How to refund gas to the smart contract caller? (What is the number?)

Question

I have this function:

    function setAccount(BaseUBI _ubi, address _user, uint256 _startTime, uint _esiaID, bool _setToZero) external {
        uint256 _refund = (gasleft() + 0/*FIXME*/) * tx.gasprice;
        require(msg.sender == server, "System function"); // don't refund otherwise
        require(_refund <= balances[_user], "Not enough balance");
        balances[_user] -= _refund; // must be called before transfer() against reentrancy attack
        server.transfer(_refund); // refund gas to the server
        _ubi.setAccount{gas: balances[_user]}(_user, _startTime, _esiaID, _setToZero);
        // We may be in a wrong state now, don't change any variables here.
    }

Which number should be in place of TODO to ensure that the server will receive back at least as much gas as it spent?

Is there a library that automates this task?

If necessary, rewrite the appropriate part of the above code in assembler.

Answer 1

I guess you can use a modifier to do this.

modifier refundGas {
uint256 gasAtStart = gasleft();
_;
uint256 gasSpent = gasAtStart - gasleft() + 28925;
msg.sender.transfer(gasSpent * tx.gasprice);
}

function anyFunction() refundGas public {
    // Do something
}

Note: The number 28925 is an approx. of the gas used for calculations + transfer at the end. I've run on remix and put that here. However, it might be slightly off as well

However, I also think if you would refund gas then someone possibly could waste all the ETH inside the contract in refunds. For e.g., server could intentionally set a very high gas price even if it's unnecessary to do so, knowing that the contract would refund them 100%.