contracts: add function "withdrawFeeToL1"

contracts/src/L2StandardBridgeBot.sol

@@ -3,6 +3,7 @@ pragma solidity 0.8.20;
 import { Ownable } from "openzeppelin-contracts/contracts/access/Ownable.sol";
 import { IERC20 } from "openzeppelin-contracts/contracts/token/ERC20/IERC20.sol";
 
+// See also https://github.com/bnb-chain/opbnb/blob/9505ae88d0ec8f593ee036284c9a13672526a232/packages/contracts-bedrock/contracts/L2/L2StandardBridge.sol#L20
 interface IL2StandardBridge {
     function withdrawTo(
         address _l2Token,
@@ -51,6 +52,7 @@ contract L2StandardBridgeBot is Ownable {
             require(approveSuccess, "BEP20 withdrawal: approve failed");
             bool transferSuccess = l2Token.transferFrom(msg.sender, address(this), _amount);
             require(transferSuccess, "BEP20 withdrawal: transferFrom failed");
+
             L2_STANDARD_BRIDGE.withdrawTo{value: 0}(_l2Token, _to, _amount, _minGasLimit, _extraData);
         }
 
@@ -66,12 +68,30 @@ contract L2StandardBridgeBot is Ownable {
         withdrawTo(_l2Token, msg.sender, _amount, _minGasLimit, _extraData);
     }
 
-    // withdrawFee withdraw the delegation fee vault to _recipient address, only owner can call this function.
+    // withdrawFee withdraw the delegation fee vault to _recipient address on L2, only owner can call this function.
     function withdrawFee(address _recipient) external onlyOwner {
         (bool sent, ) = _recipient.call{ value: address(this).balance }("");
         require(sent, "Failed to send Ether");
     }
 
+    // withdrawFeeToL1 withdraw the delegation fee vault to _recipient address on L1, only owner can call this function.
+    //
+    // NOTE: Since the `withdrawTo` function is defined as external and `_extraData` is `bytes calldata`, the
+    // `_extraData` cannot be mock from `bytes memory` by the caller. See also https://github.com/ethereum/solidity/issues/12778
+    function withdrawFeeToL1(address _recipient, bytes calldata _extraData) external onlyOwner {
+        uint256 _balance = address(this).balance;
+        require(_balance > delegationFee, "fee vault balance is insufficient to pay the required amount");
+
+        uint256 _amount = _balance - delegationFee;
+        this.withdrawTo{ value: _balance }(
+            LEGACY_ERC20_ETH,
+            _recipient,
+            _amount,
+            160000 ,        // constant acceptable minGasLimit is fine
+            _extraData
+        );
+    }
+
     // setDelegationFee set the delegation fee, only owner can call this function.
     function setDelegationFee(uint256 _delegationFee) external onlyOwner {
         delegationFee = _delegationFee;