code: func (rl *roLayer) TarStream() (io.ReadCloser, error) {
  	rc, err := rl.layerStore.getTarStream(rl)
  	if err != nil {
  		return nil, err
  	}
  
  	vrc, err := newVerifiedReadCloser(rc, digest.Digest(rl.diffID))
  	if err != nil {
  		return nil, err
  	}
  	return vrc, nil
  }
docs: // TarStream for roLayer guarantees that the data that is produced is the exact
  // data that the layer was registered with.