code: func (dh *DownloadHandler) statFiles(refs []blob.Ref) error {
  	statter, ok := dh.Fetcher.(blobserver.BlobStatter)
  	if !ok {
  		return fmt.Errorf("DownloadHandler.Fetcher %T is not a BlobStatter", dh.Fetcher)
  	}
  	statted := make(map[blob.Ref]bool)
  
  	err := statter.StatBlobs(context.TODO(), refs, func(sb blob.SizedRef) error {
  		statted[sb.Ref] = true
  		return nil
  	})
  	if err != nil {
  		log.Printf("Error statting blob files for download archive: %v", err)
  		return fmt.Errorf("error looking for files")
  	}
  	for _, v := range refs {
  		if _, ok := statted[v]; !ok {
  			return fmt.Errorf("%q was not found", v)
  		}
  	}
  	return nil
  }
docs: // statFiles stats the given refs and returns an error if any one of them is not
  // found.
  // It is the responsibility of the caller to check that dh.Fetcher is a
  // blobserver.BlobStatter.