code: func (ac *aliCDNStorageMiddleware) URLFor(ctx context.Context, path string, options map[string]interface{}) (string, error) {
  
  	if ac.StorageDriver.Name() != "oss" {
  		dcontext.GetLogger(ctx).Warn("the AliCDN middleware does not support this backend storage driver")
  		return ac.StorageDriver.URLFor(ctx, path, options)
  	}
  	acURL, err := ac.urlSigner.Sign(ac.baseURL+path, time.Now().Add(ac.duration))
  	if err != nil {
  		return "", err
  	}
  	return acURL, nil
  }
docs: // URLFor attempts to find a url which may be used to retrieve the file at the given path.