[PATCH 2/2] merge-recursive: fix merging a subdirectory into the root directory

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Elijah Newren <newren@gmail.com>

We allow renaming all entries in e.g. a directory named z/ into a
directory named y/ to be detected as a z/ -> y/ rename, so that if the
other side of history adds any files to the directory z/ in the mean
time, we can provide the hint that they should be moved to y/.

There is no reason to not allow 'y/' to be the root directory, but the
code did not handle that case correctly.  Add a testcase and the
necessary special checks to support this case.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 merge-recursive.c                   | 29 +++++++++++++++
 t/t6043-merge-rename-directories.sh | 56 +++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)

diff --git a/merge-recursive.c b/merge-recursive.c
index f80e48f623..7bd4a7cf10 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -1931,6 +1931,16 @@ static char *apply_dir_rename(struct dir_rename_entry *entry,
 		return NULL;
 
 	oldlen = strlen(entry->dir);
+	if (entry->new_dir.len == 0)
+		/*
+		 * If someone renamed/merged a subdirectory into the root
+		 * directory (e.g. 'some/subdir' -> ''), then we want to
+		 * avoid returning
+		 *     '' + '/filename'
+		 * as the rename; we need to make old_path + oldlen advance
+		 * past the '/' character.
+		 */
+		oldlen++;
 	newlen = entry->new_dir.len + (strlen(old_path) - oldlen) + 1;
 	strbuf_grow(&new_path, newlen);
 	strbuf_addbuf(&new_path, &entry->new_dir);
@@ -1980,6 +1990,25 @@ static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 	    *end_of_old == *end_of_new)
 		return; /* We haven't modified *old_dir or *new_dir yet. */
 
+	/*
+	 * If end_of_new got back to the beginning of its string, and
+	 * end_of_old got back to the beginning of some subdirectory, then
+	 * we have a rename/merge of a subdirectory into the root, which
+	 * needs slightly special handling.
+	 *
+	 * Note: There is no need to consider the opposite case, with a
+	 * rename/merge of the root directory into some subdirectory.
+	 * Our rule elsewhere that a directory which still exists is not
+	 * considered to have been renamed means the root directory can
+	 * never be renamed (because the root directory always exists).
+	 */
+	if (end_of_new == new_path &&
+	    end_of_old != old_path && end_of_old[-1] == '/') {
+		*old_dir = xstrndup(old_path, end_of_old-1 - old_path);
+		*new_dir = xstrndup(new_path, end_of_new - new_path);
+		return;
+	}
+
 	/*
 	 * We've found the first non-matching character in the directory
 	 * paths.  That means the current characters we were looking at
diff --git a/t/t6043-merge-rename-directories.sh b/t/t6043-merge-rename-directories.sh
index c966147d5d..b920bb0850 100755
--- a/t/t6043-merge-rename-directories.sh
+++ b/t/t6043-merge-rename-directories.sh
@@ -4051,6 +4051,62 @@ test_expect_success '12c-check: Moving one directory hierarchy into another w/ c
 	)
 '
 
+# Testcase 12d, Rename/merge of subdirectory into the root
+#   Commit O: a/b/{foo.c}
+#   Commit A: foo.c
+#   Commit B: a/b/{foo.c,bar.c}
+#   Expected: a/b/{foo.c,bar.c}
+
+test_expect_success '12d-setup: Rename (merge) of subdirectory into the root' '
+	test_create_repo 12d &&
+	(
+		cd 12d &&
+
+		mkdir -p a/b/subdir &&
+		test_commit a/b/subdir/foo.c &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		mkdir subdir &&
+		git mv a/b/subdir/foo.c.t subdir/foo.c.t &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		test_commit a/b/bar.c
+	)
+'
+
+test_expect_success '12d-check: Rename (merge) of subdirectory into the root' '
+	(
+		cd 12d &&
+
+		git checkout A^0 &&
+
+		git -c merge.directoryRenames=true merge -s recursive B^0 &&
+
+		git ls-files -s >out &&
+		test_line_count = 2 out &&
+
+		git rev-parse >actual \
+			HEAD:subdir/foo.c.t   HEAD:bar.c.t &&
+		git rev-parse >expect \
+			O:a/b/subdir/foo.c.t  B:a/b/bar.c.t &&
+		test_cmp expect actual &&
+
+		git hash-object bar.c.t >actual &&
+		git rev-parse B:a/b/bar.c.t >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:a/b/subdir/foo.c.t &&
+		test_must_fail git rev-parse HEAD:a/b/bar.c.t &&
+		test_path_is_missing a/
+	)
+'
+
 ###########################################################################
 # SECTION 13: Checking informational and conflict messages
 #
-- 
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