feat(ctb): MSD exit point 1

Introduces the first exit point for the MigrationSystemDictator.

packages/contracts-bedrock/contracts/deployment/MigrationSystemDictator.sol

@@ -17,16 +17,26 @@ import { BaseSystemDictator } from "./BaseSystemDictator.sol";
  *         proxies and implementations already be deployed before this contract is used.
  */
 contract MigrationSystemDictator is BaseSystemDictator {
+    /**
+     * @notice Step after which exit 1 can no longer be used.
+     */
+    uint8 constant EXIT_1_NO_RETURN_STEP = 3;
+
     /**
      * @notice Step where proxy ownership is transferred.
      */
-    uint8 constant PROXY_TRANSFER_STEP = 3;
+    uint8 constant PROXY_TRANSFER_STEP = 4;
 
     /**
      * @notice Whether or not the deployment is finalized.
      */
     bool public finalized;
 
+    /**
+     * @notice Address of the old L1CrossDomainMessenger implementation.
+     */
+    address public oldL1CrossDomainMessenger;
+
     /**
      * @param _config System configuration.
      */
@@ -63,18 +73,31 @@ contract MigrationSystemDictator is BaseSystemDictator {
      *         deposit halt flag to tell the Sequencer's DTL to stop accepting deposits.
      */
     function step2() external onlyOwner step(2) {
+        // Store the address of the old L1CrossDomainMessenger implementation. We will need this
+        // address in the case that we have to exit early.
+        oldL1CrossDomainMessenger = config.globalConfig.addressManager.getAddress(
+            "OVM_L1CrossDomainMessenger"
+        );
+
         // Temporarily brick the L1CrossDomainMessenger by setting its implementation address to
         // address(0) which will cause the ResolvedDelegateProxy to revert. Better than pausing
         // the L1CrossDomainMessenger via pause() because it can be easily reverted.
         config.globalConfig.addressManager.setAddress("OVM_L1CrossDomainMessenger", address(0));
 
-        // TODO: Set the deposit halt flag.
+        // Set the DTL shutoff block, which will tell the DTL to stop syncing new deposits from the
+        // CanonicalTransactionChain. We do this by setting an address in the AddressManager
+        // because the DTL already has a reference to the AddressManager and this way we don't also
+        // need to give it a reference to the SystemDictator.
+        config.globalConfig.addressManager.setAddress(
+            "DTL_SHUTOFF_BLOCK",
+            address(uint160(block.number))
+        );
     }
 
     /**
      * @notice Removes deprecated addresses from the AddressManager.
      */
-    function step3() external onlyOwner step(3) {
+    function step3() external onlyOwner step(EXIT_1_NO_RETURN_STEP) {
         // Remove all deprecated addresses from the AddressManager
         string[17] memory deprecated = [
             "OVM_CanonicalTransactionChain",
@@ -104,7 +127,7 @@ contract MigrationSystemDictator is BaseSystemDictator {
     /**
      * @notice Transfers system ownership to the ProxyAdmin.
      */
-    function step4() external onlyOwner step(4) {
+    function step4() external onlyOwner step(PROXY_TRANSFER_STEP) {
         // Transfer ownership of the AddressManager to the ProxyAdmin.
         config.globalConfig.addressManager.transferOwnership(
             address(config.globalConfig.proxyAdmin)
@@ -229,4 +252,23 @@ contract MigrationSystemDictator is BaseSystemDictator {
 
         finalized = true;
     }
+
+    /**
+     * @notice First exit point, can only be called before step 3 is executed.
+     */
+    function exit1() external onlyOwner {
+        require(
+            currentStep == EXIT_1_NO_RETURN_STEP,
+            "MigrationSystemDictator: can only exit1 before step 3 is executed"
+        );
+
+        // Reset the L1CrossDomainMessenger to the old implementation.
+        config.globalConfig.addressManager.setAddress(
+            "OVM_L1CrossDomainMessenger",
+            oldL1CrossDomainMessenger
+        );
+
+        // Unset the DTL shutoff block which will allow the DTL to sync again.
+        config.globalConfig.addressManager.setAddress("DTL_SHUTOFF_BLOCK", address(0));
+    }
 }