fix: Transfer excess back and reset allowance (#49)

src/periphery/contracts/adapters/paraswap/BaseParaSwapBuyAdapter.sol

@@ -39,6 +39,7 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
    * @param maxAmountToSwap Max amount to be swapped
    * @param amountToReceive Amount to be received from the swap
    * @return amountSold The amount sold during the swap
+   * @return amountBought The amount bought during the swap
    */
   function _buyOnParaSwap(
     uint256 toAmountOffset,
@@ -47,7 +48,7 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
     IERC20Detailed assetToSwapTo,
     uint256 maxAmountToSwap,
     uint256 amountToReceive
-  ) internal returns (uint256 amountSold) {
+  ) internal returns (uint256 amountSold, uint256 amountBought) {
     (bytes memory buyCalldata, IParaSwapAugustus augustus) = abi.decode(
       paraswapData,
       (bytes, IParaSwapAugustus)
@@ -75,7 +76,6 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
     uint256 balanceBeforeAssetTo = assetToSwapTo.balanceOf(address(this));
 
     address tokenTransferProxy = augustus.getTokenTransferProxy();
-    assetToSwapFrom.safeApprove(tokenTransferProxy, 0);
     assetToSwapFrom.safeApprove(tokenTransferProxy, maxAmountToSwap);
 
     if (toAmountOffset != 0) {
@@ -101,12 +101,15 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
       }
     }
 
+    // Reset allowance
+    assetToSwapFrom.safeApprove(tokenTransferProxy, 0);
+
     uint256 balanceAfterAssetFrom = assetToSwapFrom.balanceOf(address(this));
     amountSold = balanceBeforeAssetFrom - balanceAfterAssetFrom;
     require(amountSold <= maxAmountToSwap, 'WRONG_BALANCE_AFTER_SWAP');
-    uint256 amountReceived = assetToSwapTo.balanceOf(address(this)).sub(balanceBeforeAssetTo);
-    require(amountReceived >= amountToReceive, 'INSUFFICIENT_AMOUNT_RECEIVED');
+    amountBought = assetToSwapTo.balanceOf(address(this)).sub(balanceBeforeAssetTo);
+    require(amountBought >= amountToReceive, 'INSUFFICIENT_AMOUNT_RECEIVED');
 
-    emit Bought(address(assetToSwapFrom), address(assetToSwapTo), amountSold, amountReceived);
+    emit Bought(address(assetToSwapFrom), address(assetToSwapTo), amountSold, amountBought);
   }
 }

src/periphery/contracts/adapters/paraswap/BaseParaSwapSellAdapter.sol

@@ -98,6 +98,7 @@ abstract contract BaseParaSwapSellAdapter is BaseParaSwapAdapter {
         revert(0, returndatasize())
       }
     }
+
     require(
       assetToSwapFrom.balanceOf(address(this)) == balanceBeforeAssetFrom - amountToSwap,
       'WRONG_BALANCE_AFTER_SWAP'

src/periphery/contracts/adapters/paraswap/ParaSwapRepayAdapter.sol

@@ -114,7 +114,7 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
     // Pull aTokens from user
     _pullATokenAndWithdraw(address(collateralAsset), msg.sender, collateralAmount, permitSignature);
     //buy debt asset using collateral asset
-    uint256 amountSold = _buyOnParaSwap(
+    (uint256 amountSold, uint256 amountBought) = _buyOnParaSwap(
       buyAllBalanceOffset,
       paraswapData,
       collateralAsset,
@@ -127,15 +127,23 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
 
     //deposit collateral back in the pool, if left after the swap(buy)
     if (collateralBalanceLeft > 0) {
-      IERC20(collateralAsset).safeApprove(address(POOL), 0);
       IERC20(collateralAsset).safeApprove(address(POOL), collateralBalanceLeft);
       POOL.deposit(address(collateralAsset), collateralBalanceLeft, msg.sender, 0);
+      IERC20(collateralAsset).safeApprove(address(POOL), 0);
     }
 
     // Repay debt. Approves 0 first to comply with tokens that implement the anti frontrunning approval fix
-    IERC20(debtAsset).safeApprove(address(POOL), 0);
     IERC20(debtAsset).safeApprove(address(POOL), debtRepayAmount);
     POOL.repay(address(debtAsset), debtRepayAmount, debtRateMode, msg.sender);
+    IERC20(debtAsset).safeApprove(address(POOL), 0);
+
+    {
+      //transfer excess of debtAsset back to the user, if any
+      uint256 debtAssetExcess = amountBought - debtRepayAmount;
+      if (debtAssetExcess > 0) {
+        IERC20(debtAsset).safeTransfer(msg.sender, debtAssetExcess);
+      }
+    }
   }
 
   /**
@@ -170,7 +178,7 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
       initiator
     );
 
-    uint256 amountSold = _buyOnParaSwap(
+    (uint256 amountSold, uint256 amountBought) = _buyOnParaSwap(
       buyAllBalanceOffset,
       paraswapData,
       collateralAsset,
@@ -180,9 +188,9 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
     );
 
     // Repay debt. Approves for 0 first to comply with tokens that implement the anti frontrunning approval fix.
-    IERC20(debtAsset).safeApprove(address(POOL), 0);
     IERC20(debtAsset).safeApprove(address(POOL), debtRepayAmount);
     POOL.repay(address(debtAsset), debtRepayAmount, rateMode, initiator);
+    IERC20(debtAsset).safeApprove(address(POOL), 0);
 
     uint256 neededForFlashLoanRepay = amountSold.add(premium);
 
@@ -194,6 +202,14 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
       permitSignature
     );
 
+    {
+      //transfer excess of debtAsset back to the user, if any
+      uint256 debtAssetExcess = amountBought - debtRepayAmount;
+      if (debtAssetExcess > 0) {
+        IERC20(debtAsset).safeTransfer(initiator, debtAssetExcess);
+      }
+    }
+
     // Repay flashloan. Approves for 0 first to comply with tokens that implement the anti frontrunning approval fix.
     IERC20(collateralAsset).safeApprove(address(POOL), 0);
     IERC20(collateralAsset).safeApprove(address(POOL), collateralAmount.add(premium));