[PATCH v10 32/36] t6046: testcases checking whether updates can be skipped in a merge

[Elijah Newren <newren@gmail.com>]

Add several tests checking whether updates can be skipped in a merge.
Also add several similar testcases for where updates cannot be skipped in
a merge to make sure that we skip if and only if we should.

In particular:

  * Testcase 1a (particularly 1a-check-L) would have pointed out the
    problem Linus has been dealing with for year with his merges[1].

  * Testcase 2a (particularly 2a-check-L) would have pointed out the
    problem with my directory-rename-series before it broke master[2].

  * Testcases 3[ab] (particularly 3a-check-L) provide a simpler testcase
    than 12b of t6043 making that one easier to understand.

  * There are several complementary testcases to make sure we're not just
    fixing those particular issues while regressing in the opposite
    direction.

  * There are also a pair of tests for the special case when a merge
    results in a skippable update AND the user has dirty modifications to
    the path.

[1] https://public-inbox.org/git/CA+55aFzLZ3UkG5svqZwSnhNk75=fXJRkvU1m_RHBG54NOoaZPA@mail.gmail.com/
[2] https://public-inbox.org/git/xmqqmuya43cs.fsf@gitster-ct.c.googlers.com/

Signed-off-by: Elijah Newren <newren@gmail.com>
---

Stefan Beller reviewed an RFC version of this patch and gave his
Reviewed-by for it, but I've significantly modified it since then,
including:

  - new tests for dirty files being present
  - new test for a carefully crafted rename/add conflict (which I
    constructed to try see if there was still a way to get mis-merges
    with the RFC patches)
  - better test cleanup/recovery (add git clean to go with git reset)
  - added modification timestamp checking to relevant tests to verify
    that when merge-recursive claims certain file updates were skipped
    during the merge that they really were skipped.

 t/t6046-merge-skip-unneeded-updates.sh | 761 +++++++++++++++++++++++++
 1 file changed, 761 insertions(+)
 create mode 100755 t/t6046-merge-skip-unneeded-updates.sh

