Subject: Re: [RFC PATCH 1/1] Define an extended tree format

From: "brian m. carlson" <[email-masked]>
Date: Wed, 1 Oct 2025 21:45:29 +0000

On 2025-10-01 at 21:11:40, Jeff King wrote:
> So it all works as expected, but I feel like it's mostly by accident. My
> gut feeling is that we probably wanted something like this to protect us
> from confusion:
> 
> index 06ad74db22..295b0c6318 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -719,6 +719,8 @@ int add_to_index(struct index_state *istate, const char *path, struct stat *st,
>  	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);
> +		if (oid.algo != hash_algo_by_ptr(the_repository->hash_algo))
> +			return error(_("object format %s of '%s' is incompatible with this repository (%s)"), hash_algos[oid.algo].name, path, the_repository->hash_algo->name);
>  		while (namelen && path[namelen-1] == '/')
>  			namelen--;
>  	}
> 
> Of course that is strictly worse for somebody who is relying on the
> current accidental behavior. ;) And in the long run, I think this is the
> spot we'd want to hook to do whatever massaging we need (whether
> converting to the equivalent in-repo algorithm, or hacking up the name
> to store the foreign hash).

I think sending in this patch is a good first step right now.  The
intention of the transition document is that the code is in one and only
one hash algorithm at a time and it would be better to reject this case
until we're ready to wire it up correctly than end up with data that's
corrupt down the line.

> I also won't be at all surprised if you've run across this already in
> your interop work.

I have not, since it's primarily involved working with the testsuite and
some test repositories.  I know that submodules are broken in interop
mode; I didn't need to test that until I was writing code to make them
not-broken.
-- 
brian m. carlson (they/them)
Toronto, Ontario, CA