pack-objects: take lock before accessing `remaining`

When checking the conditional of "while (me->remaining)", we did not
hold the lock. Calling find_deltas would still be safe, since it checks
"remaining" (after taking the lock) and is able to handle all values. In
fact, this could (currently) not trigger any bug: a bug could happen if
`remaining` transitioning from zero to non-zero races with the evaluation
of the while-condition, but these are always separated by the
data_ready-mechanism.

Make sure we have the lock when we read `remaining`. This does mean we
release it just so that find_deltas can take it immediately again. We
could tweak the contract so that the lock should be taken before calling
find_deltas, but let's defer that until someone can actually show that
"unlock+lock" has a measurable negative impact.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -2171,7 +2171,10 @@ static void *threaded_find_deltas(void *arg)
 {
 	struct thread_params *me = arg;
 
+	progress_lock();
 	while (me->remaining) {
+		progress_unlock();
+
 		find_deltas(me->list, &me->remaining,
 			    me->window, me->depth, me->processed);
 
@@ -2193,7 +2196,10 @@ static void *threaded_find_deltas(void *arg)
 			pthread_cond_wait(&me->cond, &me->mutex);
 		me->data_ready = 0;
 		pthread_mutex_unlock(&me->mutex);
+
+		progress_lock();
 	}
+	progress_unlock();
 	/* leave ->working 1 so that this doesn't get more work assigned */
 	return NULL;
 }