[1.7.2] Please cherry-pick "upload-pack: start pack-objects before async rev-list"

[Jonathan Nieder <jrnieder@gmail.com>]

Hi Junio,

Jeff King wrote:

> Subject: [PATCH] upload-pack: start pack-objects before async rev-list
>
> In a pthread-enabled version of upload-pack, there's a race condition
> that can cause a deadlock on the fflush(NULL) we call from run-command.

This server-side deadlock started being triggered by shallow clones
when sv.gnu.org upgraded to v1.7.2.5 a couple of months ago[1].  So it
might be worth thinking about how to help upgrade-averse server admins
to fix it.

Luckily, Jeff's patch (v1.7.5.1~4^2, upload-pack: start pack-objects
before async rev-list, 2011-04-06) applies cleanly on top of the
oldest commit with the problem that I can find (v1.7.2-rc0~31^2~3,
Reimplement async procedures using pthreads, 2010-03-06).  Could you
queue it for inclusion in maint-1.7.2?

If I understand correctly, Windows with threaded async rev-list before
then would not have suffered from this deadlock because it did not
fflush(NULL) in start_command until v1.7.4.1~18 (start_command: flush
buffers in the WIN32 code path as well, 2011-02-04).

[1] http://thread.gmane.org/gmane.comp.version-control.git/172042