@0x/contracts-erc20: Add LibERC20Token.approveIfBelowMax().

`@0x/contracts-asset-proxy`: Use `LibERC20Token.approveIfBelowMax` in all DEX bridges.

contracts/asset-proxy/CHANGELOG.json

@@ -1,4 +1,12 @@
 [
+    {
+        "version": "3.3.0",
+        "changes": [
+            {
+                "note": "Use `approveIfBelowMax()` in DEX bridges for for approvals."
+            }
+        ]
+    },
     {
         "timestamp": 1583220306,
         "version": "3.2.5",

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

@@ -61,7 +61,7 @@ contract CurveBridge is
         address fromTokenAddress = exchange.underlying_coins(fromCoinIdx);
         require(toTokenAddress != fromTokenAddress, "CurveBridge/INVALID_PAIR");
         // Grant an allowance to the exchange to spend `fromTokenAddress` token.
-        LibERC20Token.approve(fromTokenAddress, address(exchange), uint256(-1));
+        LibERC20Token.approveIfBelowMax(fromTokenAddress, address(exchange));
 
         // Try to sell all of this contract's `fromTokenAddress` token balance.
         if (version == 0) {

contracts/asset-proxy/contracts/src/bridges/Eth2DaiBridge.sol

@@ -57,7 +57,7 @@ contract Eth2DaiBridge is
 
         IEth2Dai exchange = IEth2Dai(_getEth2DaiAddress());
         // Grant an allowance to the exchange to spend `fromTokenAddress` token.
-        LibERC20Token.approve(fromTokenAddress, address(exchange), uint256(-1));
+        LibERC20Token.approveIfBelowMax(fromTokenAddress, address(exchange));
 
         // Try to sell all of this contract's `fromTokenAddress` token balance.
         uint256 boughtAmount = exchange.sellAllAmount(

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

@@ -109,7 +109,7 @@ contract KyberBridge is
         } else if (state.fromTokenAddress != address(state.weth)) {
             // If the input token is not WETH, grant an allowance to the exchange
             // to spend them.
-            LibERC20Token.approve(state.fromTokenAddress, address(state.kyber), uint256(-1));
+            LibERC20Token.approveIfBelowMax(state.fromTokenAddress, address(state.kyber));
         } else {
             // If the input token is WETH, unwrap it and attach it to the call.
             state.fromTokenAddress = KYBER_ETH_ADDRESS;

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

@@ -168,7 +168,7 @@ contract UniswapBridge is
     function _grantExchangeAllowance(IUniswapExchange exchange, address tokenAddress)
         private
     {
-        LibERC20Token.approve(tokenAddress, address(exchange), uint256(-1));
+        LibERC20Token.approveIfBelowMax(tokenAddress, address(exchange));
     }
 
     /// @dev Retrieves the uniswap exchange for a given token pair.

contracts/asset-proxy/contracts/test/TestEth2DaiBridge.sol

@@ -110,6 +110,10 @@ contract TestToken {
         return true;
     }
 
+    function allowance(address, address) external view returns (uint256) {
+        return 0;
+    }
+
     /// @dev Retrieve the balance for `owner`.
     function balanceOf(address owner)
         external

contracts/asset-proxy/contracts/test/TestKyberBridge.sol

@@ -110,6 +110,10 @@ contract TestToken {
         return _testContract.wethDeposit.value(msg.value)(msg.sender);
     }
 
+    function allowance(address, address) external view returns (uint256) {
+        return 0;
+    }
+
     function balanceOf(address owner)
         external
         view

contracts/asset-proxy/contracts/test/TestUniswapBridge.sol

@@ -224,6 +224,10 @@ contract TestToken {
         TestEventsRaiser(msg.sender).raiseWethWithdraw(amount);
     }
 
+    function allowance(address, address) external view returns (uint256) {
+        return 0;
+    }
+
     /// @dev Retrieve the balance for `owner`.
     function balanceOf(address owner)
         external

contracts/erc20/CHANGELOG.json

@@ -1,4 +1,12 @@
 [
+    {
+        "version": "3.2.0",
+        "changes": [
+            {
+                "note": "Add `LibERC20Token.approveIfBelowMax()`"
+            }
+        ]
+    },
     {
         "timestamp": 1583220306,
         "version": "3.1.5",

contracts/erc20/contracts/src/LibERC20Token.sol

@@ -47,6 +47,23 @@ library LibERC20Token {
         _callWithOptionalBooleanResult(token, callData);
     }
 
+    /// @dev Calls `IERC20Token(token).approve()` and sets the allowance to the
+    ///      maximum if the current approval is not already set to the maximum.
+    ///      Reverts if `false` is returned or if the return
+    ///      data length is nonzero and not 32 bytes.
+    /// @param token The address of the token contract.
+    /// @param spender The address that receives an allowance.
+    function approveIfBelowMax(
+        address token,
+        address spender
+    )
+        internal
+    {
+        if (IERC20Token(token).allowance(address(this), spender) != uint256(-1)) {
+            approve(token, spender, uint256(-1));
+        }
+    }
+
     /// @dev Calls `IERC20Token(token).transfer()`.
     ///      Reverts if `false` is returned or if the return
     ///      data length is nonzero and not 32 bytes.