exporter: remove wrappers for oci data types

[jedevc]

opencontainers/image-spec now contains the MediaType field for all of its structs (since opencontainers/image-spec@5b82148, part of the v1.1.0-rc2 release which we already vendor).

With this update, we can avoid manually wrapping the internal data types to present this field.

[tonistiigi]

Why did this change?

[jedevc]

The order of JSON field marshalling changes 😢

Essentially, the mediaType doesn't come first anymore (since it's not defined at the top-level).

[tonistiigi]

Could you give an example. Is the problem that Version comes before Mediatype now? FYI @AkihiroSuda

[jedevc]

Previously:

{
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "schemaVersion": 2,
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:27594184c4ad11392de489e7e9eba36e4685177b3df91a7b1e13e9dbec97d427",
      "size": 1056,
      "platform": {
        "architecture": "amd64",
        "os": "linux"
      }
    },
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:5a9a7a10ff48bf889374e0bb96dfd0f817fb96b0d86d9ff738543ecf2836c3bf",
      "size": 566,
      "annotations": {
        "vnd.docker.reference.digest": "sha256:27594184c4ad11392de489e7e9eba36e4685177b3df91a7b1e13e9dbec97d427",
        "vnd.docker.reference.type": "attestation-manifest"
      },
      "platform": {
        "architecture": "unknown",
        "os": "unknown"
      }
    }
  ]
}

Now:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:eb0454aabfa3084e80359c6dd64b2e88f26ca542a2af8a36eae752168bee1317",
      "size": 1056,
      "platform": {
        "architecture": "amd64",
        "os": "linux"
      }
    },
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:113c2a57ab2a69ec43f1fab69464c613697ae288cb24126422fc309939805311",
      "size": 566,
      "annotations": {
        "vnd.docker.reference.digest": "sha256:eb0454aabfa3084e80359c6dd64b2e88f26ca542a2af8a36eae752168bee1317",
        "vnd.docker.reference.type": "attestation-manifest"
      },
      "platform": {
        "architecture": "unknown",
        "os": "unknown"
      }
    }
  ]
}

schemaVersion appears first now (I think because marshaling visits each struct recursively, starting with the top-most, then descending down into embedded structs afterwards).

(note the change in digests is caused by this property applied recursively).

[thaJeztah]

Ah, yeah, noticed that as well some time ago; originally the MediaType part of specs.Versioned;
opencontainers/image-spec@4267a18 (opencontainers/image-spec#172)

Then got removed in opencontainers/image-spec@5fc84e5 (opencontainers/image-spec#411)

And when it was added back, it was added to the manifest itself (instead of Versioned) opencontainers/image-spec@5b82148 ()