From: Junio C Hamano Date: Wed, 01 Oct 2025 21:19:09 GMT Subject: Re: [RFC PATCH 1/1] Define an extended tree format Message-ID: In-Reply-To: <20251001211140.GA140550@peff.net> Jeff King writes: > The other unexpected thing is that the sequence above works today! I > think it is mostly accidental, though. When we read the head of the > submodule, we get a "struct object_id" with a different algo field. But > we never pay attention to that field, and just treat it as if it is > using our native algorithm. Yeah, I recall that there was a discussion about this exact "yes, we get by fine with SHA-1 commit object name 0-padded to the right, when embedding SHA-1 submodule inside SHA-256 superproject", countered by brian's "no, it may happen to work but you shouldn't rely on it" recently. > 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 also won't be at all surprised if you've run across this already in > your interop work.