diff --git a/t/t6046-merge-skip-unneeded-updates.sh b/t/t6046-merge-skip-unneeded-updates.sh
new file mode 100755
index 0000000000..911e2f87a4
--- /dev/null
+++ b/t/t6046-merge-skip-unneeded-updates.sh
@@ -0,0 +1,761 @@
+#!/bin/sh
+
+test_description="merge cases"
+
+# The setup for all of them, pictorially, is:
+#
+#      A
+#      o
+#     / \
+#  O o   ?
+#     \ /
+#      o
+#      B
+#
+# To help make it easier to follow the flow of tests, they have been
+# divided into sections and each test will start with a quick explanation
+# of what commits O, A, and B contain.
+#
+# Notation:
+#    z/{b,c}   means  files z/b and z/c both exist
+#    x/d_1     means  file x/d exists with content d1.  (Purpose of the
+#                     underscore notation is to differentiate different
+#                     files that might be renamed into each other's paths.)
+
+. ./test-lib.sh
+
+
+###########################################################################
+# SECTION 1: Cases involving no renames (one side has subset of changes of
+#            the other side)
+###########################################################################
+
+# Testcase 1a, Changes on A, subset of changes on B
+#   Commit O: b_1
+#   Commit A: b_2
+#   Commit B: b_3
+#   Expected: b_2
+
+test_expect_success '1a-setup: Modify(A)/Modify(B), change on B subset of A' '
+	test_create_repo 1a &&
+	(
+		cd 1a &&
+
+		test_write_lines 1 2 3 4 5 6 7 8 9 10 >b
+		git add b &&
+		test_tick &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		test_write_lines 1 2 3 4 5 5.5 6 7 8 9 10 10.5 >b &&
+		git add b &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		test_write_lines 1 2 3 4 5 5.5 6 7 8 9 10 >b &&
+		git add b &&
+		test_tick &&
+		git commit -m "B"
+	)
+'
+
+test_expect_failure '1a-check-L: Modify(A)/Modify(B), change on B subset of A' '
+	test_when_finished "git -C 1a reset --hard" &&
+	test_when_finished "git -C 1a clean -fd" &&
+	(
+		cd 1a &&
+
+		git checkout A^0 &&
+
+		test-tool chmtime =31337 b &&
+		test-tool chmtime -v +0 b >expected-mtime &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive B^0 >out 2>err &&
+
+		test_i18ngrep "Skipped b" out &&
+		test_must_be_empty err &&
+
+		test-tool chmtime -v +0 b >actual-mtime &&
+		test_cmp expected-mtime actual-mtime &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 1 index_files &&
+
+		git rev-parse >actual HEAD:b &&
+		git rev-parse >expect A:b &&
+		test_cmp expect actual &&
+
+		git hash-object b   >actual &&
+		git rev-parse   A:b >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '1a-check-R: Modify(A)/Modify(B), change on B subset of A' '
+	test_when_finished "git -C 1a reset --hard" &&
+	test_when_finished "git -C 1a clean -fd" &&
+	(
+		cd 1a &&
+
+		git checkout B^0 &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive A^0 >out 2>err &&
+
+		test_i18ngrep "Auto-merging b" out &&
+		test_must_be_empty err &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 1 index_files &&
+
+		git rev-parse >actual HEAD:b &&
+		git rev-parse >expect A:b &&
+		test_cmp expect actual &&
+
+		git hash-object b   >actual &&
+		git rev-parse   A:b >expect &&
+		test_cmp expect actual
+	)
+'
+
+
+###########################################################################
+# SECTION 2: Cases involving basic renames
+###########################################################################
+
+# Testcase 2a, Changes on A, rename on B
+#   Commit O: b_1
+#   Commit A: b_2
+#   Commit B: c_1
+#   Expected: c_2
+
+test_expect_success '2a-setup: Modify(A)/rename(B)' '
+	test_create_repo 2a &&
+	(
+		cd 2a &&
+
+		test_seq 1 10 >b
+		git add b &&
+		test_tick &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		test_seq 1 11 > b &&
+		git add b &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		git mv b c &&
