code: func mkTokenCallback(path string) azure.TokenRefreshCallback {
  	return func(t azure.Token) error {
  		if err := azure.SaveToken(path, 0600, t); err != nil {
  			return err
  		}
  		log.Debug("Saved token to file.")
  		return nil
  	}
  }
docs: // mkTokenCallback returns a callback function that can be used to save the
  // token initially or register to the Azure SDK to be called when the token is
  // refreshed.