run-command API: don't fall back on online_cpus()

When a "jobs = 0" is passed let's BUG() out rather than fall back on
online_cpus(). The default behavior was added when this API was
implemented in c553c72eed (run-command: add an asynchronous parallel
child processor, 2015-12-15).

Most of our code in-tree that scales up to "online_cpus()" by default
calls that function by itself. Keeping this default behavior just for
the sake of two callers means that we'd need to maintain this one spot
where we're second-guessing the config passed down into pp_init().

The preceding commit has an overview of the API callers that passed
"jobs = 0". There were only two of them (actually three, but they
resolved to these two config parsing codepaths).

The "fetch.parallel" caller already had a test for the
"fetch.parallel=0" case added in 0353c68818 (fetch: do not run a
redundant fetch from submodule, 2022-05-16), but there was no such
test for "submodule.fetchJobs". Let's add one here.

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

builtin/fetch.c

@@ -122,6 +122,8 @@ static int git_fetch_config(const char *k, const char *v, void *cb)
 		fetch_parallel_config = git_config_int(k, v);
 		if (fetch_parallel_config < 0)
 			die(_("fetch.parallel cannot be negative"));
+		if (!fetch_parallel_config)
+			fetch_parallel_config = online_cpus();
 		return 0;
 	}
 

run-command.c

@@ -1564,8 +1564,8 @@ static void pp_init(struct parallel_processes *pp,
 		    task_finished_fn task_finished,
 		    void *data, int ungroup)
 {
-	if (n < 1)
-		n = online_cpus();
+	if (!n)
+		BUG("you must provide a non-zero number of processes!");
 
 	pp->max_processes = n;
 
@@ -1835,8 +1835,7 @@ void run_processes_parallel_tr2(size_t n, get_next_task_fn get_next_task,
 				task_finished_fn task_finished, void *pp_cb,
 				const char *tr2_category, const char *tr2_label)
 {
-	trace2_region_enter_printf(tr2_category, tr2_label, NULL, "max:%d",
-				   ((n < 1) ? online_cpus() : n));
+	trace2_region_enter_printf(tr2_category, tr2_label, NULL, "max:%d", n);
 
 	run_processes_parallel(n, get_next_task, start_failure,
 			       task_finished, pp_cb);

submodule-config.c

@@ -303,6 +303,8 @@ int parse_submodule_fetchjobs(const char *var, const char *value)
 	int fetchjobs = git_config_int(var, value);
 	if (fetchjobs < 0)
 		die(_("negative values not allowed for submodule.fetchJobs"));
+	if (!fetchjobs)
+		fetchjobs = online_cpus();
 	return fetchjobs;
 }
 

t/t5526-fetch-submodules.sh

@@ -714,7 +714,13 @@ test_expect_success 'fetching submodules respects parallel settings' '
 		GIT_TRACE=$(pwd)/trace.out git fetch &&
 		grep "8 tasks" trace.out &&
 		GIT_TRACE=$(pwd)/trace.out git fetch --jobs 9 &&
-		grep "9 tasks" trace.out
+		grep "9 tasks" trace.out &&
+		>trace.out &&
+
+		GIT_TRACE=$(pwd)/trace.out git -c submodule.fetchJobs=0 fetch &&
+		grep "preparing to run up to [0-9]* tasks" trace.out &&
+		! grep "up to 0 tasks" trace.out &&
+		>trace.out
 	)
 '