Anonymous blob mounting

[Kyle Tarplee]

For motivation and background see this

From the spec the from field is now optional when doing blob mounts.

The registry MAY treat the from parameter as optional, and it MAY cross-mount the blob if it can be found.

[Lixia (Sylvia) Lei]

Is this an issue of the distribution-spec? 🤔

[Kyle Tarplee]

@Wwwsylvia I'm not sure what you mean. It is not an issue with the distribution spec. The spec gives the client more flexibility in that they do not have to provide the from field if they do not want to. Maybe oras-go is handling this already but I did not see it when looking at it earlier. It seems like even in the case where the source repo is unknown, we would want to always try a POST to /v2/<name>/blobs/uploads/?mount=<digest> to try and benefit from a mount.

[Lixia (Sylvia) Lei]

@ktarplee Thanks for the clarification, I see what you mean now.
blobStore.Mount() accepts an empty value of from,

oras-go/registry/remote/repository.go

Lines 770 to 773 in 9b6f321

 fromRef.Repository = fromRepo 
 ctx = auth.AppendRepositoryScope(ctx, fromRef, auth.ActionPull) 
  
 url := buildRepositoryBlobMountURL(s.repo.PlainHTTP, s.repo.Reference, desc.Digest, fromRepo) 

and oras.Copy only checks if the sourceRepositories slice is empty or not but does not validate each sourceRepository.

oras-go/copy.go

Lines 287 to 296 in 9b6f321

 sourceRepositories, err := opts.MountFrom(ctx, desc) 
 if err != nil { 
 	// Technically this error is not fatal, we can still attempt to copy the node 
 	// But for consistency with the other callbacks we bail out. 
 	return err 
 } 
  
 if len(sourceRepositories) == 0 { 
 	return copyNode(ctx, src, dst, desc, opts) 
 } 

So oras-go technically supports this scenario. Users can do anonymous mounting through something like below:

anonymousMounting := func(ctx context.Context, desc ocispec.Descriptor) ([]string, error) {
      return []string{""}, nil
}
opts := oras.CopyOptions{
      MountFrom = anonymousMounting
}
desc, err := oras.Copy(ctx, src, tagName, dst, tagName, opts)

If needed, we can make if more user friendly though.