claimId to ensure claims can only happen once

Author: hamdiallam

ecosystem/incentivized-relays-mvp.md

@@ -75,17 +75,25 @@ Relayers that delivered messages can compensate themselves by pushing through th
 ```solidity
 contract L2ToL2CrossDomainGasTank {
 
+    mapping(bytes32 => bool) claimed;
+
     function claim(Identifier calldata id, bytes calldata payload) nonReentrant external {
         require(id.origin == address(messenger));
         ICrossL2Inbox(Predeploys.CROSS_L2_INBOX).validateMessage(id, keccak256(payload));
 
         // parse the receipt
         require(payload[:32] == RelayedMessageGasReceipt.selector);
-        (bytes32 rootMsgHash, address txOrigin, address relayer, uint256 relayCost) = _decode(payload);
+        (bytes32 rootMsgHash, address txOrigin, uint256 callDepth, address relayer, uint256 relayCost) = _decode(payload);
 
         // ensure message was sent from this chain
         require(messenger.sentMessages[rootMsgHash]);
 
+        // ensure unclaimed, and mark the claim
+        bytes32 claimId = keccak256(abi.encode(rootMsgHash, callDepth));
+        require(!claimed[claimId]);
+
+        claimed[claimId] = true
+
         // compute total cost
         uint256 claimCost = CLAIM_OVERHEAD * block.basefee;
         uint256 cost = relayCost + claimCost;
@@ -98,6 +106,8 @@ contract L2ToL2CrossDomainGasTank {
 }
 ```
 
+A claim is uniquely identified by the `(rootMsgHash, callDepth)` tuple.
+
 As relayers deliver messages, they can claim the cost against the original tx sender deposit with the gas tank. This design introduces some off-chain complexity for the relayer.
 
 1. The relayer must simulate the call and ensure the emitted tx origin has a deposit that will cover the cost on the originating chain.