Implement requestRelay in Axelar gateway

contracts/crosschain/axelar/AxelarGatewayBase.sol

@@ -3,6 +3,7 @@
 pragma solidity ^0.8.27;
 
 import {IAxelarGateway} from "@axelar-network/axelar-gmp-sdk-solidity/contracts/interfaces/IAxelarGateway.sol";
+import {IAxelarGasService} from "@axelar-network/axelar-gmp-sdk-solidity/contracts/interfaces/IAxelarGasService.sol";
 import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
 import {InteroperableAddress} from "@openzeppelin/contracts/utils/draft-InteroperableAddress.sol";
 
@@ -18,6 +19,7 @@ abstract contract AxelarGatewayBase is Ownable {
 
     /// @dev Axelar's official gateway for the current chain.
     IAxelarGateway internal immutable _axelarGateway;
+    IAxelarGasService internal immutable _axelarGasService;
 
     // Remote gateway.
     // `addr` is the isolated address part of ERC-7930. Its not a full ERC-7930 interoperable address.
@@ -41,8 +43,13 @@ abstract contract AxelarGatewayBase is Ownable {
     error RemoteGatewayAlreadyRegistered(bytes2 chainType, bytes chainReference);
 
     /// @dev Sets the local gateway address (i.e. Axelar's official gateway for the current chain).
-    constructor(IAxelarGateway _gateway) {
+    constructor(IAxelarGateway _gateway, IAxelarGasService _gasService) {
         _axelarGateway = _gateway;
+        _axelarGasService = _gasService;
+    }
+
+    function gasService() public view virtual returns (IAxelarGasService) {
+        return _axelarGasService;
     }
 
     /// @dev Returns the equivalent chain given an id that can be either either a binary interoperable address or an Axelar network identifier.

contracts/crosschain/axelar/AxelarGatewayDuplex.sol

@@ -3,7 +3,7 @@
 pragma solidity ^0.8.27;
 
 import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
-import {AxelarGatewayBase, IAxelarGateway} from "./AxelarGatewayBase.sol";
+import {AxelarGatewayBase, IAxelarGateway, IAxelarGasService} from "./AxelarGatewayBase.sol";
 import {AxelarGatewayDestination, AxelarExecutable} from "./AxelarGatewayDestination.sol";
 import {AxelarGatewaySource} from "./AxelarGatewaySource.sol";
 
@@ -16,6 +16,7 @@ contract AxelarGatewayDuplex is AxelarGatewaySource, AxelarGatewayDestination {
     /// @dev Initializes the contract with the Axelar gateway and the initial owner.
     constructor(
         IAxelarGateway gateway,
+        IAxelarGasService gasService,
         address initialOwner
-    ) Ownable(initialOwner) AxelarGatewayBase(gateway) AxelarExecutable(address(gateway)) {}
+    ) Ownable(initialOwner) AxelarGatewayBase(gateway, gasService) AxelarExecutable(address(gateway)) {}
 }

contracts/crosschain/axelar/AxelarGatewaySource.sol

@@ -5,6 +5,8 @@ pragma solidity ^0.8.27;
 import {InteroperableAddress} from "@openzeppelin/contracts/utils/draft-InteroperableAddress.sol";
 import {Strings} from "@openzeppelin/contracts/utils/Strings.sol";
 import {IERC7786GatewaySource} from "../../interfaces/IERC7786.sol";
+import {IERC7786Attributes} from "../../interfaces/IERC7786Attributes.sol";
+import {ERC7786Attributes} from "../utils/ERC7786Attributes.sol";
 import {AxelarGatewayBase} from "./AxelarGatewayBase.sol";
 
 /**
@@ -15,13 +17,21 @@ import {AxelarGatewayBase} from "./AxelarGatewayBase.sol";
  */
 abstract contract AxelarGatewaySource is IERC7786GatewaySource, AxelarGatewayBase {
     using InteroperableAddress for bytes;
-    using Strings for address;
+
+    struct MessageDetails {
+        string destination;
+        string target;
+        bytes payload;
+    }
+
+    uint256 private _lastSendId;
+    mapping(bytes32 => MessageDetails) private _details;
 
     error UnsupportedNativeTransfer();
 
     /// @inheritdoc IERC7786GatewaySource
-    function supportsAttribute(bytes4 /*selector*/) public pure returns (bool) {
-        return false;
+    function supportsAttribute(bytes4 selector) public pure returns (bool) {
+        return selector == IERC7786Attributes.requestRelay.selector;
     }
 
