perf: reset xDomainMessageSender to default value after external call (#316)

* perf: reset xDomainMessageSender to default value after external call

Fixes https://github.com/ethereum-optimism/contracts/issues/309

We also set `xDomainMessageSender in the `initialize` function in L1, and in the L2 constructor.

* test: ensure that xDomainMessageSender is always 0xdead before/after calls

* chore: use parent constructor for initializing the L2 xdomain sender

* fix: require that the xDomainSender is not the default value

packages/contracts/contracts/optimistic-ethereum/OVM/bridge/messaging/Abs_BaseCrossDomainMessenger.sol

@@ -18,6 +18,14 @@ import { Lib_ReentrancyGuard } from "../../../libraries/utils/Lib_ReentrancyGuar
  * Runtime target: defined by child contract
  */
 abstract contract Abs_BaseCrossDomainMessenger is iAbs_BaseCrossDomainMessenger, Lib_ReentrancyGuard {
+    /**************
+     *  Constants *
+     **************/
+
+    // The default x-domain message sender being set to a non-zero value makes
+    // deployment a bit more expensive, but in exchange the refund on every call to
+    // `relayMessage` by the L1 and L2 messengers will be higher.
+    address internal constant DEFAULT_XDOMAIN_SENDER = 0x000000000000000000000000000000000000dEaD;
 
     /**********************
      * Contract Variables *
@@ -27,13 +35,18 @@ abstract contract Abs_BaseCrossDomainMessenger is iAbs_BaseCrossDomainMessenger,
     mapping (bytes32 => bool) public successfulMessages;
     mapping (bytes32 => bool) public sentMessages;
     uint256 public messageNonce;
-    address override public xDomainMessageSender;
+    address internal xDomainMsgSender = DEFAULT_XDOMAIN_SENDER;
 
     /********************
      * Public Functions *
      ********************/
 
-    constructor() Lib_ReentrancyGuard() {}
+    constructor() Lib_ReentrancyGuard() internal {}
+
+    function xDomainMessageSender() public override view returns (address) {
+        require(xDomainMsgSender != DEFAULT_XDOMAIN_SENDER, "xDomainMessageSender is not set");
+        return xDomainMsgSender;
+    }
 
     /**
      * Sends a cross domain message to the target messenger.

packages/contracts/contracts/optimistic-ethereum/OVM/bridge/messaging/OVM_L1CrossDomainMessenger.sol

@@ -49,6 +49,7 @@ contract OVM_L1CrossDomainMessenger is iOVM_L1CrossDomainMessenger, Abs_BaseCros
     {
         require(address(libAddressManager) == address(0), "L1CrossDomainMessenger already intialized.");
         libAddressManager = Lib_AddressManager(_libAddressManager);
+        xDomainMsgSender = DEFAULT_XDOMAIN_SENDER;
     }
 
 
@@ -113,8 +114,9 @@ contract OVM_L1CrossDomainMessenger is iOVM_L1CrossDomainMessenger, Abs_BaseCros
             "Provided message has already been received."
         );
 
-        xDomainMessageSender = _sender;
+        xDomainMsgSender = _sender;
         (bool success, ) = _target.call(_message);
+        xDomainMsgSender = DEFAULT_XDOMAIN_SENDER;
 
         // Mark the message as received if the call was successful. Ensures that a message can be
         // relayed multiple times in the case that the call reverted.

packages/contracts/contracts/optimistic-ethereum/OVM/bridge/messaging/OVM_L2CrossDomainMessenger.sol

@@ -75,8 +75,9 @@ contract OVM_L2CrossDomainMessenger is iOVM_L2CrossDomainMessenger, Abs_BaseCros
             "Provided message has already been received."
         );
 
-        xDomainMessageSender = _sender;
+        xDomainMsgSender = _sender;
         (bool success, ) = _target.call(_message);
+        xDomainMsgSender = DEFAULT_XDOMAIN_SENDER;
 
         // Mark the message as received if the call was successful. Ensures that a message can be
         // relayed multiple times in the case that the call reverted.

packages/contracts/test/contracts/OVM/bridge/base/OVM_L1CrossDomainMessenger.spec.ts

@@ -320,6 +320,22 @@ describe('OVM_L1CrossDomainMessenger', () => {
       ).to.equal(true)
     })
 
+    it('the xDomainMessageSender is reset to the original value', async () => {
+      await expect(
+        OVM_L1CrossDomainMessenger.xDomainMessageSender()
+      ).to.be.revertedWith('xDomainMessageSender is not set')
+      await OVM_L1CrossDomainMessenger.relayMessage(
+        target,
+        sender,
+        message,
+        0,
+        proof
+      )
+      await expect(
+        OVM_L1CrossDomainMessenger.xDomainMessageSender()
+      ).to.be.revertedWith('xDomainMessageSender is not set')
+    })
+
     it('should revert if trying to send the same message twice', async () => {
       await OVM_L1CrossDomainMessenger.relayMessage(
         target,

packages/contracts/test/contracts/OVM/bridge/base/OVM_L2CrossDomainMessenger.spec.ts

@@ -137,6 +137,16 @@ describe('OVM_L2CrossDomainMessenger', () => {
       ])
     })
 
+    it('the xDomainMessageSender is reset to the original value', async () => {
+      await expect(
+        OVM_L2CrossDomainMessenger.xDomainMessageSender()
+      ).to.be.revertedWith('xDomainMessageSender is not set')
+      await OVM_L2CrossDomainMessenger.relayMessage(target, sender, message, 0)
+      await expect(
+        OVM_L2CrossDomainMessenger.xDomainMessageSender()
+      ).to.be.revertedWith('xDomainMessageSender is not set')
+    })
+
     it('should revert if trying to send the same message twice', async () => {
       Mock__OVM_L1MessageSender.smocked.getL1MessageSender.will.return.with(
         Mock__OVM_L1CrossDomainMessenger.address