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Re: Potential for a race condition with latest-repository-commit

From: Ludovic Courtès
Subject: Re: Potential for a race condition with latest-repository-commit
Date: Wed, 18 Mar 2020 17:19:05 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)


Christopher Baines <address@hidden> skribis:

> So one of the things that's currently restricted to doing by one job at
> a time in the Guix Data Service is running latest-repository-commit from
> the (guix git) module.
> Previously this was more of a problem for the Guix Data Service, as a
> large section of the work for loading information about a revision was
> serialised to avoid the potential for contention over the cached
> checkout this procedure uses. There's still a lock used now, but I
> realised when looking in to this further that it's only necessary to
> lock around this specific call, not the larger section that was
> restricted previously.
> I think that add-to-store which this procedure uses isn't atomic for a
> directory, so there's a risk of weird results if the repository is
> changed after the required revision is checked out. While it isn't
> common to run guix pull twice, I think this could happen there if you
> ran guix pull concurrently for the same repository, but two different
> profiles. I added a sleep call just prior to the add-to-store call in
> latest-repository-commit, and tested running guix pull twice at roughly
> the same time, with different branches and profiles, and I did see both
> profiles then reflecting a single branch, so one profile was mistakenly
> referring to the wrong branch.

Yes, we’ve also seen this problem with ‘static-web-site-service’¹,
whereby if several instances would try to pull from, say,
guix-artwork.git, we’d get non-deterministic results.  We worked around
it by using different cache directories…


> Is this something that is worth guarding against? Maybe
> latest-repository-commit could double check the Git repo state after
> add-to-store completes, and raise an error if it's different to what it
> expects. Or perhaps individual worktrees could be used for each process,
> which would hopefully avoid the race condition entirely.

It think it’d be worth guarding against it, yes.  What we could do is
lock the cache directory, with something like ‘with-file-lock’.



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