Cross-chain Realitio Arbitration

RealitioHomeArbitrationProxy.sol

// SPDX-License-Identifier: MIT
// File: src/dependencies/IAMB.sol
pragma solidity ^0.7.2;

interface IAMB {
    function requireToPassMessage(
        address _contract,
        bytes memory _data,
        uint256 _gas
    ) external returns (bytes32);

    function maxGasPerTx() external view returns (uint256);

    function messageSender() external view returns (address);

    function messageId() external view returns (bytes32);
}

// File: src/dependencies/RealitioInterface.sol


/* solhint-disable var-name-mixedcase */
interface RealitioInterface {
    /**
     * @dev The arbitrator contract is trusted to only call this if they've been paid, and tell us who paid them.
     * @notice Notify the contract that the arbitrator has been paid for a question, freezing it pending their decision.
     * @param question_id The ID of the question.
     * @param requester The account that requested arbitration.
     * @param max_previous If specified, reverts if a bond higher than this was submitted after you sent your transaction.
     */
    function notifyOfArbitrationRequest(
        bytes32 question_id,
        address requester,
        uint256 max_previous
    ) external;

    /**
     * @notice Cancel a previously-requested arbitration and extend the timeout
     * @dev Useful when doing arbitration across chains that can't be requested atomically
     * @param question_id The ID of the question
     */
    function cancelArbitration(bytes32 question_id) external;

    /**
     * @notice Submit the answer for a question, for use by the arbitrator, working out the appropriate winner based on the last answer details.
     * @dev Doesn't require (or allow) a bond.
     * @param question_id The ID of the question
     * @param answer The answer, encoded into bytes32
     * @param payee_if_wrong The account to by credited as winner if the last answer given is wrong, usually the account that paid the arbitrator
     * @param last_history_hash The history hash before the final one
     * @param last_answer_or_commitment_id The last answer given, or the commitment ID if it was a commitment.
     * @param last_answerer The address that supplied the last answer
     */
    function assignWinnerAndSubmitAnswerByArbitrator(
        bytes32 question_id,
        bytes32 answer,
        address payee_if_wrong,
        bytes32 last_history_hash,
        bytes32 last_answer_or_commitment_id,
        address last_answerer
    ) external;

    /**
     * @notice Report whether the answer to the specified question is finalized
     * @param question_id The ID of the question
     * @return Return true if finalized
     */
    function isFinalized(bytes32 question_id) external view returns (bool);

    /**
     * @notice Returns the current best answer.
     * @param question_id The ID of the question.
     * @return The current best answer.
     */
    function getBestAnswer(bytes32 question_id) external view returns (bytes32);
}

// File: src/ArbitrationProxyInterfaces.sol


interface IHomeArbitrationProxy {
    /**
     * @dev Recieves the requested arbitration for a question. TRUSTED.
     * @param _questionID The ID of the question.
     * @param _requesterAnswer The answer the requester deem to be correct.
     * @param _requester The address of the user that requested arbitration.
     */
    function receiveArbitrationRequest(
        bytes32 _questionID,
        bytes32 _requesterAnswer,
        address _requester
    ) external;

    /**
     * @dev Recieves a failed attempt to request arbitration. TRUSTED.
     * @param _questionID The ID of the question.
     */
    function receiveArbitrationFailure(bytes32 _questionID) external;

    /**
     * @dev Recieves the answer to a specified question. TRUSTED.
     * @param _questionID The ID of the question.
     * @param _answer The answer from the arbitratior.
     */
    function receiveArbitrationAnswer(bytes32 _questionID, bytes32 _answer) external;
}

interface IForeignArbitrationProxy {
    /**
     * @dev Requests arbitration for given question ID. TRUSTED.
     * @param _questionID The ID of the question.
     */
    function acknowledgeArbitration(bytes32 _questionID) external;

    /**
     * @dev Cancels the arbitration request. TRUSTED.
     * @param _questionID The ID of the question.
     */
    function cancelArbitration(bytes32 _questionID) external;
}

// File: src/RealitioHomeArbitrationProxy.sol





contract RealitioHomeArbitrationProxy is IHomeArbitrationProxy {
    /// @dev The contract governor. TRUSTED.
    address public governor = msg.sender;

    /// @dev The address of the Realitio contract. TRUSTED.
    RealitioInterface public realitio;

    /// @dev ArbitraryMessageBridge contract address. TRUSTED.
    IAMB public amb;

    /// @dev Address of the counter-party proxy on the Foreign Chain. TRUSTED.
    address public foreignProxy;

    enum Status {None, Pending, Accepted, AwaitingRuling, Ruled}

    struct Request {
        Status status;
        address requester;
        bytes32 requesterAnswer;
        bytes32 arbitratorAnswer;
    }

