Feedback on dai escrow contract that reimburses a relayer using Uniswap

Hey! This isn’t an OpenZeppelin specific question, but I hope that’s alright.

I’m a new contract developer, and I’m experimenting using Ethereum and dai as a possible payment method for a web application. I think I’m fairly happy with the contract architecture, though there’s still quite a bit left to figure out (specifically related to calculating gas costs.)

I’d very much appreciate a quick review, to see if I’ve missed anything major/obvious.

You can find the contract on GitHub, or here:

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at */

pragma solidity ^0.6.5;

interface Exchange {
    function tokenToEthTransferOutput(uint256 eth_bought, uint256 max_tokens, uint256 deadline, address recipient) external returns (uint256  tokens_sold);

interface DaiToken {
    function balanceOf(address tokenOwner) external view returns (uint256);

    function permit(
        address holder,
        address spender,
        uint256 nonce,
        uint256 expiry,
        bool allowed,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external;

    function pull(address usr, uint wad) external;
    function push(address usr, uint wad) external;
    function approve(address usr, uint wad) external returns (bool);

/// @author Vypo Mouse
/// @title DaiEscrowTimeouts
/// @notice Holds Dai tokens in escrow until the buyer and seller agree to
///         release them. A relayer handles paying the transaction fees, and is
///         reimbursed when the funds are released.
/// @dev First construct the contract, specifying the buyer and seller addresses.
///      Then `initialize` the contract with signatures for Dai's `permit`. This
///      transfers Dai from the buyer to the escrow contract.
///      When the seller has completed their responsibilities, the relayer
///      calls `submit` on their behalf. If the seller does not complete their
///      tasks within ~30 days, anyone may call `submitPastDue` and refund the
///      buyer.
///      Once `submit` has been called, the buyer has ~30 days to call `review`.
///      If the buyer does not call `review`, anyone may call `reviewPastDue` to
///      release the funds to the seller.
///      When `review` is called, the buyer may choose to approve the submission
///      or not approve it. If the submission is approved, the funds are released
///      to the seller. If the buyer does not approve, the funds are locked
///      forever.
contract Escrow {
    enum Status {

    // keccak256("Review(bool _approve)")
    bytes32 public constant REVIEW_TYPEHASH = 0xfa5e0016fb62b8dffda8fd95249d438edcffd3689b40ac3b4281d4cf710609ae;

    // keccak256("Submit(bytes32 _submission)")
    bytes32 public constant SUBMIT_TYPEHASH = 0x62b607caa4d4e7fcbd31bf4c033cd30888b536567fadc83710fdf15f8d5cfc9e;

    // Mainnet //
    // DaiToken constant DAI = DaiToken(0x6B175474E89094C44Da98b954EedeAC495271d0F);
    // Exchange constant UNISWAP = Exchange(0x2a1530C4C41db0B0b2bB646CB5Eb1A67b7158667);

    // Kovan //
    DaiToken constant DAI = DaiToken(0x4F96Fe3b7A6Cf9725f59d353F723c1bDb64CA6Aa);
    Exchange constant UNISWAP = Exchange(0x613639E23E91fd54d50eAfd6925AF2Ed6701A46b);

    uint constant TIMEOUT = 30 days;

    uint256 constant MAX_DAI_FOR_RELAYER = 5 ether;

    bytes32 public immutable domain_separator;

    address payable public immutable relayer;
    address immutable public seller;
    address immutable public buyer;

    uint immutable public wad;

    uint public initialized;
    uint public submitted;

    uint public relayer_owed;
    Status public status;

    modifier relayedGasCtor(uint _base) {
        uint at_start = gasleft();


        uint at_end = gasleft();

        relayer_owed += tx.gasprice * (_base + (at_start - at_end));

    modifier relayedGas(uint _base) {
        uint at_start = gasleft();


        uint at_end = gasleft();

        if (tx.origin == relayer) {
            relayer_owed += tx.gasprice * (_base + (at_start - at_end));

    modifier onlyWhen(Status _status) {
        require(status == _status, "Fn not presently valid");

        address _seller,
        address _buyer,
        uint _wad
    ) public relayedGasCtor(1039528) {
        require(_seller != address(0), "invalid seller");
        require(_buyer != address(0), "invalid buyer");

        uint8 chain_id;
        assembly {
            chain_id := chainid()

        domain_separator = keccak256(abi.encode(
            keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),

        wad = _wad;
        relayer = tx.origin;
        seller = _seller;
        buyer = _buyer;
        status = Status.AwaitingWad;

    function initialize(
        uint256 nonce,
        uint256 expiry,
        uint8 v_allow,
        bytes32 r_allow,
        bytes32 s_allow,
        uint8 v_deny,
        bytes32 r_deny,
        bytes32 s_deny
    ) external onlyWhen(Status.AwaitingWad) relayedGas(0) {
        status = Status.AwaitingSubmission;

        initialized = block.timestamp;

        // Unlock buyer's Dai balance to transfer `wad` to this contract.
        DAI.permit(buyer, address(this), nonce, expiry, true, v_allow, r_allow, s_allow);

        // Transfer Dai from `buyer` to this contract.
        DAI.pull(buyer, wad);

        // Relock Dai balance of `buyer`.
        DAI.permit(buyer, address(this), nonce + 1, expiry, false, v_deny, r_deny, s_deny);

