L1 smart contract to recursively receive L2 messages and update user's balance.

L1_message_receiver.sol

// SPDX-License-Identifier: Apache-2.0.
pragma solidity ^0.8.17;

interface IStarknetMessaging {
    /**
      Consumes a message that was sent from an L2 contract.

      Returns the hash of the message.
    */
    function consumeMessageFromL2(uint256 fromAddress, uint256[] calldata payload)
        external
        returns (bytes32);
}

contract L1L2 {
    // The StarkNet core contract.
    address public starknet_address;
    IStarknetMessaging starknetCore;

    mapping(uint256 => uint256) public userBalances;

    constructor() {
        starknet_address = 0x70c8A579aD08339cCA19d77d8646F4b6f0fd098A;
        starknetCore = IStarknetMessaging(starknet_address);
    }

    function withdraw(
        uint256 l2ContractAddress,
        uint256[] calldata payload
    ) external {

        // Consume the message from the StarkNet core contract.
        // This will revert the (Ethereum) transaction if the message does not exist.
        starknetCore.consumeMessageFromL2(l2ContractAddress, payload);
    }
}