merge-recursive: fix merging a subdirectory into the root directory

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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);
@@ -1963,8 +1973,26 @@ static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 	 */
 	end_of_old = strrchr(old_path, '/');
 	end_of_new = strrchr(new_path, '/');
-	if (end_of_old == NULL || end_of_new == NULL)
-		return; /* We haven't modified *old_dir or *new_dir yet. */
+
+	/*
+	 * If end_of_old is NULL, old_path wasn't in a directory, so there
+	 * could not be a directory rename (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_old == NULL)
+		return; /* Note: *old_dir and *new_dir are still NULL */
+
+	/*
+	 * If new_path contains no directory (end_of_new is NULL), then we
+	 * have a rename of old_path's directory to the root directory.
+	 */
+	if (end_of_new == NULL) {
+		*old_dir = xstrndup(old_path, end_of_old - old_path);
+		*new_dir = xstrdup("");
+		return;
+	}
 
 	/* Find the first non-matching character traversing backwards */
 	while (*--end_of_new == *--end_of_old &&
@@ -1978,7 +2006,25 @@ static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 	 */
 	if (end_of_old == old_path && end_of_new == new_path &&
 	    *end_of_old == *end_of_new)
-		return; /* We haven't modified *old_dir or *new_dir yet. */
+		return; /* Note: *old_dir and *new_dir are still NULL */
+
+	/*
+	 * 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
+	 * because as noted above the root directory always exists so it
+	 * cannot be considered to be renamed.
+	 */
+	if (end_of_new == new_path &&
+	    end_of_old != old_path && end_of_old[-1] == '/') {
+		*old_dir = xstrndup(old_path, --end_of_old - old_path);
+		*new_dir = xstrdup("");
+		return;
+	}
 
 	/*
 	 * We've found the first non-matching character in the directory

t/t6043-merge-rename-directories.sh

@@ -4051,6 +4051,120 @@ 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/subdir/foo
+#   Commit A: subdir/foo
+#   Commit B: a/b/subdir/foo, a/b/bar
+#   Expected: subdir/foo, bar
+
+test_expect_success '12d-setup: Rename/merge subdir into the root, variant 1' '
+	test_create_repo 12d &&
+	(
+		cd 12d &&
+
+		mkdir -p a/b/subdir &&
+		test_commit a/b/subdir/foo &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		mkdir subdir &&
+		git mv a/b/subdir/foo.t subdir/foo.t &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		test_commit a/b/bar
+	)
+'
+
+test_expect_success '12d-check: Rename/merge subdir into the root, variant 1' '
+	(
+		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.t   HEAD:bar.t &&
+		git rev-parse >expect \
+			O:a/b/subdir/foo.t  B:a/b/bar.t &&
+		test_cmp expect actual &&
+
+		git hash-object bar.t >actual &&
+		git rev-parse B:a/b/bar.t >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:a/b/subdir/foo.t &&
+		test_must_fail git rev-parse HEAD:a/b/bar.t &&
+		test_path_is_missing a/ &&
+		test_path_is_file bar.t
+	)
+'
+
+# Testcase 12e, Rename/merge of subdirectory into the root
+#   Commit O: a/b/foo
+#   Commit A: foo
+#   Commit B: a/b/foo, a/b/bar
+#   Expected: foo, bar
+
+test_expect_success '12e-setup: Rename/merge subdir into the root, variant 2' '
+	test_create_repo 12e &&
+	(
+		cd 12e &&
+
+		mkdir -p a/b &&
+		test_commit a/b/foo &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git checkout A &&
+		mkdir subdir &&
+		git mv a/b/foo.t foo.t &&
+		test_tick &&
+		git commit -m "A" &&
+
+		git checkout B &&
+		test_commit a/b/bar
+	)
+'
+
+test_expect_success '12e-check: Rename/merge subdir into the root, variant 2' '
+	(
+		cd 12e &&
+
+		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:foo.t   HEAD:bar.t &&
+		git rev-parse >expect \
+			O:a/b/foo.t  B:a/b/bar.t &&
+		test_cmp expect actual &&
+
+		git hash-object bar.t >actual &&
+		git rev-parse B:a/b/bar.t >expect &&
+		test_cmp expect actual &&
+
+		test_must_fail git rev-parse HEAD:a/b/foo.t &&
+		test_must_fail git rev-parse HEAD:a/b/bar.t &&
+		test_path_is_missing a/ &&
+		test_path_is_file bar.t
+	)
+'
+
 ###########################################################################
 # SECTION 13: Checking informational and conflict messages
 #