     /// @inheritdoc IERC7786GatewaySource
@@ -30,29 +40,71 @@ abstract contract AxelarGatewaySource is IERC7786GatewaySource, AxelarGatewayBas
         bytes calldata payload,
         bytes[] calldata attributes
     ) external payable returns (bytes32 sendId) {
-        require(msg.value == 0, UnsupportedNativeTransfer());
-        // Use of `if () revert` syntax to avoid accessing attributes[0] if it's empty
-        if (attributes.length > 0)
-            revert UnsupportedAttribute(attributes[0].length < 0x04 ? bytes4(0) : bytes4(attributes[0][0:4]));
+        // Process attributes (relay)
+        bool withRelay = false;
+        uint256 value = 0;
+        address refundRecipient = address(0);
+
+        for (uint256 i = 0; i < attributes.length; ++i) {
+            (withRelay, value, , refundRecipient) = ERC7786Attributes.tryDecodeRequestRelayCalldata(attributes[i]);
+            require(withRelay, UnsupportedAttribute(attributes[i].length < 0x04 ? bytes4(0) : bytes4(attributes[i])));
+        }
+        if (!withRelay) {
+            sendId = bytes32(++_lastSendId);
+        }
+        require(msg.value == value, UnsupportedNativeTransfer());
 
         // Create the package
         bytes memory sender = InteroperableAddress.formatEvmV1(block.chainid, msg.sender);
         bytes memory adapterPayload = abi.encode(sender, recipient, payload);
 
-        // Emit event
-        sendId = bytes32(0); // Explicitly set to 0
+        // Emit event early (stack too deep)
         emit MessageSent(sendId, sender, recipient, payload, 0, attributes);
 
         // Send the message
         (bytes2 chainType, bytes calldata chainReference, ) = recipient.parseV1Calldata();
-        string memory axelarDestination = getAxelarChain(InteroperableAddress.formatV1(chainType, chainReference, ""));
         bytes memory remoteGateway = getRemoteGateway(chainType, chainReference);
-        _axelarGateway.callContract(
-            axelarDestination,
-            address(bytes20(remoteGateway)).toChecksumHexString(), // TODO non-evm chains?
-            adapterPayload
-        );
+        string memory axelarDestination = getAxelarChain(InteroperableAddress.formatV1(chainType, chainReference, ""));
+        // TODO: How should we "stringify" addresses on non-evm chains. Axelar doesn't yet support hex format for all
+        // non evm addresses. Do we want to use Hex? Base58? Base64?
+        string memory axelarTarget = chainType == 0x0000
+            ? Strings.toChecksumHexString(address(bytes20(remoteGateway)))
+            : Strings.toHexString(remoteGateway);
 
-        return sendId;
+        if (withRelay) {
+            _axelarGasService.payNativeGasForContractCall{value: msg.value}(
+                address(this),
+                axelarDestination,
+                axelarTarget,
+                adapterPayload,
+                refundRecipient
+            );
+        } else {
+            _details[sendId] = MessageDetails(axelarDestination, axelarTarget, adapterPayload);
+        }
+
+        _axelarGateway.callContract(axelarDestination, axelarTarget, adapterPayload);
+    }
+
+    /**
+     * @dev Request relaying of a message initiated using `sendMessage`.
+     *
+     * NOTE: AxelarGasService does NOT take a gasLimit. Instead it uses the msg.value sent to determine the gas limit.
+     * This function ignores the provided `gasLimit` parameter.
+     */
+    function requestRelay(bytes32 sendId, uint256 /*gasLimit*/, address refundRecipient) external payable {
+        MessageDetails memory details = _details[sendId];
+        require(details.payload.length > 0);
+
+        // delete storage for some refund
+        delete _details[sendId];
+
+        _axelarGasService.payNativeGasForContractCall{value: msg.value}(
+            address(this),
+            details.destination,
+            details.target,
+            details.payload,
+            refundRecipient
+        );
     }
 }

