Proposal: descriptor: "dependency" property

A blob referenced via descriptor MAY be a fine-graind CAS archive. e.g. continuity manifest. (WIP: https://github.com/opencontainers/image-spec/issues/577)

This might make it difficult to implement "gc" for blob store, because the actual referenced blob set cannot be assured just by parsing OCI descriptors.

So I suggest adding a new property named dependency to the descriptor spec.

{
  "mediaType": "application/vnd.continuity.manifest.v0+json",
  "digest": "sha256:deadbeef",
  "size": 424242,
  "dependency": {
    "type": "media-specific"
  },
}

When dependency.type is media-specific, implementations can skip pruning unless it is aware of the corresponding mediaType.

Spec Proposal (for v1.0 if possible)

• dependency.type string

  This OPTIONAL property specifies the type of the content set that targeted content refers.
  Values SHOULD be one of the followings:

  • ``(empty): the set is empty
  • media-specific: the set is specific to the media type of the targeted content

Implementor's Note
Implementations MAY compute unreferenced blob set so as to prune them.
When implementations encouters a descpritor of which dependency.type is media-specific and the implementation is not aware of the corresponding media type, or dependency.type is set to unknown value, the implementation SHOULD NOT try to compute unreferenced blob set.

Post-v1

e.g. list (can be huge)

"dependency": {
  "type": "list",
  "list": [
    "sha256:f00",
    "sha256:baa",
  ]
}

e.g. list as a text file blob

"dependency": {
  "type": "blob",
  "blob": {
    "mediaType": "application/vnd.oci.dependency.v2+text",
    "digest": "sha256:ba2",
    "size": 4242
  }
}

Start by reading the descriptor specification and the linked continuity-manifest discussion in issue 577. This proposal needs a maintainer decision on the dependency model and version scope before implementation can begin; done would mean an agreed schema and corresponding specification update.