SystemContract.sol

pragma solidity 0.7.5;

contract SystemContract {
    // Address of the recipient extracted from ETH1 withdrawal credentials.
    address payable public recipient;

    // Whether the balance was already withdrawn.
    bool public withdrawn;

    // A new instance is deployed for every validator with ETH1 withdrawal credentials when it becomes withdrawable.
    constructor(address payable _recipient) payable {
        recipient = _recipient;
    }

    function withdraw() external {
        require(!withdrawn, "Already withdrawn");
        withdrawn = true;

        // forwards all available gas
        (bool success, ) = recipient.call{ value: address(this).balance }("");
        require(success, "Withdrawal has failed");
    }
}