commit-reach: replace queue_has_nonstale() scan with O(1) tracking

paint_down_to_common() and ahead_behind() call queue_has_nonstale()
on every iteration to decide whether to continue the walk.
queue_has_nonstale() performs a linear scan of the priority queue,
making the overall walk O(n*m) where n is the number of commits
walked and m is the queue size.

Introduce 'struct nonstale_queue', a thin wrapper around prio_queue
that maintains a 'max_nonstale' pointer — the lowest-priority
(oldest) non-stale commit seen so far. When this commit is popped,
every remaining queue entry is known to be stale, so the walk can
stop. This reduces the per-iteration termination check from O(m)
to O(1).

Uses <= 0 (not < 0) when comparing priorities so that among distinct
commits with equal priority (same generation and timestamp) the
last-enqueued one is tracked. Since prio_queue breaks ties by
insertion order, this ensures max_nonstale is always the last in its
priority class to be popped, making pointer equality on pop
sufficient for correctness.

The previous commit's ENQUEUED deduplication guarantees each commit
appears at most once in the queue, which is required for the pointer
equality check to be unambiguous.

On a large monorepo (3.7M commits), this yields ~2x end-to-end
speedup for merge-base calculations on deep import branches.
Profiling shows paint_down_to_common() drops from 50% to 4% of
total runtime (~27x faster), with the remaining time in commit
graph lookups and heap operations:

  Before: 8536ms / 5757ms / 4743ms  (three test cases)
  After:  3956ms / 4383ms / 1927ms

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Kristofer Karlsson <krka@spotify.com>

Author: spkrka

commit-reach.c

@@ -40,32 +40,62 @@ static int compare_commits_by_gen(const void *_a, const void *_b)
 	return 0;
 }
 
-static void prio_queue_put_dedup(struct prio_queue *queue, struct commit *c)
+/*
+ * A prio_queue with O(1) termination check.  'max_nonstale' tracks
+ * the lowest-priority non-stale commit enqueued so far; once it is
+ * popped, every remaining entry is known to be STALE.
+ */
+struct nonstale_queue {
+	struct prio_queue pq;
+	struct commit *max_nonstale;
+};
+
+static void nonstale_queue_put(struct nonstale_queue *queue,
+			       struct commit *c)
+{
+	struct commit *old = queue->max_nonstale;
+
+	prio_queue_put(&queue->pq, c);
+	if (c->object.flags & STALE)
+		return;
+	if (!old || queue->pq.compare(old, c, queue->pq.cb_data) <= 0)
+		queue->max_nonstale = c;
+}
+
+static struct commit *nonstale_queue_get(struct nonstale_queue *queue)
+{
+	struct commit *commit = prio_queue_get(&queue->pq);
+
+	if (commit == queue->max_nonstale)
+		queue->max_nonstale = NULL;
+
+	return commit;
+}
+
+static void clear_nonstale_queue(struct nonstale_queue *queue)
+{
+	clear_prio_queue(&queue->pq);
+	queue->max_nonstale = NULL;
+}
+
+static void nonstale_queue_put_dedup(struct nonstale_queue *queue,
+				     struct commit *c)
 {
 	if (c->object.flags & ENQUEUED)
 		return;
 	c->object.flags |= ENQUEUED;
-	prio_queue_put(queue, c);
+	nonstale_queue_put(queue, c);
 }
 
-static struct commit *prio_queue_get_dedup(struct prio_queue *queue)
+static struct commit *nonstale_queue_get_dedup(struct nonstale_queue *queue)
 {
-	struct commit *commit = prio_queue_get(queue);
+	struct commit *commit = nonstale_queue_get(queue);
+
 	if (commit)
 		commit->object.flags &= ~ENQUEUED;
 	return commit;
 }
 