+		test_tick &&
+		git commit -m "B"
+	)
+'
+
+test_expect_failure '2a-check-L: Modify/rename, merge into modify side' '
+	test_when_finished "git -C 2a reset --hard" &&
+	test_when_finished "git -C 2a clean -fd" &&
+	(
+		cd 2a &&
+
+		git checkout A^0 &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive B^0 >out 2>err &&
+
+		test_i18ngrep ! "Skipped c" out &&
+		test_must_be_empty err &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 1 index_files &&
+
+		git rev-parse >actual HEAD:c &&
+		git rev-parse >expect A:b &&
+		test_cmp expect actual &&
+
+		git hash-object c   >actual &&
+		git rev-parse   A:b >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:b &&
+		test_path_is_missing b
+	)
+'
+
+test_expect_success '2a-check-R: Modify/rename, merge into rename side' '
+	test_when_finished "git -C 2a reset --hard" &&
+	test_when_finished "git -C 2a clean -fd" &&
+	(
+		cd 2a &&
+
+		git checkout B^0 &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive A^0 >out 2>err &&
+
+		test_i18ngrep ! "Skipped c" out &&
+		test_must_be_empty err &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 1 index_files &&
+
+		git rev-parse >actual HEAD:c &&
+		git rev-parse >expect A:b &&
+		test_cmp expect actual &&
+
+		git hash-object c   >actual &&
+		git rev-parse   A:b >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:b &&
+		test_path_is_missing b
+	)
+'
+
+# Testcase 2b, Changed and renamed on A, subset of changes on B
+#   Commit O: b_1
+#   Commit A: c_2
+#   Commit B: b_3
+#   Expected: c_2
+
+test_expect_success '2b-setup: Rename+Mod(A)/Mod(B), B mods subset of A' '
+	test_create_repo 2b &&
+	(
+		cd 2b &&
+
+		test_write_lines 1 2 3 4 5 6 7 8 9 10 >b
+		git add b &&
+		test_tick &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		test_write_lines 1 2 3 4 5 5.5 6 7 8 9 10 10.5 >b &&
+		git add b &&
+		git mv b c &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		test_write_lines 1 2 3 4 5 5.5 6 7 8 9 10 >b &&
+		git add b &&
+		test_tick &&
+		git commit -m "B"
+	)
+'
+
+test_expect_success '2b-check-L: Rename+Mod(A)/Mod(B), B mods subset of A' '
+	test_when_finished "git -C 2b reset --hard" &&
+	test_when_finished "git -C 2b clean -fd" &&
+	(
+		cd 2b &&
+
+		git checkout A^0 &&
+
+		test-tool chmtime =31337 c &&
+		test-tool chmtime -v +0 c >expected-mtime &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive B^0 >out 2>err &&
+
+		test_i18ngrep "Skipped c" out &&
+		test_must_be_empty err &&
+
+		test-tool chmtime -v +0 c >actual-mtime &&
+		test_cmp expected-mtime actual-mtime &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 1 index_files &&
+
+		git rev-parse >actual HEAD:c &&
+		git rev-parse >expect A:c &&
+		test_cmp expect actual &&
+
+		git hash-object c   >actual &&
+		git rev-parse   A:c >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:b &&
+		test_path_is_missing b
+	)
+'
+
+test_expect_success '2b-check-R: Rename+Mod(A)/Mod(B), B mods subset of A' '
+	test_when_finished "git -C 2b reset --hard" &&
+	test_when_finished "git -C 2b clean -fd" &&
+	(
+		cd 2b &&
+
+		git checkout B^0 &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive A^0 >out 2>err &&
+
+		test_i18ngrep "Auto-merging c" out &&
+		test_must_be_empty err &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 1 index_files &&
+
+		git rev-parse >actual HEAD:c &&
+		git rev-parse >expect A:c &&
+		test_cmp expect actual &&
+
+		git hash-object c   >actual &&
+		git rev-parse   A:c >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:b &&
+		test_path_is_missing b
+	)
+'
+
+# Testcase 2c, Changes on A, rename on B
+#   Commit O: b_1
+#   Commit A: b_2, c_3
+#   Commit B: c_1
+#   Expected: rename/add conflict c_2 vs c_3
+#
+#   NOTE: Since A modified b_1->b_2, and B renamed b_1->c_1, the threeway