    /// @notice Signal that the seller has taken too long. Pays outstanding fees
    ///         to `relayer` and transfers remaining Dai to `buyer`.
    function submitPastDue() external onlyWhen(Status.AwaitingSubmission) {
        require(block.timestamp >= (initialized + TIMEOUT), "not past due");

        // TODO: Track gas for `relayer_owed`


    /// @notice Signal that the buyer has taken too long. Pays outstanding fees
    ///         to `relayer` and transfers remaining Dai to `seller`.
    function reviewPastDue() external onlyWhen(Status.AwaitingReview) {
        require(block.timestamp >= (submitted + TIMEOUT), "not past due");
        assert(submitted != 0);

        // TODO: Track gas for `relayer_owed`


    function submit(
        bytes32 _submission,
        uint8 _v,
        bytes32 _r,
        bytes32 _s
    ) external onlyWhen(Status.AwaitingSubmission) relayedGas(0) {
        bytes32 digest = keccak256(abi.encodePacked(
            keccak256(abi.encode(SUBMIT_TYPEHASH, _submission))

        require(seller == ecrecover(digest, _v, _r, _s), "invalid-permit");

        status = Status.AwaitingReview;
        submitted = block.timestamp;

    function review(
        bool _approve,
        uint8 _v,
        bytes32 _r,
        bytes32 _s
    ) external onlyWhen(Status.AwaitingReview) {
        bytes32 digest = keccak256(abi.encodePacked(
            keccak256(abi.encode(REVIEW_TYPEHASH, _approve))

        require(buyer == ecrecover(digest, _v, _r, _s), "invalid-permit");

        // TODO: Track gas for `relayer_owed`

        if (_approve) {
        } else {

    function forfeit() external {
        require(msg.sender == relayer, "relayer only");
        relayer_owed = 0;

    function resolve(address dai_target) private {
        bool locked = dai_target == address(0);

        if (locked) {
            status = Status.Locked;
        } else {
            status = Status.Complete;

        if (relayer_owed > 0) {
            uint owed = relayer_owed;
            relayer_owed = 0;

            bool approved = DAI.approve(address(UNISWAP), uint(-1));


        if (!locked) {
            DAI.push(dai_target, DAI.balanceOf(address(this)));

    function cancel() external {
        require(status == Status.AwaitingWad || status == Status.Complete, "wrong status");
        require(msg.sender == relayer, "relayer only");
        initialized = 0;
        submitted = 0;

Thanks for taking a look!


Hi @VypoMouse,

Welcome to the community forum :wave:

The forum is for discussions on smart contract development, so this is definitely the right place.

I recommend that smart contracts are appropriately tested and audited. OpenZeppelin offer security audits, see for details.

I would also recommend reading through the checklist before an audit.

:warning: The following is my opinion as a Community Manager having a quick look. (This isn’t a review nor an audit).

  • The Solidity documentation includes an example Escrow contract (though this has different incentives to yours) which might be worth looking at for ideas:
  • I didn’t see any tests in the repository.
    I would recommend writing unit tests to test all the functionality: I would suggest looking at the following guide: Test smart contracts like a rockstar
  • You have rolled your own meta transactions (which may have been part of your own learning exercise).
    You may want to consider using the Gas Station Network. (see
    This could simplify the core of your contract to just focus on the escrow.
  • I would add documentation (such as a README) explaining how the escrow works. (you could even show a badge with your test coverage).
  • You have hardcoded DAI and UniSwap addresses in the contract. You may want to pass these in to the constructor so that you can change depending on what network you deploy to.
  • My preference is not to include self destruct or any functionality in a contract only used for testing to ensure that this doesn’t make it into production.

Hey @abcoathup,

Thanks for taking a peek!

I was looking at setting up some automated tests, but found truffle to be a bit… heavy for just starting out. I’ll definitely give it another go.

Since I use Dai and Uniswap, should I be including those contracts in my testing, or would you recommend mocking them?

I hadn’t heard of the Gas Station Network, so thanks for the pointer! I did a quick google to see if it ties in with Dai’s permit functionality, but it seems like I would still need to get a signature for that to unlock the user’s Dai balance?

This contract is a snippet from a larger repository, that I’m not ready to share yet. It includes a README and all the other good stuff. However, if you have any suggestions for my natspec comment in the contract source itself, I’d very much appreciate it!

Would passing them in as a constructor arguments affect the deployment fees significantly? I’m still not entirely clear on how that whole process works.

Considering I removed it from the inline copy of the source, but not from the repository is strong evidence that this is a good idea…

1 Like

You definitely want to write unit tests (ideally 100% coverage).

I would start following the OpenZeppelin Learn guides on Writing Automated Smart Contract Tests is a great place to start.

Also use Test smart contracts like a rockstar

Feel free to ask questions about writing tests. Let me know if you get stuck.

I would mock them for unit tests.

I would also look at doing some additional testing using ganache --fork of mainnet to test against the real thing.

With an ERC20 token you would need the user to approve an allowance (or use Dai’s permit).

My thought would be to work on the Escrow functionality before adding meta transactions.

Glad to hear that you have appropriate good stuff in your repository.

You may want to look at Solidity Docgen 0.5
Nothing like generating documentation from your code to see how good your natspec is.

I assume there would be some smallish difference in deployment costs. (Feel free to try it out and compare).

My main thought is having anything specific for testing in your mainnet contract.