Subject: Re: [PATCH] object-file: disallow adding submodules of different hash algo

From: Junio C Hamano <[email-masked]>
Date: Thu, 13 Nov 2025 08:29:00 -0800

Jeff King <[email-masked]> writes:

> So this is what I'd propose on top of your patch. I can hold onto it for
> later if we don't want to muddy up what you're trying to do.

I do agree with both of the above.  The patch below makes perfect
sense to me, and it is more about the quality of implementation of
this codepath in general, than the primary theme of Brian's changes,
so there is no strong reason they have to come in a single series.

> -- >8 --
> Subject: [PATCH] read-cache: drop submodule check from add_to_cache()
>
> In add_to_cache(), we treat any directories as submodules, and complain
> if we can't resolve their HEAD. This call to resolve_gitlink_ref() was
> added by f937bc2f86 (add: error appropriately on repository with no
> commits, 2019-04-09), with the goal of improving the error message for
> empty repositories.
>
> But we already resolve the submodule HEAD in index_path(), which is
> where we find the actual oid we're going to use. Resolving it again here
> introduces some downsides:
>
>   1. It's more work, since we have to open up the submodule repository's
>      files twice.
>
>   2. There are call paths that get to index_path() without going through
>      add_to_cache(). For instance, we'd want a similar informative
>      message if "git diff empty" finds that it can't resolve the
>      submodule's HEAD. (In theory we can also get there through
>      update-index, but AFAICT it refuses to consider directories as
>      submodules at all, and just complains about them).
>
>   3. The resolution in index_path() catches more errors that we don't
>      handle here. In particular, it will validate that the object format
>      for the submodule matches that of the superproject. This isn't a
>      bug, since our call in add_to_cache() throws away the oid it gets
>      without looking at it. But it certainly caused confusion for me
>      when looking at where the object-format check should go.
>
> So instead of resolving the submodule HEAD in add_to_cache(), let's just
> teach the call in index_path() to actually produce an error message
> (which it already does for other cases). That's probably what f937bc2f86
> should have done in the first place, and it gives us a single point of
> resolution when adding a submodule to the index.
>
> The resulting output is slightly more verbose, as we propagate the error
> up the call stack, but I think that's OK (and again, matches many other
> errors we get when indexing fails).
>
> I've left the text of the error message as-is, though it is perhaps
> overly specific.  There are many reasons that resolving the submodule
> HEAD might fail, though outside of corruption or system errors it is
> probably most likely that the submodule HEAD is simply on an unborn
> branch.
>
> Signed-off-by: Jeff King <[email-masked]>
> ---
>  object-file.c  | 2 +-
>  read-cache.c   | 3 ---
>  t/t3700-add.sh | 1 +
>  3 files changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/object-file.c b/object-file.c
> index 8c43c52ed0..a7438b6205 100644
> --- a/object-file.c
> +++ b/object-file.c
> @@ -1662,7 +1662,7 @@ int index_path(struct index_state *istate, struct object_id *oid,
>  		break;
>  	case S_IFDIR:
>  		if (repo_resolve_gitlink_ref(istate->repo, path, "HEAD", oid))
> -			return -1;
> +			return error(_("'%s' does not have a commit checked out"), path);
>  		if (&hash_algos[oid->algo] != istate->repo->hash_algo)
>  			return error(_("cannot add a submodule of a different hash algorithm"));
>  		break;
> diff --git a/read-cache.c b/read-cache.c
> index 032480d0c7..990d4ead0d 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -706,7 +706,6 @@ int add_to_index(struct index_state *istate, const char *path, struct stat *st,
>  	int add_option = (ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE|
>  			  (intent_only ? ADD_CACHE_NEW_ONLY : 0));
>  	unsigned hash_flags = pretend ? 0 : INDEX_WRITE_OBJECT;
> -	struct object_id oid;
>  
>  	if (flags & ADD_CACHE_RENORMALIZE)
>  		hash_flags |= INDEX_RENORMALIZE;
> @@ -716,8 +715,6 @@ int add_to_index(struct index_state *istate, const char *path, struct stat *st,
>  
>  	namelen = strlen(path);
>  	if (S_ISDIR(st_mode)) {
> -		if (repo_resolve_gitlink_ref(the_repository, path, "HEAD", &oid) < 0)
> -			return error(_("'%s' does not have a commit checked out"), path);
>  		while (namelen && path[namelen-1] == '/')
>  			namelen--;
>  	}
> diff --git a/t/t3700-add.sh b/t/t3700-add.sh
> index b075eb9b11..d8cc0e4c66 100755
> --- a/t/t3700-add.sh
> +++ b/t/t3700-add.sh
> @@ -388,6 +388,7 @@ test_expect_success 'error on a repository with no commits' '
>  	test_must_fail git add empty >actual 2>&1 &&
>  	cat >expect <<-EOF &&
>  	error: '"'empty/'"' does not have a commit checked out
> +	error: unable to index file '"'empty/'"'
>  	fatal: adding files failed
>  	EOF
>  	test_cmp expect actual