masked]> writes:

> On Wed, Jun 29, 2022 at 11:19:09PM -0400, Taylor Blau wrote:
>> > However, I think there is yet another bug at play: running
>> > `incremental-repack` appears to be able to repack the cruft packfile.
>> > In doing so, it deletes its .mtimes file and the metadata inside.
>>
>> That sounds like a bug to me. I'll take a look into it and see what I
>> can find.
>
> I actually think that there are two bugs here.
>
> One is that we run a MIDX repack and expire, which could lead to us
> repacking the entire contents of the cruft pack and then expiring the
> metadata file. This is a bug, and we should exclude cruft packs from
> this computation.
>
> Another bug can happen when the cruft pack gets written into the MIDX
> and is MIDX-expireable (meaning that no objects are selected from the
> cruft pack). In that case, the `git multi-pack-index expire` step would
> remove the cruft pack entirely, which is also incorrect.
>
> I'll take a look at fixing both of these, and thanks for pointing them
> out!

Thanks, both.

The fact that the semantics of the .mtimes file being not equivalent
to the mtime on individual loose objects does not help thinking
about the possible ways the "cruft" pack can break, and both of the
possible issues you discuss above are indeed tricky ones.