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

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Thu, Oct 20 2022, Calvin Wan wrote:

> Output from child processes and callbacks may not be necessary to
> print out for every invoker of run_processes_parallel. Add
> hide_output as an option to not print said output.
>
> Signed-off-by: Calvin Wan <calvinwan@google.com>
> ---
>  run-command.c               | 8 +++++---
>  run-command.h               | 9 +++++++++
>  t/helper/test-run-command.c | 6 ++++++
>  t/t0061-run-command.sh      | 6 ++++++
>  4 files changed, 26 insertions(+), 3 deletions(-)
>
> diff --git a/run-command.c b/run-command.c
> index 03787bc7f5..3aa28ad6dc 100644
> --- a/run-command.c
> +++ b/run-command.c
> @@ -1603,7 +1603,8 @@ static void pp_cleanup(struct parallel_processes *pp,
>  	 * When get_next_task added messages to the buffer in its last
>  	 * iteration, the buffered output is non empty.
>  	 */
> -	strbuf_write(&pp->buffered_output, stderr);
> +	if (!opts->hide_output)
> +		strbuf_write(&pp->buffered_output, stderr);
>  	strbuf_release(&pp->buffered_output);
>  
>  	sigchain_pop_common();
> @@ -1754,7 +1755,7 @@ static int pp_collect_finished(struct parallel_processes *pp,
>  			pp->pfd[i].fd = -1;
>  		child_process_init(&pp->children[i].process);
>  
> -		if (opts->ungroup) {
> +		if (opts->ungroup || opts->hide_output) {
>  			; /* no strbuf_*() work to do here */
>  		} else if (i != pp->output_owner) {
>  			strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
> @@ -1826,7 +1827,8 @@ void run_processes_parallel(const struct run_process_parallel_opts *opts)
>  				pp.children[i].state = GIT_CP_WAIT_CLEANUP;
>  		} else {
>  			pp_buffer_stderr(&pp, opts, output_timeout);
> -			pp_output(&pp);
> +			if (!opts->hide_output)
> +				pp_output(&pp);
>  		}
>  		code = pp_collect_finished(&pp, opts);
>  		if (code) {
> diff --git a/run-command.h b/run-command.h
> index b4584c3698..4570812c08 100644
> --- a/run-command.h
> +++ b/run-command.h
> @@ -496,6 +496,11 @@ struct run_process_parallel_opts
>  	 */
>  	unsigned int ungroup:1;
>  
> +	/**
> +	 * hide_output: see 'hide_output' in run_processes_parallel() below.
> +	 */
> +	unsigned int hide_output:1;
> +
>  	/**
>  	 * get_next_task: See get_next_task_fn() above. This must be
>  	 * specified.
> @@ -547,6 +552,10 @@ struct run_process_parallel_opts
>   * NULL "struct strbuf *out" parameter, and are responsible for
>   * emitting their own output, including dealing with any race
>   * conditions due to writing in parallel to stdout and stderr.
> + * 
> + * If the "hide_output" option is set, any output in the "struct strbuf
> + * *out" parameter will not be printed by this function. This includes
> + * output from child processes as well as callbacks.
>   */
>  void run_processes_parallel(const struct run_process_parallel_opts *opts);
>  
> diff --git a/t/helper/test-run-command.c b/t/helper/test-run-command.c
> index e9b41419a0..1af443db4d 100644
> --- a/t/helper/test-run-command.c
> +++ b/t/helper/test-run-command.c
> @@ -446,6 +446,12 @@ int cmd__run_command(int argc, const char **argv)
>  		opts.ungroup = 1;
>  	}
>  
> +	if (!strcmp(argv[1], "--hide-output")) {
> +		argv += 1;
> +		argc -= 1;
> +		opts.hide_output = 1;
> +	}
> +
>  	if (!strcmp(argv[1], "--pipe-output")) {
>  		argv += 1;
>  		argc -= 1;
> diff --git a/t/t0061-run-command.sh b/t/t0061-run-command.sh
> index e50e57db89..a0219f6093 100755
> --- a/t/t0061-run-command.sh
> +++ b/t/t0061-run-command.sh
> @@ -180,6 +180,12 @@ test_expect_success 'run_command runs ungrouped in parallel with more tasks than
>  	test_line_count = 4 err
>  '
>  
> +test_expect_success 'run_command hides output when run in parallel' '
> +	test-tool run-command --hide-output run-command-parallel 4 sh -c "printf \"%s\n%s\n\" Hello World" >out 2>err &&
> +	test_must_be_empty out &&
> +	test_must_be_empty err
> +'
> +
>  cat >expect <<-EOF
>  preloaded output of a child
>  asking for a quick stop

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...