v1.ImageIndex needs a Blob(Hash) method

[jonjohnsonjr]

Given that a descriptor can point to arbitrary content, we need a way to access that content if it doesn't conform to expected media-types. layout.layoutIndex already implements this.

We may want to have an option v1.ImageIndex.Layer(Hash) as well (and use it for known layer media types), because of how MountableLayer works.

We also need to update a lot of places where we handle v1.ImageIndex. A few that spring to mind...

In mutate, you can't append non-manifests to an index:

go-containerregistry/pkg/v1/mutate/index.go

Lines 104 to 106 in 6a63025

	} else {
	logs.Warn.Printf("Unexpected index addendum: %T", add.Add)
	}

In remote.Write, we just ignore non-manifests:

go-containerregistry/pkg/v1/remote/write.go

Lines 517 to 535 in 6a63025

	switch desc.MediaType {
	case types.OCIImageIndex, types.DockerManifestList:
	ii, err := ii.ImageIndex(desc.Digest)
	if err != nil {
	return err
	}
	if err := WriteIndex(ref, ii, WithAuth(o.auth), WithTransport(o.transport)); err != nil {
	return err
	}
	case types.OCIManifestSchema1, types.DockerManifestSchema2:
	img, err := ii.Image(desc.Digest)
	if err != nil {
	return err
	}
	if err := Write(ref, img, WithAuth(o.auth), WithTransport(o.transport)); err != nil {
	return err
	}
	}

In remote.MultiWrite, we return an error:

go-containerregistry/pkg/v1/remote/multi_write.go

Line 199 in 6a63025

return nil, fmt.Errorf("unknown media type: %v", desc.MediaType)

In layout.Write, we ignore non-manifests:

go-containerregistry/pkg/v1/layout/write.go

Lines 261 to 263 in 6a63025

	default:
	// TODO: The layout could reference arbitrary things, which we should
	// probably just pass through.

In validate.Index, we log a warning:

go-containerregistry/pkg/v1/validate/index.go

Lines 78 to 79 in 6a63025

	default:
	logs.Warn.Printf("Unexpected manifest: %s", desc.MediaType)

In compare.Indexes, we don't bother:

go-containerregistry/pkg/internal/compare/index.go

Line 76 in 6a63025

// TODO(jonjohnsonjr): Iterate over Manifest and compare Image and ImageIndex results.

[jonjohnsonjr]

This came up in #814 (comment)

[jonjohnsonjr]

We might want to add a Manifests() method that mirrors v1.Image's Layers().

Unfortunately, there's not a great return type for that. We need something that can be any of v1.Image, v1.ImageIndex, v1.Layer, or just an io.ReadCloser (perhaps this can just be a v1.Layer?). Related to #835

I need to go through all the callsites to see what we actually end up doing with this stuff to see if there's an obvious common set of functionality.

In the short term, we could do some typechecking for Blob(Hash) and Layer(hash) for unexpected mediatypes.

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