+#         merge of those files should result in c_2.  We then should have a
+#         rename/add conflict between c_2 and c_3.  However, if we note in
+#         merge_content() that A had the right contents (b_2 has same
+#         contents as c_2, just at a different name), and that A had the
+#         right path present (c_3 existed) and thus decides that it can
+#         skip the update, then we're in trouble.  This test verifies we do
+#         not make that particular mistake.
+
+test_expect_success '2c-setup: Modify b & add c VS rename b->c' '
+	test_create_repo 2c &&
+	(
+		cd 2c &&
+
+		test_seq 1 10 >b
+		git add b &&
+		test_tick &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		test_seq 1 11 >b &&
+		echo whatever >c &&
+		git add b c &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		git mv b c &&
+		test_tick &&
+		git commit -m "B"
+	)
+'
+
+test_expect_failure '2c-check: Modify b & add c VS rename b->c' '
+	(
+		cd 2c &&
+
+		git checkout A^0 &&
+
+		GIT_MERGE_VERBOSITY=3 test_must_fail git merge -s recursive B^0 >out 2>err &&
+
+		test_i18ngrep "CONFLICT (rename/add): Rename b->c" out &&
+		test_i18ngrep ! "Skipped c" out &&
+		test_must_be_empty err
+
+		# FIXME: rename/add conflicts are horribly broken right now;
+		# when I get back to my patch series fixing it and
+		# rename/rename(2to1) conflicts to bring them in line with
+		# how add/add conflicts behave, then checks like the below
+		# could be added.  But that patch series is waiting until
+		# the rename-directory-detection series lands, which this
+		# is part of.  And in the mean time, I do not want to further
+		# enforce broken behavior.  So for now, the main test is the
+		# one above that err is an empty file.
+
+		#git ls-files -s >index_files &&
+		#test_line_count = 2 index_files &&
+
+		#git rev-parse >actual :2:c :3:c &&
+		#git rev-parse >expect A:b  A:c  &&
+		#test_cmp expect actual &&
+
+		#git cat-file -p A:b >>merged &&
+		#git cat-file -p A:c >>merge-me &&
+		#>empty &&
+		#test_must_fail git merge-file \
+		#	-L "Temporary merge branch 1" \
+		#	-L "" \
+		#	-L "Temporary merge branch 2" \
+		#	merged empty merge-me &&
+		#sed -e "s/^\([<=>]\)/\1\1\1/" merged >merged-internal &&
+
+		#git hash-object c               >actual &&
+		#git hash-object merged-internal >expect &&
+		#test_cmp expect actual &&
+
+		#test_path_is_missing b
+	)
+'
+
+
+###########################################################################
+# SECTION 3: Cases involving directory renames
+#
+# NOTE:
+#   Directory renames only apply when one side renames a directory, and the
+#   other side adds or renames a path into that directory.  Applying the
+#   directory rename to that new path creates a new pathname that didn't
+#   exist on either side of history.  Thus, it is impossible for the
+#   merge contents to already be at the right path, so all of these checks
+#   exist just to make sure that updates are not skipped.
+###########################################################################
+
+# Testcase 3a, Change + rename into dir foo on A, dir rename foo->bar on B
+#   Commit O: bq_1, foo/whatever
+#   Commit A: foo/{bq_2, whatever}
+#   Commit B: bq_1, bar/whatever
+#   Expected: bar/{bq_2, whatever}
+
+test_expect_success '3a-setup: bq_1->foo/bq_2 on A, foo/->bar/ on B' '
+	test_create_repo 3a &&
+	(
+		cd 3a &&
+
+		mkdir foo &&
+		test_seq 1 10 >bq &&
+		test_write_lines a b c d e f g h i j k >foo/whatever &&
+		git add bq foo/whatever &&
+		test_tick &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		test_seq 1 11 > bq &&
+		git add bq &&
+		git mv bq foo/ &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		git mv foo/ bar/ &&
+		test_tick &&
+		git commit -m "B"
+	)
+'
+
+test_expect_failure '3a-check-L: bq_1->foo/bq_2 on A, foo/->bar/ on B' '
