flash swap attempt

src/abstract/OrderBookFlashLender.sol

@@ -7,6 +7,7 @@ import {SafeERC20} from "openzeppelin-contracts/contracts/token/ERC20/utils/Safe
 
 import "../interface/ierc3156/IERC3156FlashBorrower.sol";
 import "../interface/ierc3156/IERC3156FlashLender.sol";
+import "../interface/unstable/IOrderBookV3TokenWithdrawer.sol";
 
 /// Thrown when `flashLoan` token is zero address.
 error ZeroToken();
@@ -97,11 +98,12 @@ abstract contract OrderBookFlashLender is IERC3156FlashLender {
     /// @param token The token being sent or for the debt being paid.
     /// @param receiver The receiver of the token or holder of the debt.
     /// @param sendAmount The amount to send or repay.
-    function _decreaseFlashDebtThenSendToken(address token, address receiver, uint256 sendAmount) internal {
+    function _decreaseFlashDebtThenSendToken(address token, address receiver, uint256 sendAmount, bytes memory data) internal {
         // If this token transfer matches the active debt then prioritise
         // reducing debt over sending tokens.
+        uint256 debtReduction = 0;
         if (token == _sToken && receiver == address(_sReceiver)) {
-            uint256 debtReduction = sendAmount.min(_sAmountUnpaid);
+            debtReduction = sendAmount.min(_sAmountUnpaid);
             sendAmount -= debtReduction;
 
             // Even if this completely zeros the amount the debt is considered
@@ -112,6 +114,9 @@ abstract contract OrderBookFlashLender is IERC3156FlashLender {
         if (sendAmount > 0) {
             IERC20(token).safeTransfer(receiver, sendAmount);
         }
+        if (data.length > 0) {
+            IOrderBookV3TokenWithdrawer(receiver).onWithdrawal(token, sendAmount, debtReduction, data);
+        }
     }
 
     /// @inheritdoc IERC3156FlashLender

src/concrete/OrderBook.sol

@@ -237,7 +237,7 @@ contract OrderBook is IOrderBookV3, ReentrancyGuard, Multicall, OrderBookFlashLe
     }
 
     /// @inheritdoc IOrderBookV3
-    function withdraw(address token, uint256 vaultId, uint256 targetAmount) external nonReentrant {
+    function withdraw(address token, uint256 vaultId, uint256 targetAmount, bytes memory data) external nonReentrant {
         if (targetAmount == 0) {
             revert ZeroWithdrawTargetAmount(msg.sender, token, vaultId);
         }
@@ -254,6 +254,16 @@ contract OrderBook is IOrderBookV3, ReentrancyGuard, Multicall, OrderBookFlashLe
         }
     }
 
+    function _tokenWithdrawal(address token, address receiver, uint256 amount) internal {
+        if (amount > 0) {
+            // The overflow check here is redundant with .min above, so
+            // technically this is overly conservative but we REALLY don't want
+            // withdrawals to exceed vault balances.
+            emit Withdraw(address(this), token, 0, amount, amount);
+            _decreaseFlashDebtThenSendToken(token, receiver, amount);
+        }
+    }
+
     /// @inheritdoc IOrderBookV3
     function addOrder(OrderConfigV2 calldata config) external nonReentrant returns (bool stateChanged) {
         uint256 sourceCount = LibBytecode.sourceCount(config.evaluableConfig.bytecode);
@@ -381,13 +391,14 @@ contract OrderBook is IOrderBookV3, ReentrancyGuard, Multicall, OrderBookFlashLe
             revert MinimumInput(config.minimumInput, totalInput);
         }
 
+        // Prioritise paying down any active flash loans before sending any
+        // tokens to `msg.sender`.
+        _tokenWithdrawal(config.input, msg.sender, totalInput);
+
         // We already updated vault balances before we took tokens from
         // `msg.sender` which is usually NOT the correct order of operations for
         // depositing to a vault. We rely on reentrancy guards to make this safe.
         IERC20(config.output).safeTransferFrom(msg.sender, address(this), totalOutput);
-        // Prioritise paying down any active flash loans before sending any
-        // tokens to `msg.sender`.
-        _decreaseFlashDebtThenSendToken(config.input, msg.sender, totalInput);
     }
 
     /// @inheritdoc IOrderBookV3

src/interface/unstable/IOrderBookV3.sol

@@ -330,6 +330,7 @@ interface IOrderBookV3 is IERC3156FlashLender, IInterpreterCallerV2 {
     /// target amount if the vault does not have the funds available to cover
     /// the target amount. For example an active order might move tokens before
     /// the withdraw completes.
+
     event Withdraw(address sender, address token, uint256 vaultId, uint256 targetAmount, uint256 amount);
 
     /// An order has been added to the orderbook. The order is permanently and
@@ -455,7 +456,11 @@ interface IOrderBookV3 is IERC3156FlashLender, IInterpreterCallerV2 {
     /// result in fewer tokens withdrawn if the vault balance is lower than the
     /// target amount. MAY NOT be zero, the order book MUST revert with
     /// `ZeroWithdrawTargetAmount` if the amount is zero.
-    function withdraw(address token, uint256 vaultId, uint256 targetAmount) external;
+    /// @param data Optional arbitrary data that will cause
+    /// `IOrderBookV3TokenWithdrawer.onWithdrawal` to be called on `msg.sender`
+    /// if the data has non-zero length. The data is forwarded to the
+    /// `onWithdrawal` call as-is.
+    function withdraw(address token, uint256 vaultId, uint256 targetAmount, bytes memory data) external;
 
     /// Given an order config, deploys the expression and builds the full `Order`
     /// for the config, then records it as an active order. Delegated adding an

src/interface/unstable/IOrderBookV3TokenWithdrawer.sol

@@ -0,0 +1,11 @@
+// SPDX-Licdnse-Identifier: CAL
+pragma solidity ^0.8.18;
+
+interface IOrderBookV3TokenWithdrawer {
+    function onWithdrawal(
+        address token,
+        uint256 sendAmount,
+        uint256 debtReduction,
+        bytes calldata data
+    ) external;
+}