commit-reach: terminate merge-base walk when one paint side is exhausted

Add an early termination check to paint_down_to_common() using the
per-side counters introduced earlier. Once the walk enters the
finite-generation region, terminate early when one side's exclusive
count drops to zero -- no new merge-base can form without both paint
sides meeting.

The check also waits for pending_merge_bases to reach zero, ensuring
all merge-base candidates have been dequeued and recorded before
exiting.

The INFINITY gate ensures correctness: commits without a commit-graph
entry have GENERATION_NUMBER_INFINITY and are ordered by commit date,
which is not topologically reliable. The optimization only fires
once the walk enters the finite-generation region where ordering
guarantees hold.

Step counts measured with trace2 on git.git with commit-graph:

  merge-base --all v2.0.0 v2.55.0-rc1:
    before: 72264 steps    after: 44589 steps

  merge-base --all v2.55.0-rc1 v2.55.0-rc1~5:
    before:   110 steps    after:     7 steps

Helped-by: Derrick Stolee <stolee@gmail.com>
Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Kristofer Karlsson <krka@spotify.com>

Author: spkrka

Documentation/technical/paint-down-to-common.adoc

@@ -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
 ---------------------
 

commit-reach.c

@@ -133,17 +133,30 @@ static void paint_queue_put(struct paint_state *state,
 
 static struct commit *paint_queue_get(struct paint_state *state)
 {
-	struct commit *commit;
+	struct commit *commit = prio_queue_get(&state->queue);
 
-	if (!state->p1_count && !state->p2_count &&
-	    !state->pending_merge_bases)
+	if (!commit)
 		return NULL;
 
-	commit = prio_queue_get(&state->queue);
-	if (commit) {
-		commit->object.flags &= ~ENQUEUED;
-		paint_count_update(state, commit->object.flags, -1);
+	commit->object.flags &= ~ENQUEUED;
+
+	if (!state->pending_merge_bases) {
+		if (!state->p1_count && !state->p2_count)
+			return NULL;
+		/*
+		 * Side exhaustion: a new merge-base can only form
+		 * when both PARENT1-only and PARENT2-only commits
+		 * remain in the queue. In the finite-generation
+		 * region the queue is ordered topologically, so
+		 * no future step can add paint to visited commits
+		 * and an exhausted side cannot reappear.
+		 */
+		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;
 }
 

t/t6600-test-reach.sh

@@ -332,12 +332,12 @@ test_expect_success 'merge-base --all commit-walk steps' '
 	cp commit-graph-full .git/objects/info/commit-graph &&
 	GIT_TRACE2_EVENT="$(pwd)/trace-full.txt" \
 		git merge-base --all commit-9-9 commit-9-1 >actual &&
-	test_trace2_data paint_down_to_common steps 80 <trace-full.txt &&
+	test_trace2_data paint_down_to_common steps 9 <trace-full.txt &&
 
 	cp commit-graph-half .git/objects/info/commit-graph &&
 	GIT_TRACE2_EVENT="$(pwd)/trace-half.txt" \
 		git merge-base --all commit-9-9 commit-9-1 >actual &&
-	test_trace2_data paint_down_to_common steps 81 <trace-half.txt
+	test_trace2_data paint_down_to_common steps 57 <trace-half.txt
 '
 
 test_expect_success 'reduce_heads' '