Subject: Re: [PATCH v3 2/6] run-command: add hide_output to run_processes_parallel_opts

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 25 Oct 2022 21:32:13 +0200

On Mon, Oct 24 2022, Calvin Wan wrote:

>> I may just be missing something, but doesn't "struct child_process"
>> already have e.g. "no_stderr", "no_stdout" etc. that we can use?
>> I.e. isn't this thing equivalent to running:
>>
>>         your-command >/dev/null 2>/dev/null
>>
>> Which is what the non-parallel API already supports.
>>
>> Now, IIRC if you just set that in the "get_next_task" callback it won't
>> work in the parallel API, or you'll block waiting for I/O that'll never
>> come or whatever.
>>
>> But that'll be because the parallel interface currently only suppors a
>> subset of the full "child_process" combination of options, and maybe it
>> doesn't grok this.
>>
>> But if that's the case we should just extend the API to support
>> "no_stdout", "no_stderr" etc., no?
>>
>> I.e. hypothetically the parallel one could support 100% of the "struct
>> child_process" combination of options, we just haven't bothered yet.
>>
>> But I don't see why the parallel API should grow options that we already
>> have in "struct child_process", instead we should set them there, and it
>> should gradually learn to deal with them.
>>
>> I think it's also fine to have some basic sanity checks there, e.g. I
>> could see how for something like this we don't want to support piping
>> only some children to /dev/null but not others, and that it should be
>> all or nothing (maybe it makes state management when we loop over them
>> easier).
>>
>> Or again, maybe I'm missing something...
>
> Shouldn't the options that are set in "child_process" be abstracted away
> from "parallel_processes"?

In general yes, and no :)

Our main interafce should probably be "just set
these in the 'struct child_process' we hand you", but the parallel API
might want to assert certain things about those settings, as some of
them may conflict with its assumptions.

> Setting "no_stdout", "no_stderr", etc. in a
> "child_process" shouldn't imply that we still pass the stdout and stderr to
>  "parallel_processes" and then we send the output to "/dev/null".

Sure, but if they're not producing any output because it's being piped
to /dev/null how worthwhile is it to optimize that?

We still can optimize it, but I still think the interface should just be
the equivalent of:

	parallel -k -j100% 'sleep 0.0$RANDOM && echo {} >/dev/null' ::: {1..100}

Whereas what you seem to be trying to implement is the equivalent of a:

	parallel -u -j100% 'sleep 0.0$RANDOM && echo {} ::: {1..100} >/dev/null

Except as an option to the parallel API, but the end result seems to be
equivalent.

> That being said, I can understand the aversion to adding an option like
> this that doesn't also add support for stdout and stderr. I can remove this
> patch and instead reset the buffer inside of pipe_output and task_finished
> in a later patch

I'm not necessarily opposed to it, just puzzled about it, maybe I don't
have the full picture.

In general I highly recomend looking at whatever GNU parallel is doing,
and seeing if new features in run-command.[ch] can map to that mental
model.

Our API is basically a small subset of its featureset, and I've found it
useful both to steal ideas from there, and to test
assumptions. E.g. "ungroup" is just a straight rip-off of the
"--ungroup" option, it's also had to think about combining various
options we don't have yet (but might want).

In that case the supervisor API/parallel(1) needs to do something
special, but for "I don't want output" it seems best to just do that at
the worker level, i.e. equivalent to piping to /dev/null.

Having a bias towards that approach also makes it easier to convert
things to running in parallel, i.e. you just (mostly) keep your current
"struct child_process", and don't need to find the equivalents in the
parallel API.