code: func (b *builtinInetNtoaSig) evalString(row chunk.Row) (string, bool, error) {
  	val, isNull, err := b.args[0].EvalInt(b.ctx, row)
  	if err != nil || isNull {
  		return "", true, err
  	}
  
  	if val < 0 || uint64(val) > math.MaxUint32 {
  		//not an IPv4 address.
  		return "", true, nil
  	}
  	ip := make(net.IP, net.IPv4len)
  	binary.BigEndian.PutUint32(ip, uint32(val))
  	ipv4 := ip.To4()
  	if ipv4 == nil {
  		//Not a vaild ipv4 address.
  		return "", true, nil
  	}
  
  	return ipv4.String(), false, nil
  }
docs: // evalString evals a builtinInetNtoaSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_inet-ntoa