Re: Git push race condition?

From: Jeff King <peff@peff.net>
Date: Mar 24, 2014, 22:54 UTC
Message-ID: <20140324225434.GB17080@sigill.intra.peff.net>
In-Reply-To: <CAAyEjTN53+5B9Od9wW698wODNL3hR6Upot8-ZLwEksn3ir_zjA@mail.gmail.com>

On Mon, Mar 24, 2014 at 03:18:14PM -0400, Scott Sandler wrote:

Show 8 quoted lines

> I've noticed that a few times in the past several weeks, we've had
> events where pushes have been lost when two people pushed at just
> about the same time. The scenario is that two users both have commits
> based on commit A, call them B and B'. The user with commit B pushes
> at about the same time as the user who pushes B'. Both pushes are
> determined to be fast-forwards and both succeed, but B' overwrites B
> and B is no longer on origin/master. The server does have B in its
> .git directory but the commit isn't on any branch.

What version of git are you running on the server? Is it possible that there is a simultaneous process running `git pack-refs` (e.g., a `git gc` run by a cron job or similar)?

There were some race conditions fixed last year wherein git could see stale values of refs, but I do not think they could impact writing to a ref like this. When we take the lock on the ref, we always go straight to the filesystem, so the value we see is up-to-date.

-Peff