This broadly follows the plan from #1408 (comment) :

• Reject image-specific operations (like inspect, image ID) on OCI artifacts
• Don’t change compression of artifact layers
• FIXME: Should we also reject encryption/decryption operations?

See the individual commits for details.

FIXME: At this point I’m rather unhappy about the CanChangeLayerCompression API. It’s good enough for internal use for now, exposing it in c/image/manifest is not great. OTOH I’d rather not spend time investigating the more general which-image-formats-can-support-which-compression-and-what-to-do-if-they-don’t problem space.

At this point absolutely untested apart from unit tests (which don’t cover most of the copy code); this will be the first run against Skopeo’s integration test and a real artifact.

See individual commit messages for details. Quite a lot of the refactoring could be split into a separate PR… if an existing OCI artifact didn’t break CI right now. Something to consider later.