zkSync basic Paymaster implementation

Paymaster.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.8;

import {IPaymaster, ExecutionResult, PAYMASTER_VALIDATION_SUCCESS_MAGIC} 
from  "@matterlabs/zksync-contracts/l2/system-contracts/interfaces/IPaymaster.sol";
import {IPaymasterFlow} from  "@matterlabs/zksync-contracts/l2/system-contracts/interfaces/IPaymasterFlow.sol";
import {TransactionHelper, Transaction} from "@matterlabs/zksync-contracts/l2/system-contracts/libraries/TransactionHelper.sol";

import "@matterlabs/zksync-contracts/l2/system-contracts/Constants.sol";

contract MyPaymaster is IPaymaster {

    function validateAndPayForPaymasterTransaction (
        bytes32,
        bytes32,
        Transaction calldata _transaction
    ) onlyBootloader external payable returns (bytes4 magic, bytes memory context) {
        // By default we consider the transaction as accepted.
        magic = PAYMASTER_VALIDATION_SUCCESS_MAGIC;
        
        bytes4 paymasterInputSelector = bytes4(
            _transaction.paymasterInput[0:4]
        );
        if (paymasterInputSelector == IPaymasterFlow.general.selector) {
            // Note, that while the minimal amount of ETH needed is tx.gasPrice * tx.gasLimit,
            // neither paymaster nor account are allowed to access this context variable.
            uint256 requiredETH = _transaction.gasLimit *
                _transaction.maxFeePerGas;

            // The bootloader never returns any data, so it can safely be ignored here.
            (bool success, ) = payable(BOOTLOADER_FORMAL_ADDRESS).call{value: requiredETH}("");
            require(success, "Failed to transfer funds to the bootloader");
        } else {
            revert("Unsupported paymaster flow");
        }
    }

    function postTransaction  (
        bytes calldata _context,
        Transaction calldata _transaction,
        bytes32,
        bytes32,
        ExecutionResult _txResult,
        uint256 _maxRefundedGas
    ) onlyBootloader external payable override {
        // Refunds are not supported yet.
    }
}