Subject: Re: [WIP/PATCH 3/3] submodule: helper to run foreach in parallel

From: Junio C Hamano <[email-masked]>
Date: Fri, 21 Aug 2015 12:23:37 -0700

Stefan Beller <[email-masked]> writes:

> +struct submodule_args {
> +	const char *name;
> +	const char *path;
> +	const char *sha1;
> +	const char *toplevel;
> +	const char *prefix;
> +	const char **cmd;
> +	struct submodule_output *out;
> +	sem_t *mutex;
> +};

I do not see what submodule_output looks like in the patch, but I
guess you are capturing _all_ output from a task and emitting
everything at the end, after the task is done?

I have to wonder if that is what people would expect and more
importantly if that is the most useful.  I am sympathetic to the
desire to avoid the output totally mixed up from different processes
emitting things in an uncoordinated manner, and "slurp everything
and then show everything in one go" is certainly _one_ way to do so,
but early feedback also counts.  Besides, because the order in which
tasks are dispatched and completed is unpredictable, you cannot
expect a machine parseable output _without_ assuming some help from
the command invoked by each task (e.g. by prefixing the task's output
with some string that identifies which submodule the output is about).

Once you assume that the command is _aware_ that it needs to help
the foreach-parallel infrastructure so that the user can sift their
collective outputs to make sense of them, wouldn't a line-buffered
intermixing also acceptable, and wouldn't it be a much lower impact
approach to solve the same problem?