run-command: allow stdin for run_processes_parallel

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. 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.

`some_child.process.stdout_to_stderr`, however, remains unmodifiable by
`get_next_task()` - the rest of the run_processes_parallel() API depends
on child output in stderr.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -1586,6 +1586,14 @@ static int pp_start_one(struct parallel_processes *pp,
 	if (i == opts->processes)
 		BUG("bookkeeping is hard");
 
+	/*
+	 * 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)