Solidity - 提款模式

提款模式确保不会造成安全威胁的直接转接呼叫。以下合约显示了使用转账调用发送以太币的不安全方式。

pragma solidity ^0.5.0;

contract Test {
   address payable public richest;
   uint public mostSent;

   constructor() public payable {
      richest = msg.sender;
      mostSent = msg.value;
   }
   function becomeRichest() public payable returns (bool) {
      if (msg.value > mostSent) {
         // Insecure practice
         richest.transfer(msg.value);
         richest = msg.sender;
         mostSent = msg.value;
         return true;
      } else {
         return false;
      }
   }
}

通过使最富有的合约成为后备功能失败的合约，可以使上述合约处于不可用状态。当后备函数失败时，becomeRichest()函数也会失败，合约将永远卡住。为了缓解这个问题，我们可以使用 Withdrawal Pattern。

在提款模式中，我们将在每次转账前重置待处理金额。它将确保只有调用者合约失败。

pragma solidity ^0.5.0;

contract Test {
   address public richest;
   uint public mostSent;

   mapping (address => uint) pendingWithdrawals;

   constructor() public payable {
      richest = msg.sender;
      mostSent = msg.value;
   }
   function becomeRichest() public payable returns (bool) {
      if (msg.value > mostSent) {
         pendingWithdrawals[richest] += msg.value;
         richest = msg.sender;
         mostSent = msg.value;
         return true;
      } else {
         return false;
      }
   }
   function withdraw() public {
      uint amount = pendingWithdrawals[msg.sender];
      pendingWithdrawals[msg.sender] = 0;
      msg.sender.transfer(amount);
   }
}