Merge branch 'kk/commit-reach-find-all-fix'

Fix a commit-graph v1 regression in v2.55.0: The regression was
introduced by git/git@93e5b1680e (merged as git/git@6390da42c7) and
first shipped in v2.55.0, which is why the base here is vfs-2.55.0. That
commit added a fast-path early exit in paint_down_to_common() predicated
on the priority queue being generation-ordered. The predicate holds
under the generation-then-date ordering, but the same function falls
back to plain commit-date ordering when !min_generation &&
!corrected_commit_dates_enabled(r), which is true whenever the loaded
commit-graph lacks a GDAT chunk (i.e. a v1 graph). Under that fallback
the early exit is unsound: with clock skew, a closer merge base can sit
behind deeper commits in the queue, and git merge-base (without --all)
can silently return a non-best answer. The tip commit
git/git@ae68032a8d0 threads a gen_ordered flag through and gates the
early exit on it.

The reason microsoft/git in particular needs to carry this promptly,
rather than wait the usual weeks for upstream graduation, is Scalar.
set_recommended_config() pins commitGraph.generationVersion=1
(scalar.c:191), and that pin is applied on every scalar clone, scalar
register, and scalar reconfigure. Stolee introduced it in 18580f0 as
a deliberate but temporary compatibility measure for existing
enlistments with v1 graphs on disk, with the accompanying
Documentation/scalar.adoc note calling out that v2 is preferred and the
pin is expected to change once the upgrade story solidifies. Every
Scalar-managed enlistment on microsoft/git 2.55.x therefore lands on
exactly the broken code path, and Scalar's target audience (Office
monorepos and external adopters) is precisely the large-monorepo,
many-active-branches cohort Kristofer's original optimization was aimed
at in the first place.

Sadly, even though this regression first appeared in v2.55.0, the topic
branch is unfortunately based on an older commit where a context line
changed, causing merge conflicts. Resolve them in the same way as
0444c74 (Merge branch 'kk/commit-reach-find-all-fix' into next,
2026-07-10).

Signed-off-by: Johannes Schindelin <johasc@microsoft.com>

Author: dscho

commit-reach.c

@@ -108,11 +108,14 @@ static int paint_down_to_common(struct repository *r,
 		{ compare_commits_by_gen_then_commit_date }
 	};
 	int i;
+	int gen_ordered = 1;
 	timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
 	struct commit_list **tail = result;
 
-	if (!min_generation && !corrected_commit_dates_enabled(r))
+	if (!min_generation && !corrected_commit_dates_enabled(r)) {
 		queue.pq.compare = compare_commits_by_commit_date;
+		gen_ordered = 0;
+	}
 
 	one->object.flags |= PARENT1;
 	if (!n) {
@@ -147,11 +150,12 @@ static int paint_down_to_common(struct repository *r,
 				commit->object.flags |= RESULT;
 				tail = commit_list_append(commit, tail);
 				/*
-				 * The queue is generation-ordered; no
-				 * remaining common ancestor can be a
+				 * When the queue is generation-ordered,
+				 * no remaining common ancestor can be a
 				 * descendant of this one.
 				 */
 				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
+				    gen_ordered &&
 				    generation < GENERATION_NUMBER_INFINITY)
 					break;
 			}

t/t6600-test-reach.sh

@@ -49,6 +49,42 @@ test_expect_success 'setup' '
 			git tag -a -m "$x-$i" tag-$x-$i commit-$x-$i || return 1
 		done
 	done &&
+	# Build a topology with clock skew to test the !FIND_ALL early
+	# exit in paint_down_to_common().  M2 is the correct merge base
+	# of P1 and P2, but its ancestor M1 has a higher committer date
+	# due to clock skew.  With date-only ordering (v1 commit graph
+	# without corrected commit dates), M1 pops from the queue first,
+	# gets both paint sides, and the early exit fires before M2 is
+	# ever visited.
+	#
+	#        P1     P2          @7000
+	#        |     /  \
+	#        A    B    D        @6000
+	#       / \   |    |
+	#      |  M2--+    |        @2000 (correct merge base)
+	#       \ |        |
+	#        M1--------+        @5000 (clock skew: date > M2)
+	#        |
+	#       root                @1000
+	#
+	git checkout --orphan skew-orphan &&
+	skew_tree=$(git mktree </dev/null) &&
+	skew_commit () {
+		GIT_COMMITTER_DATE="@$1 +0000" GIT_AUTHOR_DATE="@$1 +0000" \
+			git commit-tree -m "$2" "$skew_tree" $3 $4 $5 $6
+	} &&
+	skew_root=$(skew_commit 1000 root) &&
+	skew_M1=$(skew_commit 5000 M1 -p "$skew_root") &&
+	skew_M2=$(skew_commit 2000 M2 -p "$skew_M1") &&
+	skew_A=$(skew_commit 6000 A -p "$skew_M1" -p "$skew_M2") &&
+	skew_B=$(skew_commit 6000 B -p "$skew_M2") &&
+	skew_D=$(skew_commit 6000 D -p "$skew_M1") &&
+	skew_P1=$(skew_commit 7000 P1 -p "$skew_A") &&
+	skew_P2=$(skew_commit 7000 P2 -p "$skew_B" -p "$skew_D") &&
+	git branch -f skew-P1 "$skew_P1" &&
+	git branch -f skew-P2 "$skew_P2" &&
+	git tag skew-M2 "$skew_M2" &&
+
 	git commit-graph write --reachable &&
 	mv .git/objects/info/commit-graph commit-graph-full &&
 	chmod u+w commit-graph-full &&
@@ -967,4 +1003,9 @@ test_expect_success 'merge-base without --all is one of --all results' '
 	grep -F -f single all
 '
 
+test_expect_success 'merge-base without --all, clock skew, v1 commit-graph' '
+	git rev-parse skew-M2 >expect &&
+	merge_base_all_modes skew-P1 skew-P2
+'
+
 test_done