Merge pull request #446 from jakub-wojciechowski/master

Make approve method compliant with ERC20

contracts/token/StandardToken.sol

@@ -40,17 +40,15 @@ contract StandardToken is ERC20, BasicToken {
 
   /**
    * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
+   *
+   * Beware that changing an allowance with this method brings the risk that someone may use both the old
+   * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
+   * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
+   * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
    * @param _spender The address which will spend the funds.
    * @param _value The amount of tokens to be spent.
    */
   function approve(address _spender, uint256 _value) public returns (bool) {
-
-    // 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
-    // require((_value == 0) || (allowed[msg.sender][_spender] == 0));
-
     allowed[msg.sender][_spender] = _value;
     Approval(msg.sender, _spender, _value);
     return true;