    /// @dev Associates an arbitration request with a question ID.
    mapping(bytes32 => Request) public questionIDToRequest;

    /**
     * @dev To be emitted when arbitration request is received but remained pending.
     * @notice This will happen if the best answer for a given question changes between
     * the arbitration is requested on the Foreign Chain and the cross-chain message
     * reaches the home chain and becomes the same answer as the one from requester.
     * @param _questionID The ID of the question.
     * @param _requesterAnswer The answer the requester deem to be correct.
     * @param _requester The address of the user that requested arbitration.
     */
    event RequestPending(bytes32 indexed _questionID, bytes32 _requesterAnswer, address indexed _requester);

    /**
     * @dev To be emitted when the Realitio contract has been notified of an arbitration request.
     * @notice This will happen if the best answer for a given question changes between
     * the arbitration is requested on the Foreign Chain and the cross-chain message
     * reaches the home chain and becomes the same answer as the one from requester.
     * @param _questionID The ID of the question.
     * @param _requesterAnswer The answer the requester deem to be correct.
     * @param _requester The address of the user that requested arbitration.
     */
    event RequestNotified(bytes32 indexed _questionID, bytes32 _requesterAnswer, address indexed _requester);

    /**
     * @dev To be emitted when there arbitration request acknowledgement is sent to the Foreign Chain.
     * @param _questionID The ID of the question.
     */
    event RequestAcknowledged(bytes32 indexed _questionID);

    /**
     * @dev To be emitted when there arbitration request is cancelled.
     * @param _questionID The ID of the question.
     */
    event RequestCancelled(bytes32 indexed _questionID);

    /**
     * @dev To be emitted when the dispute could not be created on the Foreign Chain.
     * @notice This will happen if there is a remaining arbitration fee users fail to pay.
     * @param _questionID The ID of the question.
     */
    event ArbitrationFailed(bytes32 indexed _questionID);

    /**
     * @dev To be emitted when receiving the answer from the arbitrator.
     * @param _questionID The ID of the question.
     * @param _answer The answer from the arbitrator.
     */
    event ArbitratorAnswered(bytes32 indexed _questionID, bytes32 _answer);

    /**
     * @dev To be emitted when reporting the arbitrator answer to Realitio.
     * @param _questionID The ID of the question.
     */
    event ArbitrationCompleted(bytes32 indexed _questionID);

    modifier onlyGovernor() {
        require(msg.sender == governor, "Only governor allowed");
        _;
    }

    modifier onlyAmb() {
        require(msg.sender == address(amb), "Only AMB allowed");
        _;
    }

    modifier onlyForeignProxy() {
        require(amb.messageSender() == foreignProxy, "Only foreign proxy allowed");
        _;
    }

    /**
     * @dev Creates an arbitration proxy on the home chain.
     * @param _amb ArbitraryMessageBridge contract address.
     * @param _realitio Realitio contract address.
     */
    constructor(IAMB _amb, RealitioInterface _realitio) {
        amb = _amb;
        realitio = _realitio;
    }

    /**
     * @dev Sets the address of a new governor.
     * @param _governor The address of the new governor.
     */
    function setGovernor(address _governor) external onlyGovernor {
        governor = _governor;
    }

    /**
     * @dev Sets the address of the ArbitraryMessageBridge.
     * @param _amb The address of the new ArbitraryMessageBridge.
     */
    function setAmb(IAMB _amb) external onlyGovernor {
        amb = _amb;
    }

    /**
     * @dev Sets the address of the arbitration proxy on the Foreign Chain.
     * @param _foreignProxy The address of the proxy.
     */
    function setForeignProxy(address _foreignProxy) external onlyGovernor {
        foreignProxy = _foreignProxy;
    }

    /**
     * @dev Recieves the requested arbitration for a question.
     * @param _questionID The ID of the question.
     * @param _requesterAnswer The answer the requester deem to be correct.
     * @param _requester The address of the user that requested arbitration.
     */
    function receiveArbitrationRequest(
        bytes32 _questionID,
        bytes32 _requesterAnswer,
        address _requester
    ) external override onlyAmb onlyForeignProxy {
        Request storage request = questionIDToRequest[_questionID];
        require(request.status == Status.None, "Request already exists");

        bytes32 currentAnswer = realitio.getBestAnswer(_questionID);

        if (currentAnswer == _requesterAnswer) {
            request.status = Status.Pending;
            request.requester = _requester;
            request.requesterAnswer = _requesterAnswer;

            emit RequestPending(_questionID, _requesterAnswer, _requester);
        } else {
            request.status = Status.Accepted;

            realitio.notifyOfArbitrationRequest(_questionID, _requester, 0);

