[GSN] Is there a method that can compute the total cost to relay a transaction?

meh_blah · November 7, 2019, 5:54am

Hey,

I am trying to build a custom strategy for my application and I would like to know that given a transaction, is there a method (similar to estimateGas) that I can call to estimate how much the transaction will need (including transaction fees/ relayer service fees)?

Thanks!

abcoathup · November 7, 2019, 8:50am

Hi @meh_blah,

Welcome to the community

If you are building a custom strategy then the GSN Strategies documentation is a good place to start (if you haven’t already).

Looking at GSNRecipientERC20Fee the acceptRelayedCall function is passed the maxPossibleCharge.

github.com

OpenZeppelin/openzeppelin-contracts/blob/master/contracts/GSN/GSNRecipientERC20Fee.sol#L53


/**
 * @dev Internal function that mints the gas payment token. Derived contracts should expose this function in their public API, with proper access control mechanisms.
 */
function _mint(address account, uint256 amount) internal {
    _token.mint(account, amount);
}


/**
 * @dev Ensures that only users with enough gas payment token balance can have transactions relayed through the GSN.
 */
function acceptRelayedCall(
    address,
    address from,
    bytes calldata,
    uint256 transactionFee,
    uint256 gasPrice,
    uint256,
    uint256,
    bytes calldata,
    uint256 maxPossibleCharge
)

maxPossibleCharge is calculated in the RelayHub.

github.com

tabookey/tabookey-gasless/blob/master/contracts/RelayHub.sol#L446


        let dataPtr := add(data, 32)


        revert(dataPtr, dataSize)
    }
}


function requiredGas(uint256 relayedCallStipend) public view returns (uint256) {
    return gasOverhead + gasReserve + acceptRelayedCallMaxGas + preRelayedCallMaxGas + postRelayedCallMaxGas + relayedCallStipend;
}


function maxPossibleCharge(uint256 relayedCallStipend, uint256 gasPrice, uint256 transactionFee) public view returns (uint256) {
    return calculateCharge(requiredGas(relayedCallStipend), gasPrice, transactionFee);
}


function calculateCharge(uint256 gas, uint256 gasPrice, uint256 fee) private pure returns (uint256) {
    // The fee is expressed as a percentage. E.g. a value of 40 stands for a 40% fee, so the recipient will be
    // charged for 1.4 times the spent amount.
    return (gas * gasPrice * (100 + fee)) / 100;
}


function getChargeableGas(uint256 gasUsed, bool postRelayedCallEstimation) private pure returns (uint256) {

abcoathup · November 11, 2019, 12:21am

Hi @meh_blah,

Did the above answer your question?

meh_blah · November 13, 2019, 11:20pm

Hey @abcoathup,

Really sorry for the delay in getting back to you regarding this one. I think this should help however, I haven’t tried it yet and will confirm once I have.

I have one more question regarding integrating GSN with my smart contracts. If my recipient contract (say contract ABC is GSNRecipient) interacts with other contracts, is it already assumed that ABC contract pays gas for those calls as well?

Thanks a ton for all your help!

abcoathup · November 13, 2019, 11:39pm

Hi @meh_blah,

Yes. If a contract ABC implements GSNRecipient then any relayed call that your GSN Strategy accepts, includes the gas cost of ABC calling other contracts as part of the transaction.

abcoathup · November 14, 2019, 3:06am

A post was split to a new topic: Example of using truffle tests to test a GSN recipient contract?

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[GSN] Is there a method that can compute the total cost to relay a transaction?

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