Add InterchainGasPaymaster (#285)

* Add InterchainGasPaymaster

* Underscore leaf index parameter

* Add event section comment

solidity/core/contracts/InterchainGasPaymaster.sol

@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+pragma solidity >=0.6.11;
+
+// ============ External Imports ============
+import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
+
+/**
+ * @title InterchainGasPaymaster
+ * @notice Manages payments on a source chain to cover gas costs of proving
+ * & processing messages on destination chains.
+ * @dev This contract is only intended for paying for messages sent via a specific
+ * Outbox contract on the same source chain.
+ */
+contract InterchainGasPaymaster is Ownable {
+    // ============ Events ============
+
+    /**
+     * @notice Emitted when a payment is made for a message's gas costs.
+     * @param leafIndex The index of the message in the Outbox merkle tree.
+     * @param amount The amount of native tokens paid.
+     */
+    event GasPayment(uint256 indexed leafIndex, uint256 amount);
+
+    // ============ Constructor ============
+
+    // solhint-disable-next-line no-empty-blocks
+    constructor() Ownable() {}
+
+    // ============ External Functions ============
+
+    /**
+     * @notice Deposits the msg.value as a payment for the proving & processing
+     * of a message on its destination chain.
+     * @param _leafIndex The index of the message in the Outbox merkle tree.
+     */
+    function payGasFor(uint256 _leafIndex) external payable {
+        emit GasPayment(_leafIndex, msg.value);
+    }
+
+    /**
+     * @notice Transfers the entire native token balance to the owner of the contract.
+     * @dev The owner must be able to receive native tokens.
+     */
+    function claim() external {
+        // Transfer the entire balance to owner.
+        (bool success, ) = owner().call{value: address(this).balance}("");
+        require(success, "!transfer");
+    }
+}

solidity/core/hardhat.config.ts

@@ -1,5 +1,6 @@
 import "solidity-coverage";
 import "@typechain/hardhat";
+import "@nomiclabs/hardhat-ethers";
 import "@nomiclabs/hardhat-waffle";
 import "hardhat-gas-reporter";
 

solidity/core/test/interchainGasPaymaster.test.ts

@@ -0,0 +1,76 @@
+import { ethers } from 'hardhat';
+import { expect } from 'chai';
+import { SignerWithAddress } from '@nomiclabs/hardhat-ethers/signers';
+
+import {
+  InterchainGasPaymaster,
+  InterchainGasPaymaster__factory,
+} from '../types';
+
+const LEAF_INDEX = 4321;
+const PAYMENT_AMOUNT = 123456789;
+const OWNER = '0xdeadbeef00000000000000000000000000000000';
+
+describe('InterchainGasPaymaster', async () => {
+  let paymaster: InterchainGasPaymaster, signer: SignerWithAddress;
+
+  before(async () => {
+    [signer] = await ethers.getSigners();
+  });
+
+  beforeEach(async () => {
+    const paymasterFactory = new InterchainGasPaymaster__factory(signer);
+    paymaster = await paymasterFactory.deploy();
+  });
+
+  describe('#payGasFor', async () => {
+    it('deposits the value into the contract', async () => {
+      const paymasterBalanceBefore = await signer.provider!.getBalance(
+        paymaster.address,
+      );
+
+      await paymaster.payGasFor(LEAF_INDEX, { value: PAYMENT_AMOUNT });
+
+      const paymasterBalanceAfter = await signer.provider!.getBalance(
+        paymaster.address,
+      );
+
+      expect(paymasterBalanceAfter.sub(paymasterBalanceBefore)).equals(
+        PAYMENT_AMOUNT,
+      );
+    });
+
+    it('emits the GasPayment event', async () => {
+      await expect(paymaster.payGasFor(LEAF_INDEX, { value: PAYMENT_AMOUNT }))
+        .to.emit(paymaster, 'GasPayment')
+        .withArgs(LEAF_INDEX, PAYMENT_AMOUNT);
+    });
+  });
+
+  describe('#claim', async () => {
+    it('sends the entire balance of the contract to the owner', async () => {
+      // First pay some ether into the contract
+      await paymaster.payGasFor(LEAF_INDEX, { value: PAYMENT_AMOUNT });
+
+      // Set the owner to a different address so we aren't paying gas with the same
+      // address we want to observe the balance of
+      await paymaster.transferOwnership(OWNER);
+
+      const ownerBalanceBefore = await signer.provider!.getBalance(OWNER);
+      expect(ownerBalanceBefore).equals(0);
+      const paymasterBalanceBefore = await signer.provider!.getBalance(
+        paymaster.address,
+      );
+      expect(paymasterBalanceBefore).equals(PAYMENT_AMOUNT);
+
+      await paymaster.claim();
+
+      const ownerBalanceAfter = await signer.provider!.getBalance(OWNER);
+      expect(ownerBalanceAfter).equals(PAYMENT_AMOUNT);
+      const paymasterBalanceAfter = await signer.provider!.getBalance(
+        paymaster.address,
+      );
+      expect(paymasterBalanceAfter).equals(0);
+    });
+  });
+});