-static int queue_has_nonstale(struct prio_queue *queue)
-{
-	for (size_t i = 0; i < queue->nr; i++) {
-		struct commit *commit = queue->array[i].data;
-		if (!(commit->object.flags & STALE))
-			return 1;
-	}
-	return 0;
-}
-
 /* all input commits in one and twos[] must have been parsed! */
 static int paint_down_to_common(struct repository *r,
 				struct commit *one, int n,
@@ -74,28 +104,30 @@ static int paint_down_to_common(struct repository *r,
 				enum merge_base_flags mb_flags,
 				struct commit_list **result)
 {
-	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
+	struct nonstale_queue queue = {
+		{ compare_commits_by_gen_then_commit_date }
+	};
 	int i;
 	timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
 	struct commit_list **tail = result;
 
 	if (!min_generation && !corrected_commit_dates_enabled(r))
-		queue.compare = compare_commits_by_commit_date;
+		queue.pq.compare = compare_commits_by_commit_date;
 
 	one->object.flags |= PARENT1;
 	if (!n) {
 		commit_list_append(one, result);
 		return 0;
 	}
-	prio_queue_put_dedup(&queue, one);
+	nonstale_queue_put_dedup(&queue, one);
 
 	for (i = 0; i < n; i++) {
 		twos[i]->object.flags |= PARENT2;
-		prio_queue_put_dedup(&queue, twos[i]);
+		nonstale_queue_put_dedup(&queue, twos[i]);
 	}
 
-	while (queue_has_nonstale(&queue)) {
-		struct commit *commit = prio_queue_get_dedup(&queue);
+	while (queue.max_nonstale) {
+		struct commit *commit = nonstale_queue_get_dedup(&queue);
 		struct commit_list *parents;
 		int flags;
 		timestamp_t generation = commit_graph_generation(commit);
@@ -133,7 +165,7 @@ static int paint_down_to_common(struct repository *r,
 			if ((p->object.flags & flags) == flags)
 				continue;
 			if (repo_parse_commit(r, p)) {
-				clear_prio_queue(&queue);
+				clear_nonstale_queue(&queue);
 				commit_list_free(*result);
 				*result = NULL;
 				/*
@@ -149,11 +181,11 @@ static int paint_down_to_common(struct repository *r,
 					     oid_to_hex(&p->object.oid));
 			}
 			p->object.flags |= flags;
-			prio_queue_put_dedup(&queue, p);
+			nonstale_queue_put_dedup(&queue, p);
 		}
 	}
 
-	clear_prio_queue(&queue);
+	clear_nonstale_queue(&queue);
 	commit_list_sort_by_date(result);
 	return 0;
 }
@@ -1057,11 +1089,11 @@ struct commit_list *get_reachable_subset(struct commit **from, size_t nr_from,
 define_commit_slab(bit_arrays, struct bitmap *);
 static struct bit_arrays bit_arrays;
 
-static void insert_no_dup(struct prio_queue *queue, struct commit *c)
+static void insert_no_dup(struct nonstale_queue *queue, struct commit *c)
 {
 	if (c->object.flags & PARENT2)
 		return;
-	prio_queue_put(queue, c);
+	nonstale_queue_put(queue, c);
 	c->object.flags |= PARENT2;
 }
 
@@ -1086,7 +1118,9 @@ void ahead_behind(struct repository *r,
 		  struct commit **commits, size_t commits_nr,
 		  struct ahead_behind_count *counts, size_t counts_nr)
 {
-	struct prio_queue queue = { .compare = compare_commits_by_gen_then_commit_date };
+	struct nonstale_queue queue = {
+		{ .compare = compare_commits_by_gen_then_commit_date }
+	};
 	size_t width = DIV_ROUND_UP(commits_nr, BITS_IN_EWORD);
 
 	if (!commits_nr || !counts_nr)
@@ -1109,8 +1143,8 @@ void ahead_behind(struct repository *r,
 		insert_no_dup(&queue, c);
 	}
 
-	while (queue_has_nonstale(&queue)) {
-		struct commit *c = prio_queue_get(&queue);
+	while (queue.max_nonstale) {
+		struct commit *c = nonstale_queue_get(&queue);
 		struct commit_list *p;
 		struct bitmap *bitmap_c = get_bit_array(c, width);
 
@@ -1152,10 +1186,10 @@ void ahead_behind(struct repository *r,
 
 	/* STALE is used here, PARENT2 is used by insert_no_dup(). */
 	repo_clear_commit_marks(r, PARENT2 | STALE);
-	for (size_t i = 0; i < queue.nr; i++)
-		free_bit_array(queue.array[i].data);
+	for (size_t i = 0; i < queue.pq.nr; i++)
+		free_bit_array(queue.pq.array[i].data);
 	clear_bit_arrays(&bit_arrays);
-	clear_prio_queue(&queue);
+	clear_nonstale_queue(&queue);
 }
 
 struct commit_and_index {