run-command.c: use designated init for pp_init(), add "const"

Use a designated initializer to initialize those parts of pp_init()
that don't need any conditionals for their initialization, this sets
us on a path to pp_init() itself into mostly a validation and
allocation function.

Since we're doing that we can add "const" to some of the members of
the "struct parallel_processes", which helps to clarify and
self-document this code. E.g. we never alter the "data" pointer we
pass t user callbacks, nor (after the preceding change to stop
invoking online_cpus()) do we change "max_processes", the same goes
for the "ungroup" option.

We can also do away with a call to strbuf_init() in favor of macro
initialization, and to rely on other fields being NULL'd or zero'd.

Making members of a struct "const" rather that the pointer to the
struct itself is usually painful, as e.g. it precludes us from
incrementally setting up the structure. In this case we only set it up
with the assignment in run_process_parallel() and pp_init(), and don't
pass the struct pointer around as "const", so making individual
members "const" is worth the potential hassle for extra safety.

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

run-command.c

@@ -1498,9 +1498,9 @@ enum child_state {
 
 int run_processes_parallel_ungroup;
 struct parallel_processes {
-	void *data;
+	void *const data;
 
-	size_t max_processes;
+	const size_t max_processes;
 	size_t nr_processes;
 
 	get_next_task_fn get_next_task;
@@ -1520,7 +1520,7 @@ struct parallel_processes {
 	struct pollfd *pfd;
 
 	unsigned shutdown : 1;
-	unsigned ungroup : 1;
+	const unsigned ungroup : 1;
 
 	size_t output_owner;
 	struct strbuf buffered_output; /* of finished children */
@@ -1558,21 +1558,18 @@ static void handle_children_on_signal(int signo)
 }
 
 static void pp_init(struct parallel_processes *pp,
-		    size_t n,
 		    get_next_task_fn get_next_task,
 		    start_failure_fn start_failure,
-		    task_finished_fn task_finished,
+		    task_finished_fn task_finished)
-		    void *data, int ungroup)
 {
+	const size_t n = pp->max_processes;
+
 	if (!n)
 		BUG("you must provide a non-zero number of processes!");
 
-	pp->max_processes = n;
-
 	trace_printf("run_processes_parallel: preparing to run up to %"PRIuMAX" tasks",
 		     (uintmax_t)n);
 
-	pp->data = data;
 	if (!get_next_task)
 		BUG("you need to specify a get_next_task function");
 	pp->get_next_task = get_next_task;
@@ -1580,16 +1577,9 @@ static void pp_init(struct parallel_processes *pp,
 	pp->start_failure = start_failure ? start_failure : default_start_failure;
 	pp->task_finished = task_finished ? task_finished : default_task_finished;
 
-	pp->nr_processes = 0;
-	pp->output_owner = 0;
-	pp->shutdown = 0;
-	pp->ungroup = ungroup;
 	CALLOC_ARRAY(pp->children, n);
-	if (pp->ungroup)
+	if (!pp->ungroup)
-		pp->pfd = NULL;
-	else
 		CALLOC_ARRAY(pp->pfd, n);
-	strbuf_init(&pp->buffered_output, 0);
 
 	for (size_t i = 0; i < n; i++) {
 		strbuf_init(&pp->children[i].err, 0);
@@ -1789,13 +1779,17 @@ void run_processes_parallel(size_t n,
 	int output_timeout = 100;
 	int spawn_cap = 4;
 	int ungroup = run_processes_parallel_ungroup;
-	struct parallel_processes pp;
+	struct parallel_processes pp = {
+		.max_processes = n,
+		.data = pp_cb,
+		.buffered_output = STRBUF_INIT,
+		.ungroup = ungroup,
+	};
 
 	/* unset for the next API user */
 	run_processes_parallel_ungroup = 0;
 
-	pp_init(&pp, n, get_next_task, start_failure, task_finished, pp_cb,
+	pp_init(&pp, get_next_task, start_failure, task_finished);
-		ungroup);
 	while (1) {
 		for (i = 0;
 		    i < spawn_cap && !pp.shutdown &&