Subject: Re: [PATCH 0/2] fix http deadlock on giant ref negotiations

From: Jeff King <[email-masked]>
Date: Fri, 15 May 2015 04:38:43 -0400

On Fri, May 15, 2015 at 09:41:20AM +0200, Dennis Kaarsemaker wrote:

> > I have no clue how to write a test that would trigger this reliably
> > without requiring a gigantic test fixture. However, I did confirm that
> > it fixes the problem on the chromium case you provided (which otherwise
> > deadlocks reliably for me).
> 
> This looks similar to the failure I posted about alst year in
> http://thread.gmane.org/gmane.comp.version-control.git/258514
> 
> Though the issue is different, it has the same 'hanging git fetch'
> symptom due to the deadlock between upload-pack and http-backend.

Thanks, I think it is the same issue (in the end I was replicating not
with `--reference`, but just by doing a fetch from the other
repository). And our solutions are essentially the same. I do prefer
mine because:

  1. It keeps the buffering logic in http-backend; the half-duplex
     nature is an http detail.

  2. I think it's better to buffer the request rather than the response,
     for the reasons I stated in the commit message.

> The patch I sent back then is suboptimal, as it can cause larger packs
> than necessary (we still use it though, as the alternative is a
> non-working git), but it does include a test you may be able to use to
> verify your fix, if this is indeed the same issue.

I applied the test from your patch, but couldn't get it to fail even
with stock git.  The test above it shrunk a bit, but I was able to tweak
yours to generate tags from 2001..100000, which I thought would have
worked.  I suspect it's something silly like the size not being quite
big enough for the pipe buffer on my system, or something like that.
Though I couldn't get it to fail even with 200,000 tags, so perhaps it's
something else.

-Peff