Subject: Re: [PATCH] t1400: avoid SIGPIPE race condition on fifo

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 16 Sep 2021 06:39:37 +0200

On Wed, Sep 15, 2021 at 01:24:52PM -0400, Jeff King wrote:
> On Wed, Sep 15, 2021 at 01:13:25PM -0400, Jeff King wrote:
> 
> > I think this test may be racy. I saw a strange failure from it in CI:
> > 
> >   https://github.com/peff/git/runs/3605506649?check_suite_focus=true#step:5:6734
> > 
> > I can't reproduce the problem locally with "--stress", but the failure
> > there is on macOS (and likewise, a nearby run failed with a timeout just
> > for macOS, which could be caused by a racy deadlock).
> 
> Ah, I just wasn't trying hard enough. Using --run=1,190 lets it run a
> lot more tightly, and I got a failure pretty quickly. Here's the fix (on
> top of ps/update-ref-batch-flush).
> 
> -- >8 --
> Subject: [PATCH] t1400: avoid SIGPIPE race condition on fifo
> 
> t1400.190 sometimes fails or even hangs because of the way it uses
> fifos. Our goal is to interactively read and write lines from
> update-ref, so we have two fifos, in and out. We open a descriptor
> connected to "in" and redirect output to that, so that update-ref does
> not see EOF as it would if we opened and closed it for each "echo" call.
> 
> But we don't do the same for the output. This leads to a race where our
> "read response <out" has not yet opened the fifo, but update-ref tries
> to write to it and gets SIGPIPE. This can result in the test failing, or
> worse, hanging as we wait forever for somebody to write to the pipe.
> 
> This is the same proble we fixed in 4783e7ea83 (t0008: avoid SIGPIPE

Type: "proble" -> "problem".

> race condition on fifo, 2013-07-12), and we can fix it the same way, by
> opening a second long-running descriptor.
> 
> Before this patch, running:
> 
>   ./t1400-update-ref.sh --run=1,190 --stress
> 
> failed or hung within a few dozen iterations. After it, I ran it for
> several hundred without problems.
> 
> Signed-off-by: Jeff King <[email-masked]>
> ---
>  t/t1400-update-ref.sh | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
> index 1e754e258f..0d4f73acaa 100755
> --- a/t/t1400-update-ref.sh
> +++ b/t/t1400-update-ref.sh
> @@ -1603,19 +1603,21 @@ test_expect_success PIPE 'transaction flushes status updates' '
>  	(git update-ref --stdin <in >out &) &&
>  
>  	exec 9>in &&
> +	exec 8<out &&
>  	test_when_finished "exec 9>&-" &&
> +	test_when_finished "exec 8<&-" &&
>  
>  	echo "start" >&9 &&
>  	echo "start: ok" >expected &&
> -	read line <out &&
> +	read line <&8 &&
>  	echo "$line" >actual &&
>  	test_cmp expected actual &&
>  
>  	echo "create refs/heads/flush $A" >&9 &&
>  
>  	echo prepare >&9 &&
>  	echo "prepare: ok" >expected &&
> -	read line <out &&
> +	read line <&8 &&
>  	echo "$line" >actual &&
>  	test_cmp expected actual &&
>  
> @@ -1625,7 +1627,7 @@ test_expect_success PIPE 'transaction flushes status updates' '
>  
>  	echo commit >&9 &&
>  	echo "commit: ok" >expected &&
> -	read line <out &&
> +	read line <&8 &&
>  	echo "$line" >actual &&
>  	test_cmp expected actual
>  '
> -- 
> 2.33.0.917.g33ebf6a5f6
> 

Thanks a lot for digging and fixing my test. The patch looks good to me.

Patrick