Solidity - 提款模式

提款模式确保不会进行直接转账调用，这会带来安全风险。以下合约展示了不安全的转账调用方式来发送以太币。

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;
      }
   }
}

上述合约可以通过使最富有者成为回退函数失败的合约来使其处于不可用状态。当回退函数失败时，becomeRichest() 函数也会失败，合约将永远卡住。为了减轻这个问题，我们可以使用提款模式。

在提款模式中，我们将在每次转账前重置待处理金额。这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);
   }
}