code: func (secret hmacKey) packUploadState(lus blobUploadState) (string, error) {
  	mac := hmac.New(sha256.New, []byte(secret))
  	p, err := json.Marshal(lus)
  	if err != nil {
  		return "", err
  	}
  
  	mac.Write(p)
  
  	return base64.URLEncoding.EncodeToString(append(mac.Sum(nil), p...)), nil
  }
docs: // packUploadState packs the upload state signed with and hmac digest using
  // the hmacKey secret, encoding to url safe base64. The resulting token can be
  // used to share data with minimized risk of external tampering.