contracts/crosschain/utils/ERC7786Attributes.sol

@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.27;
+
+import {IERC7786Attributes} from "../../interfaces/IERC7786Attributes.sol";
+
+/// @dev Library of helper to parse/process ERC-7786 attributes
+library ERC7786Attributes {
+    /// @dev Parse the `requestRelay(uint256,uint256,address)` (0x4cbb573a) attribute into its components.
+    function tryDecodeRequestRelay(
+        bytes memory attribute
+    ) internal pure returns (bool success, uint256 value, uint256 gasLimit, address refundRecipient) {
+        success = bytes4(attribute) == IERC7786Attributes.requestRelay.selector && attribute.length >= 0x64;
+
+        assembly ("memory-safe") {
+            value := mul(success, mload(add(attribute, 0x24)))
+            gasLimit := mul(success, mload(add(attribute, 0x44)))
+            refundRecipient := mul(success, mload(add(attribute, 0x64)))
+        }
+    }
+
+    /// @dev Calldata variant of {tryDecodeRequestRelay}.
+    function tryDecodeRequestRelayCalldata(
+        bytes calldata attribute
+    ) internal pure returns (bool success, uint256 value, uint256 gasLimit, address refundRecipient) {
+        success = bytes4(attribute) == IERC7786Attributes.requestRelay.selector && attribute.length >= 0x64;
+
+        assembly ("memory-safe") {
+            value := mul(success, calldataload(add(attribute.offset, 0x04)))
+            gasLimit := mul(success, calldataload(add(attribute.offset, 0x24)))
+            refundRecipient := mul(success, calldataload(add(attribute.offset, 0x44)))
+        }
+    }
+}

contracts/interfaces/IERC7786Attributes.sol

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.4;
+
+/**
+ * @dev Standard attributes for ERC-7786. These attributes may be standardized in different ERCs.
+ */
+interface IERC7786Attributes {
+    function requestRelay(uint256 value, uint256 gasLimit, address refundRecipient) external;
+}

contracts/mocks/crosschain/axelar/AxelarGasServiceMock.sol

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.27;
+
+contract AxelarGasServiceMock {
+    event NativeGasPaidForContractCall(
+        address indexed sourceAddress,
+        string destinationChain,
+        string destinationAddress,
+        bytes32 indexed payloadHash,
+        uint256 gasFeeAmount,
+        address refundAddress
+    );
+
+    function payNativeGasForContractCall(
+        address sender,
+        string calldata destinationChain,
+        string calldata destinationAddress,
+        bytes calldata payload,
+        address refundAddress
+    ) external payable {
+        emit NativeGasPaidForContractCall(
+            sender,
+            destinationChain,
+            destinationAddress,
+            keccak256(payload),
+            msg.value,
+            refundAddress
+        );
+    }
+}

test/crosschain/ERC7786OpenBridge.test.js

@@ -127,7 +127,7 @@ describe('ERC7786OpenBridge', function () {
           await expect(txPromise)
             .to.emit(this.bridgeA, 'MessageSent')
             .withArgs(
-              ethers.ZeroHash,
+              anyValue,
               this.chain.toErc7930(this.sender),
               this.chain.toErc7930(this.destination),
               this.payload,
@@ -140,7 +140,7 @@ describe('ERC7786OpenBridge', function () {
             await expect(txPromise)
               .to.emit(gatewayA, 'MessageSent')
               .withArgs(
-                ethers.ZeroHash,
+                anyValue,
                 this.chain.toErc7930(this.bridgeA),
                 this.chain.toErc7930(this.bridgeB),
                 anyValue,

test/crosschain/axelar/AxelarGateway.test.js

@@ -3,17 +3,19 @@ const { expect } = require('chai');
 const { loadFixture } = require('@nomicfoundation/hardhat-network-helpers');
 const { anyValue } = require('@nomicfoundation/hardhat-chai-matchers/withArgs');
 
+const ERC7786Attributes = require('../../helpers/erc7786attributes');
+
 const AxelarHelper = require('./AxelarHelper');
 
 async function fixture() {
-  const [owner, sender, ...accounts] = await ethers.getSigners();
+  const [owner, sender, refundRecipient, ...accounts] = await ethers.getSigners();
 
   const { chain, axelar, gatewayA, gatewayB } = await AxelarHelper.deploy(owner);
 
   const receiver = await ethers.deployContract('$ERC7786ReceiverMock', [gatewayB]);
   const invalidReceiver = await ethers.deployContract('$ERC7786ReceiverInvalidMock');
 
