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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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

@@ -105,6 +105,9 @@ ends when one of the following conditions holds:
   4. Single result: the caller only needs one merge base, one has
      been found, and the walk has entered the finite-generation
      region.
+  5. 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
 ~~~~~~~~~~~~~~~~~~~~~
@@ -115,6 +118,20 @@ 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.
+
 Generation cutoff
 ~~~~~~~~~~~~~~~~~
 Some callers (notably `remove_redundant()`) supply a `min_generation`
@@ -159,12 +176,16 @@ ordering via `compare_commits_by_commit_date`.  Because commit
 dates are not monotonic (clock skew, rebases, etc.), the queue
 may visit commits out of topological order.
 
-This disables the optimization that depends on generation ordering:
+This disables the optimizations that depend on generation ordering:
 
   - *Single result*: the first merge-base candidate found may not
     be the shallowest, because a deeper ancestor with a higher
     commit date can be dequeued first.
 
+  - *Side exhaustion*: one paint side can appear to drain from the
+    queue while commits from that side are still waiting with lower
+    dates, causing premature termination.
+
 Related documentation
 ---------------------
 

commit-reach.c

@@ -132,6 +132,10 @@ 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);
@@ -141,9 +145,17 @@ static struct commit *paint_queue_get(struct paint_state *state)
 
 	commit->object.flags &= ~ENQUEUED;
 
-	if (!state->parent1_count && !state->parent2_count &&
-	    !state->mb_candidate_count)
-		return NULL;
+	if (!state->mb_candidate_count) {
+		/* only stale entries remain */
+		if (!state->parent1_count && !state->parent2_count)
+			return NULL;
+
+		/* one side is exhausted */
+		if ((!state->parent1_count || !state->parent2_count) &&
+		    state->gen_ordered &&
+		    commit_graph_generation(commit) < GENERATION_NUMBER_INFINITY)
+			return NULL;
+	}
 
 	paint_count_update(state, commit->object.flags, -1);
 	return commit;

t/t6600-test-reach.sh

@@ -297,7 +297,7 @@ 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' '
@@ -414,7 +414,7 @@ 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 81
 '
 
 test_expect_success 'merge-base --all with clock skew (side-exhaustion)' '