Needed an extra decode

src/erc3156/ERC3156Wrapper.sol

@@ -8,6 +8,7 @@ import { RevertMsgExtractor } from "../utils/RevertMsgExtractor.sol";
 import { IERC3156PPFlashLender } from "lib/erc3156pp/src/interfaces/IERC3156PPFlashLender.sol";
 import { IERC20 } from "lib/erc3156pp/src/interfaces/IERC20.sol";
 
+
 library TransferHelper {
     /// @notice Transfers tokens from msg.sender to a recipient
     /// @dev Errors with the underlying revert message if transfer fails
@@ -124,7 +125,6 @@ contract ERC3156Wrapper is IERC3156PPFlashLender, IERC3156FlashBorrower {
 
         // We pass the loan to the loan receiver and we store the callback result in storage for the the ERC3156++ flashLoan function to recover it.
         _callbackResult = _callFromData(IERC20(token), amount, fee, data);
-        _callbackResult = abi.encode(CALLBACK_SUCCESS); // TODO: Hijacking the callback result to see where the OOG error comes from
 
         IERC20(token).approve(address(lender), amount + fee);
 
@@ -147,7 +147,8 @@ contract ERC3156Wrapper is IERC3156PPFlashLender, IERC3156FlashBorrower {
             fee, // fee
             userData // data
         ));
-        require(success, RevertMsgExtractor.getRevertMsg(result));
-        return result;
+
+        if(!success) revert(RevertMsgExtractor.getRevertMsg(result));
+        return abi.decode(result, (bytes));
     }
 }

src/test/FlashBorrower.sol

@@ -12,6 +12,7 @@ contract LoanReceiver {
 }
 
 contract FlashBorrower {
+    bytes32 public constant ERC3156PP_CALLBACK_SUCCESS = keccak256("ERC3156PP_CALLBACK_SUCCESS");
     IERC3156PPFlashLender lender;
     LoanReceiver loanReceiver;
 
@@ -30,6 +31,7 @@ contract FlashBorrower {
     function onFlashLoan(address initiator, address paymentReceiver, IERC20 asset, uint256 amount, uint256 fee, bytes calldata data) external returns(bytes memory) {
         require(msg.sender == address(lender), "FlashBorrower: Untrusted lender");
         require(initiator == address(this), "FlashBorrower: External loan initiator");
+
         flashInitiator = initiator;
         flashAsset = asset;
         flashAmount = amount;
@@ -38,7 +40,7 @@ contract FlashBorrower {
         flashBalance = IERC20(asset).balanceOf(address(this));
         asset.transfer(paymentReceiver, amount + fee);
 
-        return ""; // abi.encode(data, paymentReceiver, fee); // TODO: Returning anything here causes a revert
+        return abi.encode(ERC3156PP_CALLBACK_SUCCESS);
     }
 
     function onSteal(address initiator, address paymentReceiver, IERC20 asset, uint256 amount, uint256 fee, bytes calldata data) external returns(bytes memory) {
@@ -51,7 +53,7 @@ contract FlashBorrower {
 
         // do nothing
 
-        return abi.encode(data, paymentReceiver, fee);
+        return abi.encode(ERC3156PP_CALLBACK_SUCCESS);
     }
 
     function onReenter(address initiator, address paymentReceiver, IERC20 asset, uint256 amount, uint256 fee, bytes calldata data) external returns(bytes memory) {
@@ -69,7 +71,7 @@ contract FlashBorrower {
         flashAmount += amount;
         flashFee += fee;
 
-        return abi.encode(data, paymentReceiver, fee);
+        return abi.encode(ERC3156PP_CALLBACK_SUCCESS);
     }
 
     function flashBorrow(IERC20 asset, uint256 amount) public returns(bytes memory) {

test/ERC3156Wrapper.t.sol

@@ -53,9 +53,11 @@ contract ERC3156WrapperTest is PRBTest, StdCheats {
         dai.transfer(address(borrower), fee);
         bytes memory result = borrower.flashBorrow(dai, loan);
 
-        // TODO: Temporary test to ensure we receive the correct callback result
-        assertEq(uint256(wrapper.CALLBACK_SUCCESS()), uint256(abi.decode(result, (bytes32))));
+        // Test the return values
+        (bytes32 callbackReturn) = abi.decode(result, (bytes32));
+        assertEq(uint256(callbackReturn), uint256(borrower.ERC3156PP_CALLBACK_SUCCESS()), "Callback failed");
 
+        // Test the borrower state
         assertEq(borrower.flashInitiator(), address(borrower));
         assertEq(address(borrower.flashAsset()), address(dai));
         assertEq(borrower.flashAmount(), loan);