negotiator/skipping: avoid stack overflow

mark_common() in negotiator/skipping.c may overflow the stack due to
recursive function calls. Avoid this by instead recursing using a
heap-allocated data structure.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

negotiator/skipping.c

@@ -86,21 +86,26 @@ static int clear_marks(const char *refname, const struct object_id *oid,
 /*
  * Mark this SEEN commit and all its SEEN ancestors as COMMON.
  */
-static void mark_common(struct data *data, struct commit *c)
+static void mark_common(struct data *data, struct commit *seen_commit)
 {
-	struct commit_list *p;
+	struct prio_queue queue = { NULL };
+	struct commit *c;
 
-	if (c->object.flags & COMMON)
-		return;
-	c->object.flags |= COMMON;
-	if (!(c->object.flags & POPPED))
-		data->non_common_revs--;
+	prio_queue_put(&queue, seen_commit);
+	while ((c = prio_queue_get(&queue))) {
+		struct commit_list *p;
+		if (c->object.flags & COMMON)
+			return;
+		c->object.flags |= COMMON;
+		if (!(c->object.flags & POPPED))
+			data->non_common_revs--;
 
-	if (!c->object.parsed)
-		return;
-	for (p = c->parents; p; p = p->next) {
-		if (p->item->object.flags & SEEN)
-			mark_common(data, p->item);
+		if (!c->object.parsed)
+			return;
+		for (p = c->parents; p; p = p->next) {
+			if (p->item->object.flags & SEEN)
+				prio_queue_put(&queue, p->item);
+		}
 	}
 }