t6600: add test cases for side-exhaustion edge cases

Add test cases to t6600-test-reach.sh that exercise edge cases in the
side-exhaustion optimization for paint_down_to_common():

 - in_merge_bases_many:self: commit is both A and one of the X inputs
 - get_merge_bases_many:duplicate-twos: duplicate entries in X list
 - get_merge_bases_many:pending-stale: STALE transition on an
   already-painted commit (ps-* diamond topology)
 - get_merge_bases_many:infinity-both-sides: both tips outside the
   commit-graph with non-monotonic dates (pi-* topology)

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

t/t6600-test-reach.sh

@@ -85,6 +85,61 @@ test_expect_success 'setup' '
 	git branch -f skew-P2 "$skew_P2" &&
 	git tag skew-M2 "$skew_M2" &&
 
+	# Build a small side topology to exercise the (PARENT1|PARENT2) ->
+	# (PARENT1|PARENT2|STALE) transition in paint_down_to_common(); the
+	# 10x10 grid above does not exercise it because no merge-base candidate
+	# there is a descendant of another, so STALE never reaches a
+	# still-pending candidate.
+	#
+	#       ps-X
+	#       /|\
+	#      / | \
+	#   ps-Z ps-B ps-W
+	#     |  / \  |
+	#     | /   \ |
+	#     |/     \|
+	#   ps-T1   ps-T2
+	#
+	# where ps-T1=merge(ps-Z,ps-B), ps-T2=merge(ps-W,ps-B), so
+	# merge-base(ps-T1,ps-T2) = ps-B. During the walk, ps-X transitions
+	# to (PARENT1|PARENT2) via ps-Z and ps-W before ps-B is dequeued;
+	# then the STALE-walk from ps-B transitions ps-X to
+	# (PARENT1|PARENT2|STALE).
+	git checkout --orphan ps-orphan &&
+	test_commit ps-X &&
+	git checkout -b ps-B-br ps-X && test_commit ps-B &&
+	git checkout -b ps-Z-br ps-X && test_commit ps-Z &&
+	git checkout -b ps-W-br ps-X && test_commit ps-W &&
+	git checkout -b ps-T1 ps-Z &&
+	git merge --no-ff -m ps-T1 ps-B &&
+	git checkout -b ps-T2 ps-W &&
+	git merge --no-ff -m ps-T2 ps-B &&
+
+	# Build a side topology that lives entirely outside the half
+	# commit-graph and has non-monotonic commit dates, to exercise the
+	# INFINITY-gate in paint_down_to_common. With both tips outside
+	# the graph, generation is INFINITY and the queue falls back to
+	# commit-date order, which here is non-monotonic.
+	#
+	#   pi-X (date 500, PARENT1 tip) --> pi-P, pi-D
+	#   pi-D (date 480) --> pi-C
+	#   pi-C (date 200) --> pi-B
+	#   pi-B (date 100, PARENT2 tip) --> pi-P
+	#   pi-P (date 450, root)
+	#
+	# merge-base(pi-X, pi-B) = pi-B (it is an ancestor of pi-X and is
+	# itself one of the queried tips).
+	git checkout --orphan pi-orphan &&
+	test_commit --date "@450 +0000" pi-P &&
+	test_commit --date "@100 +0000" pi-B &&
+	test_commit --date "@200 +0000" pi-C &&
+	test_commit --date "@480 +0000" pi-D &&
+	GIT_AUTHOR_DATE="@500 +0000" GIT_COMMITTER_DATE="@500 +0000" \
+		git commit-tree -p pi-D -p pi-P -m pi-X pi-D^{tree} >pi-X-oid &&
+	pi_x="$(cat pi-X-oid)" &&
+	git branch -f pi-X-br "$pi_x" &&
+	git tag pi-X "$pi_x" &&
+
 	git commit-graph write --reachable &&
 	mv .git/objects/info/commit-graph commit-graph-full &&
 	chmod u+w commit-graph-full &&
@@ -182,6 +237,16 @@ test_expect_success 'in_merge_bases_many:miss-heuristic' '
 	test_all_modes in_merge_bases_many
 '
 
+test_expect_success 'in_merge_bases_many:self' '
+	cat >input <<-\EOF &&
+	A:commit-6-8
+	X:commit-5-9
+	X:commit-6-8
+	EOF
+	echo "in_merge_bases_many(A,X):1" >expect &&
+	test_all_modes in_merge_bases_many
+'
+
 test_expect_success 'is_descendant_of:hit' '
 	cat >input <<-\EOF &&
 	A:commit-5-7
@@ -219,6 +284,51 @@ test_expect_success 'get_merge_bases_many' '
 	test_all_modes get_merge_bases_many
 '
 
+test_expect_success 'get_merge_bases_many:duplicate-twos' '
+	cat >input <<-\EOF &&
+	A:commit-5-7
+	X:commit-4-8
+	X:commit-4-8
+	X:commit-6-6
+	X:commit-6-6
+	X:commit-8-3
+	EOF
+	{
+		echo "get_merge_bases_many(A,X):" &&
+		git rev-parse commit-5-6 \
+			      commit-4-7 | sort
+	} >expect &&
+	test_all_modes get_merge_bases_many
+'
+
+test_expect_success 'get_merge_bases_many:pending-stale' '
+	# Exercises the (PARENT1|PARENT2) -> (...|STALE) transition path in
+	# paint_down_to_common(). See the topology comment in the setup test.
+	cat >input <<-\EOF &&
+	A:ps-T1
+	X:ps-T2
+	EOF
+	{
+		echo "get_merge_bases_many(A,X):" &&
+		git rev-parse ps-B
+	} >expect &&
+	test_all_modes get_merge_bases_many
+'
+
+test_expect_success 'get_merge_bases_many:infinity-both-sides' '
+	# Exercises the push-time INFINITY-gate in paint_down_to_common(). See
+	# the pi-* topology comment in the setup test.
+	cat >input <<-\EOF &&
+	A:pi-X
+	X:pi-B
+	EOF
+	{
+		echo "get_merge_bases_many(A,X):" &&
+		git rev-parse pi-B
+	} >expect &&
+	test_all_modes get_merge_bases_many
+'
+
 test_expect_success 'reduce_heads' '
 	cat >input <<-\EOF &&
 	X:commit-1-10