Migrate to OCI Artifact from ORAS Artifact

[shizhMSFT]

We will migrate to OCI Artifact from ORAS Artifact once the following PRs are merged.

• Working Group Proposal for Reference Types opencontainers/image-spec#934
• Working Group Proposal for Reference Types opencontainers/distribution-spec#335

Tasks:

• Migrate artifactspec.Descriptor to ocispec.Descriptor #307
• Add support for OCI Artifact manifest #308
• Merge oras.PackArtifact() into oras.Pack() #309
• Improve performance of filters of oras.ExtendedCopy() caused by information loss due to descriptor conversions #310
• Update implementation of Referers API #311
• Add client-side compatibility for pushing manifest with subject field #312
• Add client-side compatibility for deleting manifest with subject field #347

This issue covers #267 and #268.

[sajayantony]

Should we start a new branch for OCI now that you have go structs for OCI. Similarly, for oras - CLI?
/cc @nima @michaelb990 @shizhMSFT

[rdpsin]

Any update on this work?

[Wwwsylvia]

@rdpsin We will be working on this after releasing oras-go v2.0.0-rc3, which is scheduled for Sep. 5th.

[SteveLasker]

Just a clarification that we'll need to maintain support for ORAS artifacts, until all registries have migrated to the OCI Artifact spec.

1. Check if /referrers is supported (best future case)
2. Check if _oras/referrers is supported (limited time, as ACR, ECR and Zot migrate to /referrers)
3. Fallback to using OCI Image manifest, with a refers/subject property (see Rename refers field to subject #950)

[shizhMSFT]

Just a clarification that we'll need to maintain support for ORAS artifacts, until all registries have migrated to the OCI Artifact spec.

1. Check if /referrers is supported (best future case)
2. Check if _oras/referrers is supported (limited time, as ACR, ECR and Zot migrate to /referrers)
3. Fallback to using OCI Image manifest, with a refers/subject property (see Rename refers field to subject #950)

The initial iteration will not have the fallback to ORAS Artifact referrers. The fallback will be added in the second iteration after proper discussions.

[sajayantony]

Oras needs to consume RC2 since it includes the subject field - opencontainers/image-spec#958

[michaelb990]

The fallback will be added in the second iteration after proper discussions.

What fallback are we talking about here? From what I read, it seemed like falling back to ORAS Artifacts wouldn't be compatible with the tag-based fallback already proposed in the OCI distribution-spec, now an RC.

[sajayantony]

On the call today we discussed that the fall back to ORAS is not going to be supported. This issues needs to be updated to reflect that and the next update to the libraries will remove the ORAS artifact types and move to OCI with OCI image fallback.

[shizhMSFT]

Closing as all sub tasks are resolved.