commit-reach: move min_generation check into paint_queue_get()

Consolidate the min_generation termination condition into
paint_queue_get(), alongside the existing stale-entry and
side-exhaustion checks.

Move last_gen into struct paint_state so that
commit_graph_generation() is called exactly once per dequeued commit
and the result is shared across all termination checks and the
monotonicity BUG assertion.  The loop body in paint_down_to_common()
reads state.last_gen instead of recomputing the generation number.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>

Author: spkrka

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

@@ -97,6 +97,8 @@ ends when one of the following conditions holds:
   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.
+  4. Generation cutoff: the dequeued commit's generation is below
+     a caller-supplied `min_generation` threshold.
 
 Stale entry condition
 ~~~~~~~~~~~~~~~~~~~~~
@@ -121,6 +123,13 @@ 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`
+threshold -- the minimum generation of the input commits. No merge
+base can have a generation below this threshold, so the walk
+terminates as soon as it dequeues such a commit.
+
 Related documentation
 ---------------------
 

commit-reach.c

@@ -89,6 +89,8 @@ struct paint_state {
 	int p1_count;
 	int p2_count;
 	int pending_merge_bases;
+	timestamp_t min_generation;
+	timestamp_t last_gen;
 };
 
 static void paint_count_update(struct paint_state *state,
@@ -138,11 +140,23 @@ static void paint_queue_put(struct paint_state *state,
 static struct commit *paint_queue_get(struct paint_state *state)
 {
 	struct commit *commit = prio_queue_get(&state->queue);
+	timestamp_t generation;
 
 	if (!commit)
 		return NULL;
 
 	commit->object.flags &= ~ENQUEUED;
+	generation = commit_graph_generation(commit);
+
+	if (generation > state->last_gen)
+		BUG("bad generation skip %"PRItime" > %"PRItime" at %s",
+		    generation, state->last_gen,
+		    oid_to_hex(&commit->object.oid));
+	state->last_gen = generation;
+
+	/* generation cutoff */
+	if (state->min_generation && generation < state->min_generation)
+		return NULL;
 
 	if (!state->pending_merge_bases) {
 		/* only stale entries remain */
@@ -151,7 +165,7 @@ static struct commit *paint_queue_get(struct paint_state *state)
 
 		/* one side is exhausted */
 		if ((!state->p1_count || !state->p2_count) &&
-		    commit_graph_generation(commit) < GENERATION_NUMBER_INFINITY)
+		    generation < GENERATION_NUMBER_INFINITY)
 			return NULL;
 	}
 
@@ -177,9 +191,10 @@ static int paint_down_to_common(struct repository *r,
 	struct commit *commit;
 	int i;
 	int steps = 0;
-	timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
 	struct commit_list **tail = result;
 
+	state.min_generation = min_generation;
+	state.last_gen = GENERATION_NUMBER_INFINITY;
 	if (!min_generation && !corrected_commit_dates_enabled(r))
 		state.queue.compare = compare_commits_by_commit_date;
 
@@ -196,18 +211,8 @@ static int paint_down_to_common(struct repository *r,
 	while ((commit = paint_queue_get(&state))) {
 		struct commit_list *parents;
 		int flags;
-		timestamp_t generation = commit_graph_generation(commit);
 		steps++;
 
-		if (generation > last_gen)
-			BUG("bad generation skip %"PRItime" > %"PRItime" at %s",
-			    generation, last_gen,
-			    oid_to_hex(&commit->object.oid));
-		last_gen = generation;
-
-		if (generation < min_generation)
-			break;
-
 		flags = commit->object.flags & (PARENT1 | PARENT2 | STALE);
 		if (flags == (PARENT1 | PARENT2)) {
 			if (!(commit->object.flags & RESULT)) {
@@ -219,7 +224,7 @@ static int paint_down_to_common(struct repository *r,
 				 * descendant of this one.
 				 */
 				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
-				    generation < GENERATION_NUMBER_INFINITY)
+				    state.last_gen < GENERATION_NUMBER_INFINITY)
 					break;
 			}
 			/* Mark parents of a found merge stale */