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@xxxxxxxxxxxxxx/
>>
>> Reported-by: Anthony Sottile <asottile@xxxxxxxxx>
>> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
>> ---
>> 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.
