Hi folks,

I run a private Git repository (using Gitlab) with about 200 users
doing about 100 pushes per day.

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.

I'm confident nobody is force pushing (we have a hook to disallow it
on master branches and I've seen screenshots of both user's clients
after they pushed). Both git clients say "successfully pushed A..B
master -> master" (or A..B') in the output of their push commands.
However, when the user that had B does a fetch, it shows master as
having been force updated.

We have a few pre-receive hooks and post-receive hooks that run on
pushes, and Gitlab has an update hook as well. My original theory was
that this was happening because Git checks if it's a fast-forward
before running hooks, and that the hooks taking a few seconds creates
more opportunity for a race condition to occur.

However, after reading
http://git.661346.n2.nabble.com/push-race-td7569254.html and doing
some of my own testing (creating a hook that runs for 60 seconds and
pushing from two locations to a test repo) this theory seems to be
wrong. With the 60 second sleep hook (tried as an update hook and a
pre-receive hook), I wasn't able to reproduce the problem. The second
pusher always got an error like this:

error: Ref refs/heads/master is at
4584c1f34e07cea2df6abc8e0d407fe016017130 but expected
remote: error: failed to lock refs/heads/master

Which looks like exactly what I'd want Git to be doing in this
scenario, and supports what that archived thread says about how this
should work.

So the question is, how might this be happening and what can I do about it?

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