Merge pull request #219 from TokenMarketNet/feat/approve-mitigation

approve() race condition mitigation

contracts/token/StandardToken.sol

@@ -29,6 +29,13 @@ contract StandardToken is BasicToken, ERC20 {
   }
 
   function approve(address _spender, uint _value) {
+
+    // To change the approve amount you first have to reduce the addresses`
+    //  allowance to zero by calling `approve(_spender, 0)` if it is not
+    //  already 0 to mitigate the race condition described here:
+    //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
+    if ((_value != 0) && (allowed[msg.sender][_spender] != 0)) throw;
+
     allowed[msg.sender][_spender] = _value;
     Approval(msg.sender, _spender, _value);
   }