            emit RequestNotified(_questionID, _requesterAnswer, _requester);
        }
    }

    /**
     * @dev Handles arbitration request after it has been notified to Realitio for a given question.
     * @notice Sends the arbitration acknowledgement to the Foreign Chain.
     * @param _questionID The ID of the question.
     */
    function handleNotifiedRequest(bytes32 _questionID) external {
        Request storage request = questionIDToRequest[_questionID];
        require(request.status == Status.Accepted, "Invalid request status");

        request.status = Status.AwaitingRuling;

        bytes4 selector = IForeignArbitrationProxy(0).acknowledgeArbitration.selector;
        bytes memory data = abi.encodeWithSelector(selector, _questionID);
        amb.requireToPassMessage(foreignProxy, data, amb.maxGasPerTx());

        emit RequestAcknowledged(_questionID);
    }

    /**
     * @dev Handles changed answer for a given question.
     * @notice Sends the arbitration acknowledgement to the Foreign Chain.
     * @param _questionID The ID of the question.
     */
    function handleChangedAnswer(bytes32 _questionID) external {
        Request storage request = questionIDToRequest[_questionID];
        require(request.status == Status.Pending, "Invalid request status");

        bytes32 currentAnswer = realitio.getBestAnswer(_questionID);
        require(request.requesterAnswer != currentAnswer, "Answers are the same");

        request.status = Status.AwaitingRuling;

        realitio.notifyOfArbitrationRequest(_questionID, request.requester, 0);

        emit RequestNotified(_questionID, request.requesterAnswer, request.requester);

        bytes4 selector = IForeignArbitrationProxy(0).acknowledgeArbitration.selector;
        bytes memory data = abi.encodeWithSelector(selector, _questionID);
        amb.requireToPassMessage(foreignProxy, data, amb.maxGasPerTx());

        emit RequestAcknowledged(_questionID);
    }

    /**
     * @dev Handles a given question being finalized.
     * @notice Sends the arbitration cancellation to the Foreign Chain.
     * @param _questionID The ID of the question.
     */
    function handleFinalizedQuestion(bytes32 _questionID) external {
        Request storage request = questionIDToRequest[_questionID];
        require(request.status == Status.Pending, "Invalid request status");

        bool isFinalized = realitio.isFinalized(_questionID);
        require(isFinalized, "Question not finalized");

        delete questionIDToRequest[_questionID];

        bytes4 selector = IForeignArbitrationProxy(0).cancelArbitration.selector;
        bytes memory data = abi.encodeWithSelector(selector, _questionID);
        amb.requireToPassMessage(foreignProxy, data, amb.maxGasPerTx());

        emit RequestCancelled(_questionID);
    }

    /**
     * @dev Recieves a failed attempt to request arbitration.
     * @param _questionID The ID of the question.
     */
    function receiveArbitrationFailure(bytes32 _questionID) external override onlyAmb onlyForeignProxy {
        Request storage request = questionIDToRequest[_questionID];
        require(request.status == Status.AwaitingRuling, "Invalid request status");

        delete questionIDToRequest[_questionID];

        realitio.cancelArbitration(_questionID);

        emit ArbitrationFailed(_questionID);
    }

    /**
     * @dev Recieves the answer to a specified question.
     * @param _questionID The ID of the question.
     * @param _answer The answer from the arbitratior.
     */
    function receiveArbitrationAnswer(bytes32 _questionID, bytes32 _answer) external override onlyAmb onlyForeignProxy {
        Request storage request = questionIDToRequest[_questionID];
        require(request.status == Status.AwaitingRuling, "Invalid request status");

        request.status = Status.Ruled;
        request.arbitratorAnswer = _answer;

        emit ArbitratorAnswered(_questionID, _answer);
    }

    /**
     * @dev Report the answer provided by the arbitrator to a specified question.
     * @param _questionID The ID of the question.
     * @param _lastHistoryHash The history hash given with the last answer to the question in the Realitio contract.
     * @param _lastAnswerOrCommitmentID The last answer given, or its commitment ID if it was a commitment, to the question in the Realitio contract.
     * @param _lastAnswerer The last answerer to the question in the Realitio contract.
     */
    function reportAnswer(
        bytes32 _questionID,
        bytes32 _lastHistoryHash,
        bytes32 _lastAnswerOrCommitmentID,
        address _lastAnswerer
    ) external {
        Request storage request = questionIDToRequest[_questionID];
        require(request.status == Status.Ruled, "Arbitrator has not ruled yet");

        realitio.assignWinnerAndSubmitAnswerByArbitrator(
            _questionID,
            request.arbitratorAnswer,
            request.requester,
            _lastHistoryHash,
            _lastAnswerOrCommitmentID,
            _lastAnswerer
        );

        delete questionIDToRequest[_questionID];

        emit ArbitrationCompleted(_questionID);
    }
}