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From: Junio C Hamano <gitster@pobox.com>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: git@vger.kernel.org, Emily Shaffer <emilyshaffer@google.com>,
	Eric Sunshine <sunshine@sunshineco.com>,
	Taylor Blau <me@ttaylorr.com>,
	Michael Strawbridge <michael.strawbridge@amd.com>,
	Phillip Wood <phillip.wood123@gmail.com>
Subject: Re: [PATCH v2 2/5] run-command: allow stdin for run_processes_parallel
Date: Wed, 08 Feb 2023 13:09:26 -0800	[thread overview]
Message-ID: <xmqq357frhix.fsf@gitster.g> (raw)
In-Reply-To: <patch-v2-2.5-9a178577dcc-20230208T191924Z-avarab@gmail.com> (=?utf-8?B?IsOGdmFyIEFybmZqw7Zyw7A=?= Bjarmason"'s message of "Wed, 8 Feb 2023 20:21:12 +0100")

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> From: Emily Shaffer <emilyshaffer@google.com>
>
> While it makes sense not to inherit stdin from the parent process to
> avoid deadlocking, it's not necessary to completely ban stdin to
> children.

I do not think deadlock avoidance is an issue.  Unpredictable
feeding of pieces of input into multiple children is.  One possible
semantics is to grab the input and dup/tee into all the children, so 
that each child gets its own copy to process.  A hook like "pre-receive",
when it has more than one scripts listening to the event, may want
such a semantics.  Another possible semantics is to give priority
among the children running simultaneously and feed only one child,
while starving others.

> An informed user should be able to configure stdin safely. By
> setting `some_child.process.no_stdin=1` before calling `get_next_task()`
> we provide a reasonable default behavior but enable users to set up
> stdin streaming for themselves during the callback.

I _think_ this alludes to the latter, e.g. "only one child is
allowed, and the one that controls what children are spawned sets
no_stdin for everybody but the chosen one".  We may want to be a bit
more explicit in the proposed log message and definitely in the
documentation.

The implementation is "nice".

> +	/*
> +	 * By default, do not inherit stdin from the parent process - otherwise,
> +	 * all children would share stdin! Users may overwrite this to provide
> +	 * something to the child's stdin by having their 'get_next_task'
> +	 * callback assign 0 to .no_stdin and an appropriate integer to .in.
> +	 */
> +	pp->children[i].process.no_stdin = 1;
> +
>  	code = opts->get_next_task(&pp->children[i].process,
>  				   opts->ungroup ? NULL : &pp->children[i].err,
>  				   opts->data,
> @@ -1601,7 +1609,6 @@ static int pp_start_one(struct parallel_processes *pp,
>  		pp->children[i].process.err = -1;
>  		pp->children[i].process.stdout_to_stderr = 1;
>  	}
> -	pp->children[i].process.no_stdin = 1;
>  
>  	if (start_command(&pp->children[i].process)) {
>  		if (opts->start_failure)

  reply	other threads:[~2023-02-08 21:09 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-23 17:15 [PATCH 0/5] hook API: support stdin, convert post-rewrite Ævar Arnfjörð Bjarmason
2023-01-23 17:15 ` [PATCH 1/5] run-command.c: remove dead assignment in while-loop Ævar Arnfjörð Bjarmason
2023-01-23 22:48   ` Junio C Hamano
2023-01-23 17:15 ` [PATCH 2/5] run-command: allow stdin for run_processes_parallel Ævar Arnfjörð Bjarmason
2023-01-23 22:52   ` Junio C Hamano
2023-01-23 17:15 ` [PATCH 3/5] hook API: support passing stdin to hooks, convert am's 'post-rewrite' Ævar Arnfjörð Bjarmason
2023-01-23 23:10   ` Junio C Hamano
2023-01-23 23:13   ` Junio C Hamano
2023-01-23 17:15 ` [PATCH 4/5] sequencer: use the new hook API for the simpler "post-rewrite" call Ævar Arnfjörð Bjarmason
2023-01-24 14:46   ` Phillip Wood
2023-01-27 15:08     ` Phillip Wood
2023-01-23 17:15 ` [PATCH 5/5] hook: support a --to-stdin=<path> option for testing Ævar Arnfjörð Bjarmason
2023-01-24  0:55   ` Junio C Hamano
2023-01-24  0:59     ` Michael Strawbridge
2023-02-08 19:21 ` [PATCH v2 0/5] hook API: support stdin, convert post-rewrite Ævar Arnfjörð Bjarmason
2023-02-08 19:21   ` [PATCH v2 1/5] run-command.c: remove dead assignment in while-loop Ævar Arnfjörð Bjarmason
2023-02-08 21:03     ` Junio C Hamano
2023-02-08 19:21   ` [PATCH v2 2/5] run-command: allow stdin for run_processes_parallel Ævar Arnfjörð Bjarmason
2023-02-08 21:09     ` Junio C Hamano [this message]
2023-02-08 19:21   ` [PATCH v2 3/5] hook API: support passing stdin to hooks, convert am's 'post-rewrite' Ævar Arnfjörð Bjarmason
2023-02-08 21:12     ` Junio C Hamano
2023-02-08 19:21   ` [PATCH v2 4/5] sequencer: use the new hook API for the simpler "post-rewrite" call Ævar Arnfjörð Bjarmason
2023-02-08 21:17     ` Junio C Hamano
2023-02-08 19:21   ` [PATCH v2 5/5] hook: support a --to-stdin=<path> option Ævar Arnfjörð Bjarmason
2023-02-08 21:22     ` Junio C Hamano
2023-02-09  1:56       ` Ævar Arnfjörð Bjarmason
2023-02-08 21:23   ` [PATCH v2 0/5] hook API: support stdin, convert post-rewrite Junio C Hamano
2023-02-03 12:15 Ævar Arnfjörð Bjarmason
2023-02-03 12:15 ` [PATCH v2 2/5] run-command: allow stdin for run_processes_parallel Ævar Arnfjörð Bjarmason

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