Cannot copy buildkit cache images

[multun]

I've tried copying an image with a manifest with mediaType application/vnd.buildkit.cacheconfig.v0. These images are used by buildkit when caching to a registry, which is the prefered setup.

Reproducer

$ skopeo inspect docker://ghcr.io/osrd-project/unstable/osrd-core:dev-cache
FATA[0000] Error parsing manifest for image: fetching target platform image selected from image index: reading manifest sha256:0bfe9587aa2bf5b903546ccce960c67e492e2991ed1f17f849d31ab9ce90c7ac in ghcr.io/osrd-project/unstable/osrd-core: manifest unknown 

$ skopeo copy docker://ghcr.io/osrd-project/unstable/osrd-core:dev-cache XXXX
[fails with the same error]

Adding --raw to skopeo inspect makes it "work", as skopeo then doesn't need to parse the manifest

[mtrmac]

Thanks for your report.

The title says copy (and that also fails) but the reproducer uses inspect. Which one matters to you?

---

The manifest structure is

{
    "schemaVersion": 2,
    "mediaType": "application/vnd.oci.image.index.v1+json",
    "manifests": [
        {
            "mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
            "digest": "sha256:0bfe9587aa2bf5b903546ccce960c67e492e2991ed1f17f849d31ab9ce90c7ac",
            "size": 734,
            "annotations": {
                "buildkit/createdat": "2023-11-29T15:55:24.502779316Z",
                "containerd.io/distribution.source.ghcr.io": "osrd-project/unstable/osrd-core",
                "containerd.io/uncompressed": "sha256:393a68c8324ccea2b51ac02caf3f03f7b906040f8d8c2ffef1cc3bbca4b64acc"
            }
        },
…

and that’s just invalid. https://github.com/opencontainers/image-spec/blob/main/image-index.md says that the manifests entries must point to manifests, and these are layers.

I’m not saying we must fail handling this, but it’s a new structure not anticipated by the existing code.

[multun]

The title says copy (and that also fails) but the reproducer uses inspect. Which one matters to you?

copy is the one I'm interested in, but I figured that making inspect work would probably be one of the intermediary steps in implementing copy support