code: func (s *Sentinel) MasterAddr() (string, error) {
  	res, err := s.doUntilSuccess(func(c redis.Conn) (interface{}, error) {
  		return queryForMaster(c, s.MasterName)
  	})
  	if err != nil {
  		return "", err
  	}
  	return res.(string), nil
  }
docs: // MasterAddr returns an address of current Redis master instance.