Subject: Re: [PATCH] t4053: avoid race when killing background processes

From: Junio C Hamano <[email-masked]>
Date: Thu, 10 Aug 2023 10:40:03 -0700

"Phillip Wood via GitGitGadget" <[email-masked]> writes:

> From: Phillip Wood <[email-masked]>
>
> The test 'diff --no-index reads from pipes' starts a couple of
> background processes that write to the pipes that are passed to "diff
> --no-index". If the test passes then we expect these processes to exit
> as all their output will have been read. However if the test fails
> then we want to make sure they do not hang about on the users machine
> and the test remembers they should be killed by calling
>
>       test_when_finished  "! kill $!"
>
> after each background process is created. Unfortunately there is a
> race where test_when_finished may run before the background process
> exits even when all its output has been read resulting in the kill
> command succeeding which causes the test to fail. Fix this by ignoring
> the exit status of the kill command. If the diff is successful we
> could instead wait for the background process to exit and check their
> status but that feels like it is testing the platform's printf
> implementation rather than git's code.
>
> Reported-by: Jeff King <[email-masked]>
> Signed-off-by: Phillip Wood <[email-masked]>
> ---
>     t4053: avoid race when killing background processes
>     
>     Thanks to Peff for reporting this. Junio - this fixes a regression
>     introduced in the current cycle. It is fairly minor though so I'm not
>     sure if you want to pick it up now or wait until 2.42.0 is out.

This did not cut -rc1 that was scheduled for yesterday, but a fix
for a new test added during the cycle is a very welcome addition.

While I can see that "kill" in the when-finished handler may or may
not find the backgrounded process by the time it is run, and
ignoring its exit status (hence keeping test_when_finished happy)
would be a reasonable thing to do.  I can understand if this patch
is to fix a different symptom, namely, when-finished handler
sometimes fails and makes the test fail.

But I am not sure how this causes the test to "hang", which
presumably is a symptom that somebody is trying to read from
a pipe that nobody is making progress to write into?  We will
send a signal either way to the writers, and the only difference is
that we ignore the exit code.

Granted, when-finished handlers are concatenated with "&&-", and one
"kill"s failure will cause the other "kill" not to run, so we may
send a signal to only one but not to the other, but that should all
happen after "diff --no-index" returns, so it still does not explain
the "hang".

Puzzled...

> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1571%2Fphillipwood%2Fdiff-no-index-pipes-fixes-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1571/phillipwood/diff-no-index-pipes-fixes-v1
> Pull-Request: https://github.com/gitgitgadget/git/pull/1571
>
>  t/t4053-diff-no-index.sh | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/t/t4053-diff-no-index.sh b/t/t4053-diff-no-index.sh
> index a28b9ff2434..1fb7d334620 100755
> --- a/t/t4053-diff-no-index.sh
> +++ b/t/t4053-diff-no-index.sh
> @@ -248,11 +248,11 @@ test_expect_success PIPE,SYMLINKS 'diff --no-index reads from pipes' '
>  	{
>  		(test_write_lines a b c >old) &
>  	} &&
> -	test_when_finished "! kill $!" &&
> +	test_when_finished "kill $! || :" &&
>  	{
>  		(test_write_lines a x c >new) &
>  	} &&
> -	test_when_finished "! kill $!" &&
> +	test_when_finished "kill $! || :" &&
>  
>  	cat >expect <<-EOF &&
>  	diff --git a/old b/new-link
>
> base-commit: a82fb66fed250e16d3010c75404503bea3f0ab61