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

> 
> 
> 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/CA+dzEBn108QoMA28f0nC8K21XT+Afua0V2Qv8XkR8rAeqUCCZw@mail.gmail.com/
> >> 
> >> Reported-by: Anthony Sottile <asottile@umich.edu>
> >> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> >> ---
> >>  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.