t6600: add clock-skew topologies and step counts for edge cases

Add topologies and tests exercising paint_down_to_common() under
clock skew, where commit-date ordering (v1 commit-graph without
corrected commit dates) violates the topological invariant that
children are dequeued before parents:

 - se-*: side-exhaustion fires too early when one paint side fully
   drains from the queue while a low-date ancestor on the other
   side is still queued

 - se2-*: side-exhaustion returns a too-deep merge base because
   the correct (closer) base never receives both paint sides

Also add step counts to the edge-case tests from the previous
commit, a mixed finite/INFINITY generation topology exercising
the transition from INFINITY-generation commits to graph-backed
commits, and step counts for the grid-based merge-base test.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6600-test-reach.sh

@@ -140,6 +140,48 @@ test_expect_success 'setup' '
 	git branch -f pi-X-br "$pi_x" &&
 	git tag pi-X "$pi_x" &&
 
+	# Clock-skew topology for side-exhaustion testing.
+	# D is the correct merge base but has a higher committer date
+	# than C (its child).  With date ordering, D would be dequeued
+	# before C, causing side-exhaustion to fire too early.
+	# Generation ordering prevents this by visiting children
+	# before parents regardless of dates.
+	#
+	#   se-A (date 7000) --> se-C (date 3000) --> se-D (date 5000) --> se-root (date 4000)
+	#   se-B (date 6000) --> se-D
+	#
+	se_root=$(skew_commit 4000 se-root) &&
+	se_D=$(skew_commit 5000 se-D -p "$se_root") &&
+	se_C=$(skew_commit 3000 se-C -p "$se_D") &&
+	se_A=$(skew_commit 7000 se-A -p "$se_C") &&
+	se_B=$(skew_commit 6000 se-B -p "$se_D") &&
+	git branch -f se-A "$se_A" &&
+	git branch -f se-B "$se_B" &&
+	git tag se-D "$se_D" &&
+
+	# Clock-skew topology with redundant ancestor for
+	# side-exhaustion testing.  MB1 is the correct merge base;
+	# MB2 is its parent.  A reaches MB2 via E (high date) and
+	# MB1 via C (low date).  B reaches MB1 via D.  With date
+	# ordering, side-exhaustion would fire before C is dequeued,
+	# missing MB1.  Generation ordering ensures both are found.
+	#
+	#   se2-A (date 8000) --> se2-C (date 2000) --> se2-MB1 (date 5000) --> se2-MB2 (date 4000) --> se2-root (date 1000)
+	#   se2-A              --> se2-E (date 6500) --> se2-MB2
+	#   se2-B (date 7000) --> se2-D (date 6000) --> se2-MB1
+	#
+	se2_root=$(skew_commit 1000 se2-root) &&
+	se2_MB2=$(skew_commit 4000 se2-MB2 -p "$se2_root") &&
+	se2_MB1=$(skew_commit 5000 se2-MB1 -p "$se2_MB2") &&
+	se2_C=$(skew_commit 2000 se2-C -p "$se2_MB1") &&
+	se2_D=$(skew_commit 6000 se2-D -p "$se2_MB1") &&
+	se2_E=$(skew_commit 6500 se2-E -p "$se2_MB2") &&
+	se2_A=$(skew_commit 8000 se2-A -p "$se2_C" -p "$se2_E") &&
+	se2_B=$(skew_commit 7000 se2-B -p "$se2_D") &&
+	git branch -f se2-A "$se2_A" &&
+	git branch -f se2-B "$se2_B" &&
+	git tag se2-MB1 "$se2_MB1" &&
+
 	git commit-graph write --reachable &&
 	mv .git/objects/info/commit-graph commit-graph-full &&
 	chmod u+w commit-graph-full &&
@@ -323,7 +365,8 @@ test_expect_success 'get_merge_bases_many:pending-stale' '
 		echo "get_merge_bases_many(A,X):" &&
 		git rev-parse ps-B
 	} >expect &&
-	test_all_modes get_merge_bases_many
+	test_all_modes get_merge_bases_many &&
+	test_paint_down_steps 6 6 6 6
 '
 
 test_expect_success 'get_merge_bases_many:infinity-both-sides' '
@@ -337,7 +380,34 @@ test_expect_success 'get_merge_bases_many:infinity-both-sides' '
 		echo "get_merge_bases_many(A,X):" &&
 		git rev-parse pi-B
 	} >expect &&
-	test_all_modes get_merge_bases_many
+	test_all_modes get_merge_bases_many &&
+	test_paint_down_steps 5 5 5 5
+'
+
+test_expect_success 'setup mixed finite/INFINITY topology' '
+	# Create a commit outside all saved commit-graph files so it always
+	# has INFINITY generation, while its parent (ps-X) is in the graph
+	# with a finite generation. Use the ps-* orphan topology so we do
+	# not pollute the grid-based rev-list tests.
+	git checkout ps-X &&
+	test_env GIT_TEST_COMMIT_GRAPH= test_commit pm-INF
+'
+
+test_expect_success 'get_merge_bases_many:mixed-finite-infinity' '
+	# One tip (pm-INF) is outside the commit-graph with INFINITY
+	# generation; the other (ps-B) is in the graph with finite
+	# generation. The walk starts in the INFINITY region and crosses
+	# into the finite region where side-exhaustion can fire.
+	cat >input <<-\EOF &&
+	A:pm-INF
+	X:ps-B
+	EOF
+	{
+		echo "get_merge_bases_many(A,X):" &&
+		git rev-parse ps-X
+	} >expect &&
+	test_all_modes get_merge_bases_many &&
+	test_paint_down_steps 3 3 3 3
 '
 
 test_expect_success 'merge-base --all commit-walk steps' '
@@ -347,6 +417,30 @@ test_expect_success 'merge-base --all commit-walk steps' '
 	test_paint_down_steps 81 80 81 81
 '
 
+test_expect_success 'merge-base --all with clock skew (side-exhaustion)' '
+	# Verify correct merge base under clock skew.  se-D (the
+	# merge base) has a higher date than its child se-C.
+	# Generation ordering ensures se-C is visited before se-D,
+	# so P1 paint propagates correctly and se-D is found.
+	>input &&
+	git rev-parse se-D >expect &&
+	run_all_modes git merge-base --all se-A se-B &&
+	test_paint_down_steps 6 4 6 6
+'
+
+test_expect_success 'merge-base --all with clock skew and redundant ancestor (side-exhaustion)' '
+	# Verify correct merge base when clock skew could cause a
+	# too-deep result.  MB1 is the correct merge base; MB2 is
+	# its ancestor.  A reaches MB2 via E (high date) and MB1
+	# via C (low date).  Generation ordering ensures C is
+	# visited before side-exhaustion fires, so MB1 is found
+	# and remove_redundant correctly discards MB2.
+	>input &&
+	git rev-parse se2-MB1 >expect &&
+	run_all_modes git merge-base --all se2-A se2-B &&
+	test_paint_down_steps 8 7 8 8
+'
+
 test_expect_success 'reduce_heads' '
 	cat >input <<-\EOF &&
 	X:commit-1-10