Theodore Tso <tytso@xxxxxxx> writes: > Long-standing mis-feature in git's logic in deciding what to push. > It's been reported a few times, but apparently it's hard to fix, or at > least it never hsa been fixed as far as I know. This comes from an early (mis)design of git. Background. * A git repository and the object store it uses can be separate. From the beginning, you can have a"objects/" directory (aka "object store") that is shared by more than one repositories. There is no Porcelain level support to set up two repositories that physically share the same object store, but the result of "git init; rm -rf .git/objects; ln -s $other/.git/objects .git/objects" was supposed to work (and it still largely works, until you gc) in the original design. The alternate object store does not even have to be a git repository, which makes things worse. You can have everybody pointing at /var/cache/objects, and /var/cache does not have to be a git repository (i.e. no var/cache/refs). * The existing alternates mechanism is not about alternate repositories. It is about alternate object stores. That is why each line of this file points at "objects" directory elsewhere, not the ".git" directory that is typically at one level above that "objects" directory. The fact your repository's object store points at the object store that happens to be inside Linus's repository does not imply that Linus's object store is associated with refs in Linus's repository in any way (that's the early _mis_design part). * An existing ref in a git repository is meant to be a guarantee that all objects the object referenced by the ref is found somewhere in the object store(s) the repository uses. Object transfers in git (i.e. fetch and push) use this guarantee to tell what a repository has to the other side. What happens in your case is that github end knows that the repository you are pushing into have up to the refs you have there. Alternate may point at object store that holds objects from Linus's repository, but there is no information as to what the latest commits you do not see in your refs namespace (namely, "what's Linus's latest" is not something you can learn from your repository that has alternates). A possible fix would involve: - Deprecate objects/info/alternates file, and GIT_OBJECT_DIRECTORY and GIT_ALTERNATE_OBJECT_DIRECTORIES environment variables; - Introduce info/alternates that points at alternate _repositories_ (as opposed to objects/info/alternates that points at alternate object stores); - Teach fetch and push to include refs from alternate _repositories_ into what local side considers complete. The above won't break existing setups, but it won't help them either. All the borrowing repositoies need to be converted if we go that route. We could instead redefine the semantics of the existing alternates mechanism. This technically *breaks* backward compatibility, but I suspect it won't hurt many existing installations: - Declare that a freestanding object store is illegal. In other words, if a directory "$D/objects" is (1) used as $GIT_OBJECT_DIRECTORY's value, (2) pointed by some repository's "alternates" file, or (3) listed in $GIT_ALTERNATE_OBJECT_DIRECTORIES's value, this change makes it illegal for "$D" not being a proper git repository. This will not break your example of your repository's object store borrowing from the object store inside Linus's repository. - When you have "$D/objects" in alternates, start relying on "$D/refs" being correct (i.e. repository $D is not corrupt). This technically makes the system slightly less robust, as we are depending on _other people's_ good behaviour even more when you use alternates, but you are already depending on them having good objects in $D/objects anyway, so it is not a big deal. - Now that we declared that everything reachable from "$D/refs" do not have to be transferred from elsewhere when a push sends things into us (or a fetch gets things from elsewhere into us) when you have "$D/objects" in your alternates. In your "borrowing from Linus" example, Linus's latest will be reachable from somewhere in "$D/refs", when you are borrowing from him by having "$D/objects" in your alternates. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
