Re: Git push race condition?

From: Jeff King <peff@peff.net>
Date: Mar 24, 2014, 22:51 UTC
Message-ID: <20140324225136.GA17080@sigill.intra.peff.net>
In-Reply-To: <CACBZZX4ZEPA3sBp4-3QF6de0EWXzPkcOiqSxH3_CXV27Z=gxtw@mail.gmail.com>

On Mon, Mar 24, 2014 at 10:16:52PM +0100, Ævar Arnfjörð Bjarmason wrote:

Show 11 quoted lines

> > error: Ref refs/heads/master is at
> > 4584c1f34e07cea2df6abc8e0d407fe016017130 but expected
> > 61b79b6d35b066d054fb3deab550f1c51598cf5f
> > remote: error: failed to lock refs/heads/master
> 
> I also see this error once in a while. I read the code a while back
> and it's basically because there's two levels of locks that
> receive-pack tries to get, and it's possible for two pushers to get
> the first lock due to a race condition.
> 
> I've never seen data loss due to this though, because the inner lock is atomic.

The reason is that there are not 2 locks. Each side remembers the "old" value when it started the operation, and only takes a lock when it comes time to write the ref (and then checks that the old value is still current). Two pushes happening simultaneously do not have any idea that the other is occurring.

-Peff