Re: [PATCH 0/6] deadlock regression in v2.11.0 with failed mkdtemp

From: Jeff King <peff@peff.net>
Date: Mar 7, 2017, 13:50 UTC
Message-ID: <20170307135012.fdk6jhhl4zspdk2v@sigill.intra.peff.net>
In-Reply-To: <20170307133437.qee2jtynbiwf6uzr@sigill.intra.peff.net>

On Tue, Mar 07, 2017 at 08:34:37AM -0500, Jeff King wrote:

Show 13 quoted lines

> Yuck. In the original, the error was generated by the child index-pack,
> and we relayed it over the sideband. But we don't even get as far as
> running index-pack in the newer version; we fail trying to make the
> tmpdir. The error ends up in the "unpack" protocol field, but the client
> side does a bad job of showing that. With the rest of the patches, it
> looks like:
> 
>   $ git push ~/tmp/foo.git HEAD
>   Counting objects: 210973, done.
>   Delta compression using up to 8 threads.
>   Compressing objects: 100% (52799/52799), done.
>   error: remote unpack failed: unable to create temporary object directory
>   error: failed to push some refs to '/home/peff/tmp/foo.git'

There are two other options here I should mention.

One is that when pack-objects dies, we suppress the ref-status table entirely. Which is fair, because it doesn't have anything interesting to say. But for a case like this where the other side stops reading our pack early but still produces status reports, we could actually read them all and have a status table like:

  $ git push ~/tmp/foo.git HEAD
  Counting objects: 209843, done.
  Delta compression using up to 8 threads.
  Compressing objects: 100% (52186/52186), done.
  error: remote unpack failed: unable to create temporary object directory
  To /home/peff/tmp/foo.git
   ! [remote rejected]     HEAD -> jk/push-index-pack-deadlock (unpacker error)
  error: failed to push some refs to '/home/peff/tmp/foo.git'

We'd have to take some care to make sure we handle the case when the remote _doesn't_ manage to give us the status (e.g., when there's a complete hangup). I don't know if it's worth the effort. There's no new information there.

The second thing is that I think the design of the "unpack <reason>" report is a bit dated. These days everybody supports the sideband protocol, so it would probably make more sense to just issue the "<reason>" report via the sideband (at which point it gets a nice "remote: " prefix).

We could do something like this:

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index f2c6953a3..6204d3d00 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1670,6 +1670,8 @@ static const char *unpack(int err_fd, struct shallow_info *si)
 	if (!tmp_objdir) {
 		if (err_fd > 0)
 			close(err_fd);
+		rp_error("unable to create temporary object directory: %s",
+			 strerror(errno));
 		return "unable to create temporary object directory";
 	}
 	child.env = tmp_objdir_env(tmp_objdir);

and drop the "try to show the unpack failure" parts of my series, and
you'd end up with:

  $ git push ~/tmp/foo.git HEAD
  Counting objects: 209843, done.
  Delta compression using up to 8 threads.
  Compressing objects: 100% (52186/52186), done.
  remote: error: unable to create temporary object directory: Permission denied
  error: failed to push some refs to '/home/peff/tmp/foo.git'

but in cases where pack-objects _doesn't_ fail, existing git versions
do show the "unpack" error. So you'd see it twice.

I don't know if it's worth trying to hack around.

-Peff