feat: null message telling the Fast Bridge to ignore the claim for a ticketID

Author: jaybuidl

contracts/src/bridge/FastBridgeReceiverOnEthereum.sol

@@ -148,6 +148,12 @@ contract FastBridgeReceiverOnEthereum is SafeBridgeReceiverOnEthereum, IFastBrid
         require(_relay(_messageData), "Failed to call contract"); // Checks-Effects-Interaction
     }
 
+    function ignoreClaim(uint256 _ticketID) external override {
+        require(isSentBySafeBridge(), "Access not allowed: SafeBridgeSender only.");
+
+        delete tickets[_ticketID]; // nobody gets rewarded for this and the Claimant loses his deposit
+    }
+
     function withdrawClaimDeposit(uint256 _ticketID) external override {
         Ticket storage ticket = tickets[_ticketID];
         require(ticket.relayed == true, "Message not relayed yet");

contracts/src/bridge/FastBridgeSenderToEthereum.sol

@@ -107,7 +107,7 @@ contract FastBridgeSenderToEthereum is SafeBridgeSenderToEthereum, IFastBridgeSe
      * @param _receiver The L1 contract address who will receive the calldata
      * @param _calldata The receiving domain encoded message data.
      */
-    function sendSafeFallback(
+    function sendSafeTicket(
         uint256 _ticketID,
         address _receiver,
         bytes memory _calldata
@@ -132,6 +132,18 @@ contract FastBridgeSenderToEthereum is SafeBridgeSenderToEthereum, IFastBridgeSe
         _sendSafe(address(fastBridgeReceiver), safeMessageData);
     }
 
+    function sendSafeNullTicket(uint256 _ticketID) external payable override {
+        Ticket storage ticket = tickets[_ticketID];
+        require(ticket.messageHash == 0, "Ticket does exist.");
+
+        // Safe Bridge message envelope
+        bytes4 methodSelector = IFastBridgeReceiver.ignoreClaim.selector;
+        bytes memory safeMessageData = abi.encodeWithSelector(methodSelector, _ticketID);
+
+        // TODO: how much ETH should be provided for bridging? add an ISafeBridgeSender.bridgingCost() if needed
+        _sendSafe(address(fastBridgeReceiver), safeMessageData);
+    }
+
     // ************************ //
     // *      Governance      * //
     // ************************ //

contracts/src/bridge/interfaces/IFastBridgeReceiver.sol

@@ -19,6 +19,8 @@ interface IFastBridgeReceiver {
         bytes calldata _messageData
     ) external;
 
+    function ignoreClaim(uint256 _ticketID) external;
+
     function withdrawClaimDeposit(uint256 _ticketID) external;
 
     function withdrawChallengeDeposit(uint256 _ticketID) external;

contracts/src/bridge/interfaces/IFastBridgeSender.sol

@@ -20,9 +20,17 @@ interface IFastBridgeSender {
      * @param _receiver The L1 contract address who will receive the calldata
      * @param _calldata The receiving domain encoded message data.
      */
-    function sendSafeFallback(
+    function sendSafeTicket(
         uint256 _ticketID,
         address _receiver,
         bytes memory _calldata
     ) external payable;
+
+    /**
+     * Sends a special message to the counterparty contract indicating to ignore
+     * a specific claim about a `ticketID`
+     *
+     * @param _ticketID The identifier to provide to sendSafeFallback()
+     */
+    function sendSafeNullTicket(uint256 _ticketID) external payable;
 }