-  return { owner, sender, accounts, chain, axelar, gatewayA, gatewayB, receiver, invalidReceiver };
+  return { owner, sender, refundRecipient, accounts, chain, axelar, gatewayA, gatewayB, receiver, invalidReceiver };
 }
 
 describe('AxelarGateway', function () {
@@ -22,14 +24,16 @@ describe('AxelarGateway', function () {
   });
 
   it('initial setup', async function () {
-    await expect(this.gatewayA.gateway()).to.eventually.equal(this.axelar);
+    await expect(this.gatewayA.gateway()).to.eventually.equal(this.axelar.gateway);
+    await expect(this.gatewayA.gasService()).to.eventually.equal(this.axelar.gasService);
     await expect(this.gatewayA.getAxelarChain(this.chain.erc7930)).to.eventually.equal('local');
     await expect(this.gatewayA.getErc7930Chain('local')).to.eventually.equal(this.chain.erc7930);
     await expect(this.gatewayA.getRemoteGateway(this.chain.erc7930)).to.eventually.equal(
       this.gatewayB.target.toLowerCase(),
     );
 
-    await expect(this.gatewayB.gateway()).to.eventually.equal(this.axelar);
+    await expect(this.gatewayB.gateway()).to.eventually.equal(this.axelar.gateway);
+    await expect(this.gatewayB.gasService()).to.eventually.equal(this.axelar.gasService);
     await expect(this.gatewayB.getAxelarChain(this.chain.erc7930)).to.eventually.equal('local');
     await expect(this.gatewayB.getErc7930Chain('local')).to.eventually.equal(this.chain.erc7930);
     await expect(this.gatewayB.getRemoteGateway(this.chain.erc7930)).to.eventually.equal(
@@ -47,13 +51,48 @@ describe('AxelarGateway', function () {
       [erc7930Sender, erc7930Recipient, payload],
     );
 
+    const sendId = '0x0000000000000000000000000000000000000000000000000000000000000001';
+
     await expect(this.gatewayA.connect(this.sender).sendMessage(erc7930Recipient, payload, attributes))
       .to.emit(this.gatewayA, 'MessageSent')
-      .withArgs(ethers.ZeroHash, erc7930Sender, erc7930Recipient, payload, 0n, attributes)
-      .to.emit(this.axelar, 'ContractCall')
+      .withArgs(sendId, erc7930Sender, erc7930Recipient, payload, 0n, attributes)
+      .to.emit(this.axelar.gateway, 'ContractCall')
+      .withArgs(this.gatewayA, 'local', this.gatewayB, ethers.keccak256(encoded), encoded)
+      .to.emit(this.axelar.gateway, 'MessageExecuted')
+      .withArgs(anyValue)
+      .to.emit(this.receiver, 'MessageReceived')
+      .withArgs(this.gatewayB, anyValue, erc7930Sender, payload);
+
+    await expect(this.gatewayA.connect(this.sender).requestRelay(sendId, 0n, this.refundRecipient, { value: 1000n }))
+      .to.emit(this.axelar.gasService, 'NativeGasPaidForContractCall')
+      .withArgs(this.gatewayA, 'local', this.gatewayB, ethers.keccak256(encoded), 1000n, this.refundRecipient);
+  });
+
+  it('workflow (with requestRelay attribute)', async function () {
+    const erc7930Sender = this.chain.toErc7930(this.sender);
+    const erc7930Recipient = this.chain.toErc7930(this.receiver);
+    const payload = ethers.randomBytes(128);
+    const attributes = [
+      ERC7786Attributes.encodeFunctionData('requestRelay', [1000n, 0n, this.refundRecipient.address]),
+    ];
+    const encoded = ethers.AbiCoder.defaultAbiCoder().encode(
+      ['bytes', 'bytes', 'bytes'],
+      [erc7930Sender, erc7930Recipient, payload],
+    );
+
+    const sendId = '0x0000000000000000000000000000000000000000000000000000000000000000';
+
+    await expect(
+      this.gatewayA.connect(this.sender).sendMessage(erc7930Recipient, payload, attributes, { value: 1000n }),
+    )
+      .to.emit(this.gatewayA, 'MessageSent')
+      .withArgs(sendId, erc7930Sender, erc7930Recipient, payload, 0n, attributes)
+      .to.emit(this.axelar.gateway, 'ContractCall')
       .withArgs(this.gatewayA, 'local', this.gatewayB, ethers.keccak256(encoded), encoded)
-      .to.emit(this.axelar, 'MessageExecuted')
+      .to.emit(this.axelar.gateway, 'MessageExecuted')
       .withArgs(anyValue)
+      .to.emit(this.axelar.gasService, 'NativeGasPaidForContractCall')
+      .withArgs(this.gatewayA, 'local', this.gatewayB, ethers.keccak256(encoded), 1000n, this.refundRecipient)
       .to.emit(this.receiver, 'MessageReceived')
       .withArgs(this.gatewayB, anyValue, erc7930Sender, payload);
   });