Subject: Re: [PATCH v2 8/8] hook API: fix v2.36.0 regression: hooks should be connected to a TTY

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Thu, 26 May 2022 20:23:23 +0200

On Thu, May 26 2022, Emily Shaffer wrote:

> On Wed, May 18, 2022 at 10:05:24PM +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 "stdout_to_stderr=1" assignment
>> as being redundant. This change brings it 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          |  5 +++++
>>  t/t1800-hook.sh | 37 +++++++++++++++++++++++++++++++++++++
>>  2 files changed, 42 insertions(+)
>> 
>> diff --git a/hook.c b/hook.c
>> index dc498ef5c39..5f31b60384a 100644
>> --- a/hook.c
>> +++ b/hook.c
>> @@ -54,6 +54,7 @@ static int pick_next_hook(struct child_process *cp,
>>  		return 0;
>>  
>>  	strvec_pushv(&cp->env_array, hook_cb->options->env.v);
>> +	cp->stdout_to_stderr = 1; /* because of .ungroup = 1 */
>>  	cp->trace2_hook_name = hook_cb->hook_name;
>>  	cp->dir = hook_cb->options->dir;
>>  
>> @@ -126,6 +127,7 @@ int run_hooks_opt(const char *hook_name, struct run_hooks_opt *options)
>>  		.tr2_label = hook_name,
>>  
>>  		.jobs = jobs,
>> +		.ungroup = jobs == 1,
>
> I mentioned it on patch 5, but I actually do not see a reason why we
> shouldn't do this logic in run_processes_parallel instead of just for
> the hooks. If someone can mention a reason we want to buffer child
> processes we're running in series I'm all ears, of course.
>
>>  
>>  		.get_next_task = pick_next_hook,
>>  		.start_failure = notify_start_failure,
>> @@ -136,6 +138,9 @@ int run_hooks_opt(const char *hook_name, struct run_hooks_opt *options)
>>  	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");
>
> Doesn't this mean we're actually disallowing parallel hooks entirely? I
> don't think that's necessary or desired. I guess right now when the
> config isn't used, there's not really a way to provide parallel hooks,
> but I also think this will cause unnecessary conflicts for Google who is
> carrying config hooks downstream. I know that's not such a great reason.
> But it seems weird to be explicitly using the parallel processing
> framework, but then say, "oh, but we actually don't want to run in
> parallel, that's a BUG()".

I can just drop this paranoia. I figured it was prudent to leave this
landmine in place so we'd definitely remember to re-visit this aspect of
it, but I think there's 0% that we'll forget. So I'll make it less
paranoid.