[PATCH v6] contrib/subtree: fix "subtree split" skipped-merge bug

[Dave Ware <davidw@realtimegenomics.com>]

'git subtree split' can incorrectly skip a merge even when both parents
act on the subtree, provided the merge results in a tree identical to
one of the parents. Fix by copying the merge if at least one parent is
non-identical, and the non-identical parent is not an ancestor of the
identical parent.

Also, add a test case which checks that a descendant remains a
descendent on the subtree in this case.

Signed-off-by: Dave Ware <davidw@realtimegenomics.com>
---
 contrib/subtree/git-subtree.sh     | 12 ++++++--
 contrib/subtree/t/t7900-subtree.sh | 60 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+), 2 deletions(-)

diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
index edf36f8..5c83727 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/contrib/subtree/git-subtree.sh
@@ -479,8 +479,16 @@ copy_or_skip()
 			p="$p -p $parent"
 		fi
 	done
-	
-	if [ -n "$identical" ]; then
+
+	copycommit=
+	if [ -n "$identical" ] && [ -n "$nonidentical" ]; then
+		extras=$(git rev-list --count $identical..$nonidentical)
+		if [ "$extras" -ne 0 ]; then
+			# we need to preserve history along the other branch
+			copycommit=1
+		fi
+	fi
+	if [ -n "$identical" ] && [ -z "$copycommit" ]; then
 		echo $identical
 	else
 		copy_commit $rev $tree "$p" || exit $?
diff --git a/contrib/subtree/t/t7900-subtree.sh b/contrib/subtree/t/t7900-subtree.sh
index 751aee3..c5089c3 100755
--- a/contrib/subtree/t/t7900-subtree.sh
+++ b/contrib/subtree/t/t7900-subtree.sh
@@ -1014,4 +1014,64 @@ test_expect_success 'push split to subproj' '
 	)
 '
 
+#
+# This test covers 2 cases in subtree split copy_or_skip code
+# 1) Merges where one parent is a superset of the changes of the other
+#    parent regarding changes to the subtree, in this case the merge
+#    commit should be copied
+# 2) Merges where only one parent operate on the subtree, and the merge
+#    commit should be skipped
+#
+# (1) is checked by ensuring subtree_tip is a descendent of subtree_branch
+# (2) should have a check added (not_a_subtree_change shouldn't be present
+#     on the produced subtree)
+#
+# Other related cases which are not tested (or currently handled correctly)
+# - Case (1) where there are more than 2 parents, it will sometimes correctly copy
+#   the merge, and sometimes not
+# - Merge commit where both parents have same tree as the merge, currently
+#   will always be skipped, even if they reached that state via different
+#   set of commits.
+#
+
+next_test
+test_expect_success 'subtree descendant check' '
+	subtree_test_create_repo "$subtree_test_count" &&
+	test_create_commit "$subtree_test_count" folder_subtree/a &&
+	(
+		cd "$subtree_test_count" &&
+		git branch branch
+	) &&
+	test_create_commit "$subtree_test_count" folder_subtree/0 &&
+	test_create_commit "$subtree_test_count" folder_subtree/b &&
+	cherry=$(cd "$subtree_test_count"; git rev-parse HEAD) &&
+	(
+		cd "$subtree_test_count" &&
+		git checkout branch
+	) &&
+	test_create_commit "$subtree_test_count" commit_on_branch &&
+	(
+		cd "$subtree_test_count" &&
+		git cherry-pick $cherry &&
+		git checkout master &&
+		git merge -m "merge should be kept on subtree" branch &&
+		git branch no_subtree_work_branch
+	) &&
+	test_create_commit "$subtree_test_count" folder_subtree/d &&
+	(
+		cd "$subtree_test_count" &&
+		git checkout no_subtree_work_branch
+	) &&
+	test_create_commit "$subtree_test_count" not_a_subtree_change &&
+	(
+		cd "$subtree_test_count" &&
+		git checkout master
+		git merge -m "merge should be skipped on subtree" no_subtree_work_branch
+
+		git subtree split --prefix folder_subtree/ --branch subtree_tip master &&
+		git subtree split --prefix folder_subtree/ --branch subtree_branch branch
+		check_equal $(git rev-list --count subtree_tip..subtree_branch) 0
+	)
+'
+
 test_done
-- 
1.9.1