Okay, I've adjusted the tests and this should be good to go, although I don't love the hackiness of it all. I guess we can rip out all the special casing whenever registry.fedoraproject.org gets retired.

Yeah, seems ok to me...

On a quick eyeball review I get the idea and it looks fine. If I get a bit of time I'll try to refresh my memory on what the different bits of this are called and suggest a docstring and maybe some changed variable names.

https://specs.opencontainers.org/image-spec/manifest/ refers to the outer manifest as a "fat manifest" (quotation marks in original!) and the index as the "image index", so we could maybe disambiguate a bit using that terminology? i.e. something like this:

class FatManifest:
    """
    A "fat" OCI container manifest, per
    https://specs.opencontainers.org/image-spec/manifest/ . It contains
    an "image index" pointing to multiple individual image manifests,
    one per arch, so forming a multiple-architecture manifest.
    """

Thanks, that's much better than my placeholder docs. I've adjusted things and if you're comfortable with the change I can merge things and roll it out shortly.

now we know the correct name for this, maybe we should call it image_index just for clarity/consistency?

just that one minor note, then I think it's good. thanks a lot for doing this!