chore: adding bridge spec

specs/protocol/bridges.md

@@ -6,6 +6,7 @@
 
 - [Overview](#overview)
 - [Token Depositing](#token-depositing)
+- [ERC20 Unlocking](#erc20-unlocking)
 - [Upgradability](#upgradability)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
@@ -29,15 +30,19 @@ interface StandardBridge {
     event ERC20BridgeInitiated(address indexed localToken, address indexed remoteToken, address indexed from, address to, uint256 amount, bytes extraData);
     event ETHBridgeFinalized(address indexed from, address indexed to, uint256 amount, bytes extraData);
     event ETHBridgeInitiated(address indexed from, address indexed to, uint256 amount, bytes extraData);
+    event ERC20Unlocked(address indexed localToken, address indexed remotetoken, address indexed from, uint256 amount, bytes32 messageHash);
 
     function bridgeERC20(address _localToken, address _remoteToken, uint256 _amount, uint32 _minGasLimit, bytes memory _extraData) external;
     function bridgeERC20To(address _localToken, address _remoteToken, address _to, uint256 _amount, uint32 _minGasLimit, bytes memory _extraData) external;
     function bridgeETH(uint32 _minGasLimit, bytes memory _extraData) payable external;
     function bridgeETHTo(address _to, uint32 _minGasLimit, bytes memory _extraData) payable external;
     function deposits(address, address) view external returns (uint256);
+    function processedMessages(bytes32 _messageHashes) view external returns (uint256);
+    function unlockERC20s(address _localToken, address _remoteToken, address _from, address _to, uint256 _amount, bytes calldata _extraData, uint240 _nonce, uint32 _minGasLimit) external;
     function finalizeBridgeERC20(address _localToken, address _remoteToken, address _from, address _to, uint256 _amount, bytes memory _extraData) external;
     function finalizeBridgeETH(address _from, address _to, uint256 _amount, bytes memory _extraData) payable external;
     function messenger() view external returns (address);
+    function ROLLBACK_INBOX() view external returns (address);
     function OTHER_BRIDGE() view external returns (address);
 }
 ```
@@ -49,6 +54,9 @@ domain. An `OptimismMintableERC20` token contract must exist on the remote
 domain to be able to deposit tokens to that domain. One of these tokens can be
 deployed using the `OptimismMintableERC20Factory` contract.
 
+## ERC20 Unlocking
+The `ERC20Unlocked` function is used to unlock tokens stuck due to failure in relaying prior ERC20 bridging actions. A `messageHash` must exist in the `ROLLBACK_INBOX` contract to certify the message hash corresponding to an ERC20 bridging action started from the standard bridged failed on the other domain.
+
 ## Upgradability
 
 Both the L1 and L2 standard bridges should be behind upgradable proxies.

specs/protocol/messengers.md

@@ -6,6 +6,7 @@
 
 - [Overview](#overview)
 - [Message Passing](#message-passing)
+- [Rolling a Message Back](#rolling-a-message-back)
 - [Upgradability](#upgradability)
 - [Message Versioning](#message-versioning)
   - [Message Version 0](#message-version-0)
@@ -38,15 +39,18 @@ interface CrossDomainMessenger {
     event RelayedMessage(bytes32 indexed msgHash);
     event SentMessage(address indexed target, address sender, bytes message, uint256 messageNonce, uint256 gasLimit);
     event SentMessageExtension1(address indexed sender, uint256 value);
+    event MessageSentToRollbackInbox(bytes32 indexed messageHash, address indexed target, address sender, bytes message, uint256 messageNonce, uint256 gasLimit);
 
     function MESSAGE_VERSION() external view returns (uint16);
     function MIN_GAS_CALLDATA_OVERHEAD() external view returns (uint64);
     function MIN_GAS_CONSTANT_OVERHEAD() external view returns (uint64);
     function MIN_GAS_DYNAMIC_OVERHEAD_DENOMINATOR() external view returns (uint64);
     function MIN_GAS_DYNAMIC_OVERHEAD_NUMERATOR() external view returns (uint64);
     function OTHER_MESSENGER() external view returns (address);
+    function ROLLBACK_INBOX() external view returns (address);
+    function ROLLBACK_INBOX_DELAY() external view returns (uint256);
     function baseGas(bytes memory _message, uint32 _minGasLimit) external pure returns (uint64);
-    function failedMessages(bytes32) external view returns (bool);
+    function failedMessages(bytes32) external view returns (uint256);
     function messageNonce() external view returns (uint256);
     function relayMessage(
         uint256 _nonce,
@@ -57,7 +61,8 @@ interface CrossDomainMessenger {
         bytes memory _message
     ) external payable;
     function sendMessage(address _target, bytes memory _message, uint32 _minGasLimit) external payable;
-    function successfulMessages(bytes32) external view returns (bool);
+    function sendHashToRollbackInbox(bytes32 _versionedHash, bytes32 _minGasLimit) external;
+    function successfulMessages(bytes32) external view returns (uint256);
     function xDomainMessageSender() external view returns (address);
 }
 ```
@@ -79,6 +84,9 @@ then waits for the finalization window to pass, and then finalizes the withdrawa
 on the OptimismPortal, which calls `relayMessage` on the
 `L1CrossDomainMessenger` to finalize the withdrawal.
 
+## Rolling a Message Back
+The `sendHashToRollbackInbox` function is used to send the hashes of failed and unsuccesful messages to the `ROLLBACK_INBOX` contract on the other domain after a given delay, ensuring the message corresponding to the rolled back hash is never processed in the current domain. The main purpose of this function is to inform the other domain of failed messages so contracts living in this other domain can handle the message failure. This allows functionalities such as unlocking stuck ERC20s.
+
 ## Upgradability
 
 The L1 and L2 cross domain messengers should be deployed behind upgradable

specs/protocol/rollback-inboxes.md

@@ -0,0 +1,34 @@
+# Rollback Inboxes
+
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+**Table of Contents**
+
+- [Overview](#overview)
+- [Message Hash Reception](#message-hash-reception)
+- [Upgradability](#upgradability)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+## Overview
+
+The rollback inbox contracts are responsible for storing the hashes of messages that failed on the other domain. They are built with the purpose of allowing contracts on the same domain to have the capability of handling what actions to perform when a message failed on the other domain and was rolled-back to it's origin domain.
+
+```solidity
+interface RollbackInbox {
+    event MessageHashReceived(bytes32 indexed messageHash, uint256 indexed timestamp);
+
+    function receiveMessageHash(bytes32 _messageHash, address _sender) external;
+    function messenger() view external returns (address);
+    function otherMessenger() view external returns (address);
+    function messageHashes(bytes32 _messageHash) view external returns (uint256);
+}
+```
+
+## Message Hash Reception
+
+The `receiveMessageHash` function is used to receive message hashes from the other domain. It must ensure the caller is the `CrossDomainMessenger` from this domain, and that the sender is the `CrossDomainMessenger` from the other domain.
+
+## Upgradability
+
+Both the L1 and L2 rollback inboxes should be behind upgradable proxies.