Re: on Amazon EFS (NFS): "Reference directory conflict: refs/heads/" with status code 128

From: Michael Haggerty <mhagger@alum.mit.edu>
Date: Aug 25, 2016, 06:28 UTC
Message-ID: <67cde0aa-89fc-b574-d5f5-7e754d77b5bb@alum.mit.edu>
In-Reply-To: <20160824213900.gcnwxu46zvgpjr5a@sigill.intra.peff.net>

On 08/24/2016 11:39 PM, Jeff King wrote:

Show 23 quoted lines

> On Wed, Aug 24, 2016 at 04:52:33PM -0400, Alex Nauda wrote:
> 
>> Elastic File System (EFS) is Amazon's scalable filesystem product that
>> is exposed to the OS as an NFS mount. We're using EFS to host the
>> filesystem used by a Jenkins CI server. Sometimes when Jenkins tries
>> to git fetch, we get this error:
>> $ git -c core.askpass=true fetch --tags --progress
>> git@github.com:mediasilo/dodo.git
>> +refs/pull/*:refs/remotes/origin/pr/*
>> fatal: Reference directory conflict: refs/heads/
>> $ echo $? 128
>>
>> Has anyone seen anything like this before? Any tips on how to troubleshoot it?
> 
> No, I haven't seen it before. That's an internal assertion in the refs
> code that shouldn't ever happen. It looks like it happens when the loose
> refs end up with duplicate directory entries. While a bug in git is an
> obvious culprit, I wonder if it's possible that your filesystem might
> expose the same name twice in one set of readdir() results.
> 
> +cc Michael, who added this assertion long ago (and since this is the
> first report in all these years, it does make me suspect that the
> filesystem is a critical part of reproducing).

Thanks for the CC.

I've never heard of this problem before.

What Git version are you using?

I tried to provoke the problem by hand-corrupting the packed-refs file, but wasn't successful.

So Peff's suggestion that the problem originates in your filesystem seems to be to be the most likely cause. A quick Google search found, for example,

    https://bugzilla.redhat.com/show_bug.cgi?id=739222

http://superuser.com/questions/640419/how-can-i-have-two-files-with-the-same-name-in-a-directory-when-mounted-with-nfs

though these reports seem connected with having lots of files in the directory, which seems unlikely for `$GIT_DIR/refs/`. But I didn't do a more careful search, and it is easily possible that there are other bugs in NFS (or EFS) that could be affecting you.

If this were repeatable, you could run Git under strace to test Peff's hypothesis. But I suppose it only happens rarely, right?

Is it possible that multiple clients have the same NFS filesystem mounted while Git is running? That would seem like an especially bad idea and I could imagine it leading to problems like this.

It's surprising that you are seeing this problem in directory `refs`, because (1) that directory is unlikely to have very many entries, and (2) as far as I remember, Git will never delete the directories `refs/heads` and `refs/tags`.

Michael