Gas Tokens in Bridges (#2523)

* Gas Tokens in Bridges

* Added freesGasTokensFromCollector.

To handle the case where GST2 is being traded and the balanceOf checks become confusing

* Update CHANGELOGs

contracts/asset-proxy/contracts/src/bridges/CurveBridge.sol

@@ -26,6 +26,7 @@ import "@0x/contracts-exchange-libs/contracts/src/IWallet.sol";
 import "@0x/contracts-utils/contracts/src/DeploymentConstants.sol";
 import "../interfaces/IERC20Bridge.sol";
 import "../interfaces/ICurve.sol";
+import "./MixinGasToken.sol";
 
 
 // solhint-disable not-rely-on-time
@@ -33,7 +34,8 @@ import "../interfaces/ICurve.sol";
 contract CurveBridge is
     IERC20Bridge,
     IWallet,
-    DeploymentConstants
+    DeploymentConstants,
+    MixinGasToken
 {
     struct CurveBridgeData {
         address curveAddress;
@@ -60,6 +62,7 @@ contract CurveBridge is
         bytes calldata bridgeData
     )
         external
+        freesGasTokensFromCollector
         returns (bytes4 success)
     {
         // Decode the bridge data to get the Curve metadata.

contracts/asset-proxy/contracts/src/bridges/MixinGasToken.sol

@@ -0,0 +1,55 @@
+/*
+
+  Copyright 2019 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.5.16;
+
+import "@0x/contracts-utils/contracts/src/DeploymentConstants.sol";
+import "../interfaces/IGasToken.sol";
+
+
+contract MixinGasToken is
+    DeploymentConstants
+{
+
+    /// @dev Frees gas tokens based on the amount of gas consumed in the function
+    modifier freesGasTokens {
+        uint256 gasBefore = gasleft();
+        _;
+        IGasToken gst = IGasToken(_getGstAddress());
+        if (address(gst) != address(0)) {
+            // (gasUsed + FREE_BASE) / (2 * REIMBURSE - FREE_TOKEN)
+            //            14154             24000        6870
+            uint256 value = (gasBefore - gasleft() + 14154) / 41130;
+            gst.freeUpTo(value);
+        }
+    }
+
+    /// @dev Frees gas tokens using the balance of `from`. Amount freed is based
+    ///     on the gas consumed in the function
+    modifier freesGasTokensFromCollector() {
+        uint256 gasBefore = gasleft();
+        _;
+        IGasToken gst = IGasToken(_getGstAddress());
+        if (address(gst) != address(0)) {
+            // (gasUsed + FREE_BASE) / (2 * REIMBURSE - FREE_TOKEN)
+            //            14154             24000        6870
+            uint256 value = (gasBefore - gasleft() + 14154) / 41130;
+            gst.freeFromUpTo(_getGstCollectorAddress(), value);
+        }
+    }
+}

contracts/asset-proxy/contracts/src/interfaces/IGasToken.sol

@@ -0,0 +1,40 @@
+/*
+
+  Copyright 2019 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.5.15;
+
+import "@0x/contracts-erc20/contracts/src/interfaces/IERC20Token.sol";
+
+
+contract IGasToken is IERC20Token {
+
+    /// @dev Frees up to `value` sub-tokens
+    /// @param value The amount of tokens to free
+    /// @return How many tokens were freed
+    function freeUpTo(uint256 value) external returns (uint256 freed);
+
+    /// @dev Frees up to `value` sub-tokens owned by `from`
+    /// @param from The owner of tokens to spend
+    /// @param value The amount of tokens to free
+    /// @return How many tokens were freed
+    function freeFromUpTo(address from, uint256 value) external returns (uint256 freed);
+
+    /// @dev Mints `value` amount of tokens
+    /// @param value The amount of tokens to mint
+    function mint(uint256 value) external;
+}