Feature request: Adding repo.ReferrersCapability() in oras-go

[patrickzheng200]

As a user, I'd like to check if a remote registry supports the Referrers API. Currently, I need the following code to do so:

const zeroDigest = "sha256:0000000000000000000000000000000000000000000000000000000000000000"
if err := remoteRepo.SetReferrersCapability(true); err != nil {
	return nil, err
}
var checkReferrerDesc ocispec.Descriptor
checkReferrerDesc.Digest = zeroDigest
err := remoteRepo.Referrers(ctx, checkReferrerDesc, "", func(referrers []ocispec.Descriptor) error {
	return nil
})
if err != nil {
	return nil, fmt.Errorf("failed to ping Referrers API with error: %v", err)
}

the remoteRepo.Referrers is an overhead. Applications that want this check need to have something similar, so probably adding a repo.ReferrersCapability() feature in oras-go might save resources for users?

[Wwwsylvia]

We may export pingReferrers() if there is a need.

[shizhMSFT]

Since there are multiple solutions / options, why not start a discussion on the design?

[shizhMSFT]

We may export pingReferrers() if there is a need.

Exporting pingReferrers() is a good candidate as we already have an API named Ping().

[patrickzheng200]

We may export pingReferrers() if there is a need.

Yeah, exporting pingReferrers() would simplify the above logic to a 1-line code since it also sets the repo's referrersState.

[Wwwsylvia]

This feature was requested for notation to determine which manifest type to pack (OCI Artifact Manifest or OCI Image Manifest).
Given that OCI Artifact Manifest has been deprecated since image-spec v1.1.0-rc4 (See #494), this scenario is not valid anymore.
Hence, we can close this issue for now.
Please reopen it if the feature is still needed.