sent message context

sentmessage.context

Author: hamdiallam

specs/interop/predeploys.md

@@ -2,6 +2,7 @@
 
 <!-- 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)
@@ -390,7 +391,7 @@ which is used to track whether the message has successfully been relayed.
 It also emits a `SentMessage` event with the necessary metadata to execute when relayed on the destination chain.
 
 ```solidity
-event SentMessage(uint256 indexed destination, address indexed target, uint256 indexed messageNonce, address sender, bytes message);
+event SentMessage(uint256 indexed destination, address indexed target, uint256 indexed messageNonce, address sender, bytes message, bytes context);
 ```
 
 An explicit `_destination` chain and `nonce` are used to ensure that the message can only be played on a single remote
@@ -406,6 +407,8 @@ it will be the `L2ToL2CrossDomainMessenger` and users will need to callback to g
 The `_destination` MUST NOT be the chain-ID of the local chain and a locally defined `nonce` MUST increment on
 every call to `sendMessage`.
 
+Messages might also contain some internal context with metadata, analogous to headers with HTTP requests.
+
 Note that `sendMessage` is not `payable`.
 
 ### Re-sending Messages
@@ -421,6 +424,7 @@ mapping prior to emitting the `SentMessage` event.
         address _sender,
         address _target,
         bytes calldata _message
+        bytes calldata _context
     )
         external;
 ```