+	test_when_finished "git -C 3a reset --hard" &&
+	test_when_finished "git -C 3a clean -fd" &&
+	(
+		cd 3a &&
+
+		git checkout A^0 &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive B^0 >out 2>err &&
+
+		test_i18ngrep ! "Skipped bar/bq" out &&
+		test_must_be_empty err &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 2 index_files &&
+
+		git rev-parse >actual HEAD:bar/bq HEAD:bar/whatever &&
+		git rev-parse >expect A:foo/bq    A:foo/whatever &&
+		test_cmp expect actual &&
+
+		git hash-object bar/bq   bar/whatever   >actual &&
+		git rev-parse   A:foo/bq A:foo/whatever >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:bq HEAD:foo/bq &&
+		test_path_is_missing bq foo/bq foo/whatever
+	)
+'
+
+test_expect_success '3a-check-R: bq_1->foo/bq_2 on A, foo/->bar/ on B' '
+	test_when_finished "git -C 3a reset --hard" &&
+	test_when_finished "git -C 3a clean -fd" &&
+	(
+		cd 3a &&
+
+		git checkout B^0 &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive A^0 >out 2>err &&
+
+		test_i18ngrep ! "Skipped bar/bq" out &&
+		test_must_be_empty err &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 2 index_files &&
+
+		git rev-parse >actual HEAD:bar/bq HEAD:bar/whatever &&
+		git rev-parse >expect A:foo/bq    A:foo/whatever &&
+		test_cmp expect actual &&
+
+		git hash-object bar/bq   bar/whatever   >actual &&
+		git rev-parse   A:foo/bq A:foo/whatever >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:bq HEAD:foo/bq &&
+		test_path_is_missing bq foo/bq foo/whatever
+	)
+'
+
+# Testcase 3b, rename into dir foo on A, dir rename foo->bar + change on B
+#   Commit O: bq_1, foo/whatever
+#   Commit A: foo/{bq_1, whatever}
+#   Commit B: bq_2, bar/whatever
+#   Expected: bar/{bq_2, whatever}
+
+test_expect_success '3b-setup: bq_1->foo/bq_2 on A, foo/->bar/ on B' '
+	test_create_repo 3b &&
+	(
+		cd 3b &&
+
+		mkdir foo &&
+		test_seq 1 10 >bq &&
+		test_write_lines a b c d e f g h i j k >foo/whatever &&
+		git add bq foo/whatever &&
+		test_tick &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		git mv bq foo/ &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		test_seq 1 11 > bq &&
+		git add bq &&
+		git mv foo/ bar/ &&
+		test_tick &&
+		git commit -m "B"
+	)
+'
+
+test_expect_success '3b-check-L: bq_1->foo/bq_2 on A, foo/->bar/ on B' '
+	test_when_finished "git -C 3b reset --hard" &&
+	test_when_finished "git -C 3b clean -fd" &&
+	(
+		cd 3b &&
+
+		git checkout A^0 &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive B^0 >out 2>err &&
+
+		test_i18ngrep ! "Skipped bar/bq" out &&
+		test_must_be_empty err &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 2 index_files &&
+
+		git rev-parse >actual HEAD:bar/bq HEAD:bar/whatever &&
+		git rev-parse >expect B:bq        A:foo/whatever &&
+		test_cmp expect actual &&
+
+		git hash-object bar/bq bar/whatever   >actual &&
+		git rev-parse   B:bq   A:foo/whatever >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:bq HEAD:foo/bq &&
+		test_path_is_missing bq foo/bq foo/whatever
+	)
+'
+
+test_expect_failure '3b-check-R: bq_1->foo/bq_2 on A, foo/->bar/ on B' '
+	test_when_finished "git -C 3b reset --hard" &&
+	test_when_finished "git -C 3b clean -fd" &&
+	(
+		cd 3b &&
+
+		git checkout B^0 &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive A^0 >out 2>err &&
+
+		test_i18ngrep ! "Skipped bar/bq" out &&
+		test_must_be_empty err &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 2 index_files &&
+
+		git rev-parse >actual HEAD:bar/bq HEAD:bar/whatever &&
+		git rev-parse >expect B:bq        A:foo/whatever &&
+		test_cmp expect actual &&
+
+		git hash-object bar/bq bar/whatever   >actual &&
+		git rev-parse   B:bq   A:foo/whatever >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:bq HEAD:foo/bq &&
+		test_path_is_missing bq foo/bq foo/whatever
+	)
+'
+
