merge-ort: fix incorrect file handling

We have multiple bugs here -- accidental silent file deletion,
accidental silent file retention for files that should be deleted,
and incorrect number of entries left in the index.

The series merged at commit d3b88be (Merge branch
'en/merge-dir-rename-corner-case-fix', 2021-07-16) introduced testcase
12i-12k in t6423 which checked for rename-to-self cases, and fixed bugs
that merge-ort and merge-recursive had with these testcases.  At the
time, I noted that merge-ort had one bug for these cases, while
merge-recursive had two.  It turns out that merge-ort did in fact have
another bug, but the "relevant renames" optimizations were masking it.
If we modify testcase 12i from t6423 to modify the file in the commit
that renames it (but only modify it enough that it can still be detected
as a rename), then we can trigger silent deletion of the file.

Tweak testcase 12i slightly to make the file in question have more than
one line in it, but which doesn't change how it operates.  Make this
change to otherwise minimize the changes between this testcase and a new
one that we want to add.  Then duplicate the testcase as 12i2, changing
it so that it adds a single line to the file in question when it is
renamed, as a testcase for this bug.

Further, commit 98a1a00 (t6423: add a testcase causing a failed
assertion in process_renames, 2025-03-06), fixed an issue with
rename-to-self but added a new testcase, 12n, that only checked for
whether the merge ran to completion.  A few commits ago, we modified
this test to check for the number of entries in the index -- but noted
that the number was wrong.  And we also noted a
silently-keep-instead-of-delete bug at the same time in the new testcase
12n2.

Fix to merge-ort to prevent multiple bugs with rename-to-self cases:
  * silent deletion of file expected to be kept (t6423 testcase 12i2)
  * silent retention of file expected to be removed (t6423 testcase 12n2)
  * wrong number of extries left in the index (t6423 testcase 12n)

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

merge-ort.c

@@ -2873,6 +2873,17 @@ static int process_renames(struct merge_options *opt,
 			newinfo = new_ent->value;
 		}
 
+		/*
+		 * Directory renames can result in rename-to-self, which we
+		 * want to skip so we don't mark oldpath for deletion.
+		 *
+		 * Note that we can avoid strcmp here because of prior
+		 * diligence in apply_directory_rename_modifications() to
+		 * ensure we reused existing paths from opt->priv->paths.
+		 */
+		if (oldpath == newpath)
+			continue;
+
 		/*
 		 * If pair->one->path isn't in opt->priv->paths, that means
 		 * that either directory rename detection removed that

t/t6423-merge-rename-directories.sh

@@ -4731,7 +4731,7 @@ test_setup_12i () {
 
 		mkdir -p source/subdir &&
 		echo foo >source/subdir/foo &&
-		echo bar >source/bar &&
+		printf "%d\n" 1 2 3 4 5 6 7 >source/bar &&
 		echo baz >source/baz &&
 		git add source &&
 		git commit -m orig &&
@@ -4778,6 +4778,69 @@ test_expect_success '12i: Directory rename causes rename-to-self' '
 	)
 '
 
+# Testcase 12i2, Identical to 12i except that source/subdir/bar modified on unrenamed side
+#   Commit O: source/{subdir/foo, bar, baz_1}
+#   Commit A: source/{foo, bar_2, baz_1}
+#   Commit B: source/{subdir/{foo, bar}, baz_2}
+#   Expected: source/{foo, bar, baz_2}, with conflicts on
+#                source/bar vs. source/subdir/bar
+
+test_setup_12i2 () {
+	git init 12i2 &&
+	(
+		cd 12i2 &&
+
+		mkdir -p source/subdir &&
+		echo foo >source/subdir/foo &&
+		printf "%d\n" 1 2 3 4 5 6 7 >source/bar &&
+		echo baz >source/baz &&
+		git add source &&
+		git commit -m orig &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git switch A &&
+		git mv source/subdir/foo source/foo &&
+		echo 8 >> source/bar &&
+		git add source/bar &&
+		git commit -m A &&
+
+		git switch B &&
+		git mv source/bar source/subdir/bar &&
+		echo more baz >>source/baz &&
+		git add source/baz &&
+		git commit -m B
+	)
+}
+
+test_expect_success '12i2: Directory rename causes rename-to-self' '
+	test_setup_12i2 &&
+	(
+		cd 12i2 &&
+
+		git checkout A^0 &&
+
+		test_must_fail git -c merge.directoryRenames=conflict merge -s recursive B^0 &&
+
+		test_path_is_missing source/subdir &&
+		test_path_is_file source/bar &&
+		test_path_is_file source/baz &&
+
+		git ls-files >actual &&
+		uniq <actual >tracked &&
+		test_line_count = 3 tracked &&
+
+		git status --porcelain -uno >actual &&
+		cat >expect <<-\EOF &&
+		UU source/bar
+		M  source/baz
+		EOF
+		test_cmp expect actual
+	)
+'
+
 # Testcase 12j, Directory rename to root causes rename-to-self
 #   Commit O: {subdir/foo, bar, baz_1}
 #   Commit A: {foo, bar, baz_1}
@@ -5106,7 +5169,7 @@ test_setup_12n () {
 	)
 }
 
-test_expect_failure '12n: Directory rename transitively makes rename back to self' '
+test_expect_success '12n: Directory rename transitively makes rename back to self' '
 	test_setup_12n &&
 	(
 		cd 12n &&
@@ -5166,7 +5229,7 @@ test_setup_12n2 () {
 	)
 }
 
-test_expect_failure '12n2: Directory rename transitively makes rename back to self' '
+test_expect_success '12n2: Directory rename transitively makes rename back to self' '
 	test_setup_12n2 &&
 	(
 		cd 12n2 &&