code: func (serv *v2MetadataService) GetDiffID(dgst digest.Digest) (layer.DiffID, error) {
  	if serv.store == nil {
  		return layer.DiffID(""), errors.New("no metadata storage")
  	}
  	diffIDBytes, err := serv.store.Get(serv.digestNamespace(), serv.digestKey(dgst))
  	if err != nil {
  		return layer.DiffID(""), err
  	}
  
  	return layer.DiffID(diffIDBytes), nil
  }
docs: // GetDiffID finds a layer DiffID from a digest.