+###########################################################################
+# SECTION 4: Cases involving dirty changes
+###########################################################################
+
+# Testcase 4a, Changed on A, subset of changes on B, locally modified
+#   Commit O: b_1
+#   Commit A: b_2
+#   Commit B: b_3
+#   Working copy: b_4
+#   Expected: b_2 for merge, b_4 in working copy
+
+test_expect_success '4a-setup: Change on A, change on B subset of A, dirty mods present' '
+	test_create_repo 4a &&
+	(
+		cd 4a &&
+
+		test_write_lines 1 2 3 4 5 6 7 8 9 10 >b
+		git add b &&
+		test_tick &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		test_write_lines 1 2 3 4 5 5.5 6 7 8 9 10 10.5 >b &&
+		git add b &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		test_write_lines 1 2 3 4 5 5.5 6 7 8 9 10 >b &&
+		git add b &&
+		test_tick &&
+		git commit -m "B"
+	)
+'
+
+# NOTE: For as long as we continue using unpack_trees() without index_only
+#   set to true, it will error out on a case like this claiming the the locally
+#   modified file would be overwritten by the merge.  Getting this testcase
+#   correct requires doing the merge in-memory first, then realizing that no
+#   updates to the file are necessary, and thus that we can just leave the path
+#   alone.
+test_expect_failure '4a-check: Change on A, change on B subset of A, dirty mods present' '
+	test_when_finished "git -C 4a reset --hard" &&
+	test_when_finished "git -C 4a clean -fd" &&
+	(
+		cd 4a &&
+
+		git checkout A^0 &&
+		echo "File rewritten" >b &&
+
+		test-tool chmtime =31337 b &&
+		test-tool chmtime -v +0 b >expected-mtime &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive B^0 >out 2>err &&
+
+		test_i18ngrep "Skipped b" out &&
+		test_must_be_empty err &&
+
+		test-tool chmtime -v +0 b >actual-mtime &&
+		test_cmp expected-mtime actual-mtime &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 1 index_files &&
+
+		git rev-parse >actual :0:b &&
+		git rev-parse >expect A:b &&
+		test_cmp expect actual &&
+
+		git hash-object b >actual &&
+		echo "File rewritten" | git hash-object --stdin >expect &&
+		test_cmp expect actual
+	)
+'
+
+# Testcase 4b, Changed+renamed on A, subset of changes on B, locally modified
+#   Commit O: b_1
+#   Commit A: c_2
+#   Commit B: b_3
+#   Working copy: c_4
+#   Expected: c_2
+
+test_expect_success '4b-setup: Rename+Mod(A)/Mod(B), change on B subset of A, dirty mods present' '
+	test_create_repo 4b &&
+	(
+		cd 4b &&
+
+		test_write_lines 1 2 3 4 5 6 7 8 9 10 >b
+		git add b &&
+		test_tick &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		test_write_lines 1 2 3 4 5 5.5 6 7 8 9 10 10.5 >b &&
+		git add b &&
+		git mv b c &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		test_write_lines 1 2 3 4 5 5.5 6 7 8 9 10 >b &&
+		git add b &&
+		test_tick &&
+		git commit -m "B"
+	)
+'
+
+test_expect_success '4b-check: Rename+Mod(A)/Mod(B), change on B subset of A, dirty mods present' '
+	test_when_finished "git -C 4b reset --hard" &&
+	test_when_finished "git -C 4b clean -fd" &&
+	(
+		cd 4b &&
+
+		git checkout A^0 &&
+		echo "File rewritten" >c &&
+
+		test-tool chmtime =31337 c &&
+		test-tool chmtime -v +0 c >expected-mtime &&
+
+		GIT_MERGE_VERBOSITY=3 git merge -s recursive B^0 >out 2>err &&
+
+		test_i18ngrep "Skipped c" out &&
+		test_must_be_empty err &&
+
+		test-tool chmtime -v +0 c >actual-mtime &&
+		test_cmp expected-mtime actual-mtime &&
+
+		git ls-files -s >index_files &&
+		test_line_count = 1 index_files &&
+
+		git rev-parse >actual :0:c &&
+		git rev-parse >expect A:c &&
+		test_cmp expect actual &&
+
+		git hash-object c >actual &&
+		echo "File rewritten" | git hash-object --stdin >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:b &&
+		test_path_is_missing b
+	)
+'
+
+test_done
-- 
2.17.0.290.ge988e9ce2a