[General] Paymaster - How to send all the refunded ETH back to the user

[lutr0]

Environment

Mainnet

Provide a brief description of the functionality you're trying to implement.

I would like to implement a simple paymaster flow in postTransaction that sends all the the ETH refunded back to the user.
My expectation was that _maxRefundedGas covers uint256 requiredETH = _transaction.gasLimit *_transaction.maxFeePerGas;,

instead from my experimentation below it seems to give only refer to the gasLimit used.

So my questions are:

1. What is the recommended wat to send all the the ETH refunded back to the user?
2. Is recommended to set maxFeePerGas = gasPrice?

What is the specific issue or error you're encountering?

Sending back only _maxRefundedGas

function postTransaction(
        bytes calldata _context,
        Transaction calldata _transaction,
        bytes32,
        bytes32,
        ExecutionResult _txResult,
        uint256 _maxRefundedGas
    ) external payable override onlyBootloader {
        console.log("maxRefundedGas: ", _maxRefundedGas);
        address userAddress = address(uint160(_transaction.from));
        uint gasUsed = _txResult.gasUsed;
        (bool success, ) = userAddress.call{value: _maxRefundedGas}("");
        require(success, "Failed to refund after the transaction.");
    }

has the user refunded 0.000000000002682814 ETH
while the paymaster received back 0.0006881845 ETH (see below)

Multiplying by _transaction.maxFeePerGas seems to give us a good approximation:
(bool success, ) = userAddress.call{value: _maxRefundedGas * _transaction.maxFeePerGas}("");

User refunded 0.0006706995 ETH, paymaster refunded 0.00068476375

However, it requires to use the current gasPrice as maxFeePerGas, which could lead to failed transactions if the gas spikes.

Can you share the error messages or logs you're receiving, if any?

N/A

Have you made any recent changes to the contract before encountering this issue?

N/A

Are there any external libraries or contracts that your contract interacts with?

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Can you provide the relevant portions of your contract code where the issue is occurring?

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Have you tried to isolate the problem, and if so, what were the results?

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What steps have you already taken to try to resolve the issue?

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Repo Link (Optional)

No response

Additional Details

No response

[StanislavBreadless]

Hey! The _maxRefundedGas is the number of how much gas (not ETH) has been left after all the execution steps.

It does not include:

• The fact that paymaster itself will consume a certain amount of gas in the postOp.
• Potential refunds that might come from places like a storage slot that has not been edited.

If you try to refund the user with anything above, then _maxRefundedGas there might be situations when the paymaster will not get compensated.

A more correct way for implementing functionality like these is: to send something around block.gasprice * (_maxRefundedGas - PAYMASTER_GAS to the user), where PAYMASTER_GAS is some constant of how much gas a paymaster is expected to consume. However, we do not recommend relying on any sort of constants regarding gas costs as these might change during an upgrade (so to maintain this paymaster you'll have to carefully watch out for upgrades).

Overall, while returning all the refunded eth to the user offers a better UX in some cases, there is no canonical easy way to do it