From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 13 Mar 2019 01:43:57 GMT
Subject: Re: BUG: Race condition due to reflog expiry in "gc"
Message-ID: <xmqq1s3bh7ky.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <87tvg7brlm.fsf@evledraar.gmail.com>

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

> I'm still working on fixing a race condition I encountered in "gc"
> recently, but am not 100% sure of the fix. So I thought I'd send a
> braindump of what I have so far in case it jolts any memories.
>
> The problem is that when we "gc" we run "reflog expire --all". This
> iterates over the reflogs, and takes a *.lock for each reference.
>
> It'll fail intermittendly in two ways:
>
>  1. If something is concurrently committing to the repo it'll fail
>     because we for a tiny amount of time hold a HEAD.lock file, so HEAD
>     can't be updated.
>
>  2. On a repository that's just being "git fetch"'d by some concurrent
>     process the "gc" will fail, because the ref's SHA1 has changed,
>     which we inspect as we aquire the lock.

Both sounds very much expected and expectable outcome.  I am not
sure how they need to be called bugs.