Subject: Re: [PATCH] submodule: prevent overwriting .gitmodules entry on path reuse

From: Junio C Hamano <[email-masked]>
Date: Mon, 12 May 2025 05:32:08 -0700

K Jayatheerth <[email-masked]> writes:

> When a submodule is added at a path that previously hosted another submodule
> (e.g., 'child'), Git reuses the submodule name derived from the path and
> updates the corresponding entry in .gitmodules. This can silently overwrite
> existing configuration if the old submodule was only moved (e.g., to
> 'child_old') without renaming the submodule.
>
> This patch improves the `module_add()` logic by checking whether the
> submodule name already exists in the config but maps to a different path.

Quite sensible description of the problem and the proposed course of
improvement.

I do not think `--force` that allows the same name to be reused a
good idea at all, though.  We shouldn't encourage its use to resolve
such a case.  If what used to be called `child` now sits elsewhere,
perhaps because the tree structure was reorganized due to mass
renaming, but if it still is being used in the project, there is no
good reason to nuke the configuration recorded for that existing
module.

The module name used in .git/config is purely local so the user
should just give a new one a name that does not conflict, or even
better yet, perhaps the tool should pick a unique and nonconflicting
name automatically, no?

> In such a case, Git now errors out unless `--force` is specified, thus
> preventing accidental overwrites. To proceed safely, the user can provide
> a new name via `--name` or use `--force`.

>
> Signed-off-by: K Jayatheerth <[email-masked]>
> ---
>  builtin/submodule--helper.c | 45 ++++++++++++++++++++++++++++---------
>  t/t7400-submodule-basic.sh  | 23 +++++++++++++++++++
>  2 files changed, 57 insertions(+), 11 deletions(-)
>
> diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
> index 53da2116dd..0f98ef122b 100644
> --- a/builtin/submodule--helper.c
> +++ b/builtin/submodule--helper.c
> @@ -32,6 +32,8 @@
>  #include "advice.h"
>  #include "branch.h"
>  #include "list-objects-filter-options.h"
> +#include "wildmatch.h"
> +
>  
>  #define OPT_QUIET (1 << 0)
>  #define OPT_CACHED (1 << 1)
> @@ -3323,6 +3325,23 @@ static int config_submodule_in_gitmodules(const char *name, const char *var, con
>  	return ret;
>  }
>  
> +static int submodule_active_matches_path(const char *path)
> +{
> +	const struct string_list *values;
> +	size_t i;
> +
> +	if (git_config_get_string_multi("submodule.active", &values))
> +		return 0;
> +
> +	for (i = 0; i < values->nr; i++) {
> +		const char *pat = values->items[i].string;
> +		if (!wildmatch(pat, path, 0))
> +			return 1;
> +	}
> +
> +	return 0;
> +}
> +
>  static void configure_added_submodule(struct add_data *add_data)
>  {
>  	char *key;
> @@ -3370,17 +3389,7 @@ static void configure_added_submodule(struct add_data *add_data)
>  	 * is_submodule_active(), since that function needs to find
>  	 * out the value of "submodule.active" again anyway.
>  	 */
> -	if (!git_config_get("submodule.active")) {
> -		/*
> -		 * If the submodule being added isn't already covered by the
> -		 * current configured pathspec, set the submodule's active flag
> -		 */
> -		if (!is_submodule_active(the_repository, add_data->sm_path)) {
> -			key = xstrfmt("submodule.%s.active", add_data->sm_name);
> -			git_config_set_gently(key, "true");
> -			free(key);
> -		}
> -	} else {
> +	if (!submodule_active_matches_path(add_data->sm_path)) {
>  		key = xstrfmt("submodule.%s.active", add_data->sm_name);
>  		git_config_set_gently(key, "true");
>  		free(key);
> @@ -3443,6 +3452,7 @@ static int module_add(int argc, const char **argv, const char *prefix,
>  	int force = 0, quiet = 0, progress = 0, dissociate = 0;
>  	struct add_data add_data = ADD_DATA_INIT;
>  	const char *ref_storage_format = NULL;
> +	const struct submodule *existing;
>  	char *to_free = NULL;
>  	struct option options[] = {
>  		OPT_STRING('b', "branch", &add_data.branch, N_("branch"),
> @@ -3546,6 +3556,19 @@ 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' "
> +			"(use --name to choose another or --force to overwrite)"),
> +			add_data.sm_name, existing->path);
> +		}
> +		add_data.sm_name = xstrfmt("%s.%s", add_data.sm_name, basename(add_data.sm_path));
> +	}
> +
>  	if (check_submodule_name(add_data.sm_name))
>  		die(_("'%s' is not a valid submodule name"), add_data.sm_name);
>  
> diff --git a/t/t7400-submodule-basic.sh b/t/t7400-submodule-basic.sh
> index d6a501d453..5c3f471338 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 "initial commit" &&
> +
> +		git init ../child-origin &&
> +		git -C ../child-origin commit --allow-empty -m "initial commit" &&
> +
> +		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" &&
> +
> +		# Create another submodule repo
> +		git init ../child2-origin &&
> +		git -C ../child2-origin commit --allow-empty -m "initial commit" &&
> +
> +		test_must_fail git submodule add ../child2-origin child
> +	)
> +'
> +
>  test_done