[PATCH v8 1/2] submodule: prevent overwriting .gitmodules entry on path reuse

[K Jayatheerth <jayatheerthkulkarni2005@gmail.com>]

Adding a submodule at a path that previously hosted another submodule
(e.g., 'child') reuses the submodule name derived from the path. If the
original submodule was only moved (e.g., to 'child_old') and not renamed,
this silently overwrites its configuration in .gitmodules.

This behavior loses user configuration and causes confusion when the
original submodule is expected to remain intact. It assumes that the
path-derived name is always safe to reuse, even though the name might
still be in use elsewhere in the repository.

Teach `module_add()` to check if the computed submodule name already
exists in the repository's submodule config, and if so, refuse the
operation unless the user explicitly renames or uses force to auto increment.

Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
---
 builtin/submodule--helper.c | 28 ++++++++++++++++++++++++++++
 t/t7400-submodule-basic.sh  | 23 +++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 53da2116dd..9f6df833f0 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -3444,6 +3444,10 @@ static int module_add(int argc, const char **argv, const char *prefix,
 	struct add_data add_data = ADD_DATA_INIT;
 	const char *ref_storage_format = NULL;
 	char *to_free = NULL;
+	const struct submodule *existing;
+	struct strbuf buf = STRBUF_INIT;
+	int i;
+	char *sm_name_to_free = NULL;
 	struct option options[] = {
 		OPT_STRING('b', "branch", &add_data.branch, N_("branch"),
 			   N_("branch of repository to add as submodule")),
@@ -3546,6 +3550,29 @@ static int module_add(int argc, const char **argv, const char *prefix,
 	if(!add_data.sm_name)
 		add_data.sm_name = add_data.sm_path;
 
+	existing = submodule_from_name(the_repository,
+					null_oid(the_hash_algo),
+					add_data.sm_name);
+
+	if (existing && strcmp(existing->path, add_data.sm_path)) {
+		if (!force) {
+			die(_("submodule name '%s' already used for path '%s'"),
+			add_data.sm_name, existing->path);
+		}
+
+		/* --force: build <name><n> until unique */
+		for (i = 1; ; i++) {
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "%s%d", add_data.sm_name, i);
+			if (!submodule_from_name(the_repository,
+						null_oid(the_hash_algo),
+						buf.buf)) {
+				break;
+			}
+		}
+
+		add_data.sm_name = sm_name_to_free = strbuf_detach(&buf, NULL);
+	}
 	if (check_submodule_name(add_data.sm_name))
 		die(_("'%s' is not a valid submodule name"), add_data.sm_name);
 
@@ -3561,6 +3588,7 @@ static int module_add(int argc, const char **argv, const char *prefix,
 
 	ret = 0;
 cleanup:
+	free(sm_name_to_free);
 	free(add_data.sm_path);
 	free(to_free);
 	strbuf_release(&sb);
diff --git a/t/t7400-submodule-basic.sh b/t/t7400-submodule-basic.sh
index d6a501d453..f5514decab 100755
--- a/t/t7400-submodule-basic.sh
+++ b/t/t7400-submodule-basic.sh
@@ -1482,4 +1482,27 @@ test_expect_success '`submodule init` and `init.templateDir`' '
 	)
 '
 
+test_expect_success 'submodule add fails when name is reused' '
+	git init test-submodule &&
+	(
+		cd test-submodule &&
+		git commit --allow-empty -m init &&
+
+		git init ../child-origin &&
+		git -C ../child-origin commit --allow-empty -m init &&
+
+		git submodule add ../child-origin child &&
+		git commit -m "Add submodule child" &&
+
+		git mv child child_old &&
+		git commit -m "Move child to child_old" &&
+
+		# Now adding a *new* repo at the old name must fail
+		git init ../child2-origin &&
+		git -C ../child2-origin commit --allow-empty -m init &&
+		test_must_fail git submodule add ../child2-origin child
+	)
+'
+
+
 test_done
-- 
2.49.GIT