[PATCH v3 7/8] commit-reach: terminate merge-base walk when one paint side is exhausted

@@ -94,6 +94,9 @@ ends when one of the following conditions holds:
 
   1. The queue is empty.
   2. The queue contains only stale entries.
+  3. Side exhaustion: no pure PARENT1 or pure PARENT2 commits
+     remain in the queue, no pending merge-base candidates exist,
+     and the walk has entered the finite-generation region.
 
 Stale entry condition
 ~~~~~~~~~~~~~~~~~~~~~

@@ -104,6 +107,20 @@ existing candidates by proving one is an ancestor of another, but
 `remove_redundant()` handles that as a post-processing step, so it
 is safe to exit early.
 
+Side-exhaustion condition
+~~~~~~~~~~~~~~~~~~~~~~~~~
+A new merge-base requires commits from both sides to meet. When one
+side's exclusive counter reaches zero and there are no pending
+merge-base candidates, no future traversal step can produce a new
+candidate.
+
+This optimization only activates in the finite-generation region
+where topological ordering holds. In that region, children are
+always visited before parents, so paint flags are final at visit
+time and an exhausted side cannot reappear. In the INFINITY region,
+commit-date ordering can violate this guarantee, so the check is
+skipped.
+
 Related documentation
 ---------------------
 

@@ -131,6 +131,10 @@ static void paint_queue_put(struct paint_state *state,
 	}
 }
 
+/*
+ * Dequeue the next commit for the paint walk, or return NULL when
+ * no more merge bases can be discovered.
+ */
 static struct commit *paint_queue_get(struct paint_state *state)
 {
 	struct commit *commit = prio_queue_get(&state->queue);

@@ -140,9 +144,16 @@ static struct commit *paint_queue_get(struct paint_state *state)
 
 	commit->object.flags &= ~ENQUEUED;
 
-	if (!state->p1_count && !state->p2_count &&
-	    !state->pending_merge_bases)
-		return NULL;
+	if (!state->pending_merge_bases) {
+		/* only stale entries remain */
+		if (!state->p1_count && !state->p2_count)
+			return NULL;
+
+		/* one side is exhausted */
+		if ((!state->p1_count || !state->p2_count) &&
+		    commit_graph_generation(commit) < GENERATION_NUMBER_INFINITY)
+			return NULL;
+	}
 
 	paint_count_update(state, commit->object.flags, -1);
 	return commit;

@@ -188,7 +199,7 @@ static int paint_down_to_common(struct repository *r,
 		timestamp_t generation = commit_graph_generation(commit);
 		steps++;
 
-		if (min_generation && generation > last_gen)
+		if (generation > last_gen)
 			BUG("bad generation skip %"PRItime" > %"PRItime" at %s",
 			    generation, last_gen,
 			    oid_to_hex(&commit->object.oid));

@@ -220,7 +220,7 @@ test_expect_success 'in_merge_bases_many:self' '
 	EOF
 	echo "in_merge_bases_many(A,X):1" >expect &&
 	test_all_modes in_merge_bases_many &&
-	test_paint_down_steps 45 2 25 3
+	test_paint_down_steps 45 1 25 1
 '
 
 test_expect_success 'is_descendant_of:hit' '

@@ -337,7 +337,7 @@ test_expect_success 'merge-base --all commit-walk steps' '
 	>input &&
 	git rev-parse commit-9-1 >expect &&
 	run_all_modes git merge-base --all commit-9-9 commit-9-1 &&
-	test_paint_down_steps 81 80 81 81
+	test_paint_down_steps 81 9 57 10
 '
 
 test_expect_success 'reduce_heads' '