Validating successor descriptor during copy

[Wwwsylvia]

Problem

When copying a manifest, it's possible for the manifest to reference a malformed descriptor. For example, the following manifest includes a subject descriptor that is missing the required size field:

{
  "artifactType": "application/vnd.dev.sigstore.bundle.v0.3+json",
  "config": {
    "digest": "sha256:b198fd23d763339899149b9d9c96bf43d4ba9fa97e6c81d502c9fbbb58eb319f",
    "mediaType": "application/vnd.oci.image.config.v1+json",
    "size": 233
  },
  "layers": [
    {
      "digest": "sha256:eb14f1aeeecfb76481f4ff70b2d523fe065d947932768317bb7469581cd0a0a5",
      "mediaType": "application/vnd.dev.cosign.simplesigning.v1+json",
      "size": 432
    }
  ],
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "schemaVersion": 2,
"subject": {
    "digest": "sha256:60ac88fe91aee1a990c7f89f8003e391ac80c847321ccf697986320a6e9717c8",
    "mediaType": "application/vnd.oci.image.manifest.v1+json"
  }
}

The relationship between the manifest and its subject can be visualized as:

graph TD;

Manifest--subject-->Subject["Subject (sha256:60ac88...)"]

Loading

During the copy process, the Successors method is invoked on the manifest. This returns the subject descriptor, which is then passed to a goroutine responsible for copying the subject:

{
    "digest": "sha256:60ac88fe91aee1a990c7f89f8003e391ac80c847321ccf697986320a6e9717c8",
    "mediaType": "application/vnd.oci.image.manifest.v1+json"
}

When processing the subject, since this descriptor lacks a size field, it may fail subsequent validation, especially when compared against the Content-Length header from a remote registry. This results in an error like:

Error: GET "https://{registry}/v2/{repo}/manifests/sha256:60ac88fe91aee1a990c7f89f8003e391ac80c847321ccf697986320a6e9717c8": mismatch Content-Length

This error is not intuitive and makes it difficult for users to identify the root cause.

Proposal

To address this, we could consider validating the successor descriptors earlier in the copy process and returning a clear error message when issues are detected.

One possible error message could be:

invalid successor descriptor for {manifest_digest}: successor media type: application/vnd.oci.image.manifest.v1+json; successor size: 0; successor digest: sha256:60ac88fe91aee1a990c7f89f8003e391ac80c847321ccf697986320a6e9717c8