Subject: Re: [PATCH 6/6] hook API: fix v2.36.0 regression: hooks should be connected to a TTY

From: Emily Shaffer <[email-masked]>
Date: Thu, 28 Apr 2022 16:31:48 -0700

On Thu, Apr 21, 2022 at 02:25:31PM +0200, Ævar Arnfjörð Bjarmason wrote:
> 
> Fix a regression reported[1] in f443246b9f2 (commit: convert
> {pre-commit,prepare-commit-msg} hook to hook.h, 2021-12-22): Due to
> using the run_process_parallel() API in the earlier 96e7225b310 (hook:
> add 'run' subcommand, 2021-12-22) we'd capture the hook's stderr and
> stdout, and thus lose the connection to the TTY in the case of
> e.g. the "pre-commit" hook.
> 
> As a preceding commit notes GNU parallel's similar --ungroup option
> also has it emit output faster. While we're unlikely to have hooks
> that emit truly massive amounts of output (or where the performance
> thereof matters) it's still informative to measure the overhead. In a
> similar "seq" test we're now ~30% faster:
> 
> 	$ cat .git/hooks/seq-hook; git hyperfine -L rev origin/master,HEAD~0 -s 'make CFLAGS=-O3' './git hook run seq-hook'
> 	#!/bin/sh
> 
> 	seq 100000000
> 	Benchmark 1: ./git hook run seq-hook' in 'origin/master
> 	  Time (mean ± σ):     787.1 ms ±  13.6 ms    [User: 701.6 ms, System: 534.4 ms]
> 	  Range (min … max):   773.2 ms … 806.3 ms    10 runs
> 
> 	Benchmark 2: ./git hook run seq-hook' in 'HEAD~0
> 	  Time (mean ± σ):     603.4 ms ±   1.6 ms    [User: 573.1 ms, System: 30.3 ms]
> 	  Range (min … max):   601.0 ms … 606.2 ms    10 runs
> 
> 	Summary
> 	  './git hook run seq-hook' in 'HEAD~0' ran
> 	    1.30 ± 0.02 times faster than './git hook run seq-hook' in 'origin/master'
> 
> In the preceding commit we removed the "no_stdin=1" and
> "stdout_to_stderr=1" assignments. This change brings them back as with
> ".ungroup=1" the run_process_parallel() function doesn't provide them
> for us implicitly.
> 
> As an aside omitting the stdout_to_stderr=1 here would have all tests
> pass, except those that test "git hook run" itself in
> t1800-hook.sh. But our tests passing is the result of another test
> blind spot, as was the case with the regression being fixed here. The
> "stdout_to_stderr=1" for hooks is long-standing behavior, see
> e.g. 1d9e8b56fe3 (Split back out update_hook handling in receive-pack,
> 2007-03-10) and other follow-up commits (running "git log" with
> "--reverse -p -Gstdout_to_stderr" is a good start).
> 
> 1. https://lore.kernel.org/git/[email-masked]/
> 
> Reported-by: Anthony Sottile <[email-masked]>
> Signed-off-by: Ævar Arnfjörð Bjarmason <[email-masked]>
> ---
>  hook.c          |  8 +++++++-
>  t/t1800-hook.sh | 37 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/hook.c b/hook.c
> index 68ee4030551..f5eef1d561b 100644
> --- a/hook.c
> +++ b/hook.c
> @@ -53,7 +53,9 @@ static int pick_next_hook(struct child_process *cp,
>  	if (!hook_path)
>  		return 0;
>  
> +	cp->no_stdin = 1;
>  	strvec_pushv(&cp->env_array, hook_cb->options->env.v);
> +	cp->stdout_to_stderr = 1;
>  	cp->trace2_hook_name = hook_cb->hook_name;
>  	cp->dir = hook_cb->options->dir;
>  
> @@ -119,16 +121,20 @@ int run_hooks_opt(const char *hook_name, struct run_hooks_opt *options)
>  		.options = options,
>  	};
>  	const char *const hook_path = find_hook(hook_name);
> -	int jobs = 1;
> +	const int jobs = 1;
>  	int ret = 0;
>  	struct run_process_parallel_opts run_opts = {
>  		.tr2_category = "hook",
>  		.tr2_label = hook_name,
> +		.ungroup = jobs == 1,

So here we do set .ungroup based only on the job count - but we do that
only for hooks, which means someone else could conceivably come across
similar bug in their later use of run_processes_parallel. Is the reason
for doing this in the context of the hook library instead of in the
context of run_processes_parallel library just because we are not sure
if it will break other parallel callers? Or some other reason?

 - Emily

>  	};
>  
>  	if (!options)
>  		BUG("a struct run_hooks_opt must be provided to run_hooks");
>  
> +	if (jobs != 1 || !run_opts.ungroup)
> +		BUG("TODO: think about & document order & interleaving of parallel hook output");
> +
>  	if (options->invoked_hook)
>  		*options->invoked_hook = 0;
>  
> diff --git a/t/t1800-hook.sh b/t/t1800-hook.sh
> index 1e4adc3d53e..f22754deccc 100755
> --- a/t/t1800-hook.sh
> +++ b/t/t1800-hook.sh
> @@ -4,6 +4,7 @@ test_description='git-hook command'
>  
>  TEST_PASSES_SANITIZE_LEAK=true
>  . ./test-lib.sh
> +. "$TEST_DIRECTORY"/lib-terminal.sh
>  
>  test_expect_success 'git hook usage' '
>  	test_expect_code 129 git hook &&
> @@ -120,4 +121,40 @@ test_expect_success 'git -c core.hooksPath=<PATH> hook run' '
>  	test_cmp expect actual
>  '
>  
> +test_hook_tty() {
> +	local fd="$1" &&
> +
> +	cat >expect &&
> +
> +	test_when_finished "rm -rf repo" &&
> +	git init repo &&
> +
> +	test_hook -C repo pre-commit <<-EOF &&
> +	{
> +		test -t 1 && echo >&$fd STDOUT TTY || echo >&$fd STDOUT NO TTY &&
> +		test -t 2 && echo >&$fd STDERR TTY || echo >&$fd STDERR NO TTY
> +	} $fd>actual
> +	EOF
> +
> +	test_commit -C repo A &&
> +	test_commit -C repo B &&
> +	git -C repo reset --soft HEAD^ &&
> +	test_terminal git -C repo commit -m"B.new" &&
> +	test_cmp expect repo/actual
> +}
> +
> +test_expect_success TTY 'git hook run: stdout and stderr are connected to a TTY: STDOUT redirect' '
> +	test_hook_tty 1 <<-\EOF
> +	STDOUT NO TTY
> +	STDERR TTY
> +	EOF
> +'
> +
> +test_expect_success TTY 'git hook run: stdout and stderr are connected to a TTY: STDERR redirect' '
> +	test_hook_tty 2 <<-\EOF
> +	STDOUT TTY
> +	STDERR NO TTY
> +	EOF
> +'
> +
>  test_done
> -- 
> 2.36.0.893.g80a51c675f6
>