Use case is wanting to set up a server with a git-annex repository which is a mirror of an original repository. All git refs are mirrored, using eg git push --mirror.

So, the git-annex branch cannot diverge. Or if it does, any divergence will get overwritten the next time the refs mirror is updated.

The mirror can contain some, but not necessarily all annex objects as the original repository. If a client requents an annex object that is not present in the mirror, it transparently proxies the request to the original repository. It may also, optionally, cache the object in the mirror (if the mirror's preferred content configuration wants it).

A client dropping an object from the mirror would have it proxy the drop to the original repository, and only delete its cache if that drop succeeded.

It seems at first that the mirror repository would need to have the same uuid as the original repository. Since the client sees the original git-annex branch, it only knows about the uuid in that branch for location tracking.

But using the same uuid would bring in a ton of special cases and problems. Just a few of them are: * git-annex get in the mirror repository would fail to get files from the original repository, because the local repo uuid is filtered out of the remote list in keyPossibilities'. * The mirror couldn't have its own preferred content configuration.

The mirror repository could have the same uuid as the original set in annex.uuid, so the client thinks it has the same uuid, but actually have a different uuid that is configured elsewhere. Make git-annex use that other uuid, and with annex.private set, it will never reach the git-annex branch.

In the P2P protocol, AUTH-SUCCESS includes the server's uuid. That would need to be the original repository's uuid, to avoid a client thinking it's talking to the wrong repository.

Concretely, a mirror could be configured like this:

git remote add original http://example.com/foo
git pull --prune mirror
git config annex.mirroring original
git config annex.private true
git-annex init
# that sets annex.uuid to the discovered uuid of original
# and it sets annex.mirror-uuid to the repo's true uuid
git-annex mirror --from=original # optional