Make mailbox client upgrade safe (#2984)

### Description Pulls in changes from #2904 ### Related issues - Fixes hyperlane-xyz/issues#593 ### Backward compatibility No, existing v3 MailboxClients (Routers) will be storage incompatible ### Testing Unit Tests --------- Co-authored-by: Michalis Kargakis <[email protected]>
hyperlane-xyz · Nov 28, 2023 · df34198 · df34198
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diff --git a/.changeset/hip-jobs-smoke.md b/.changeset/hip-jobs-smoke.md
@@ -0,0 +1,5 @@
+---
+'@hyperlane-xyz/core': minor
+---
+
+Includes storage gap in Mailbox Client for forwards compatibility
diff --git a/solidity/contracts/client/MailboxClient.sol b/solidity/contracts/client/MailboxClient.sol
@@ -22,6 +22,8 @@ abstract contract MailboxClient is OwnableUpgradeable {
 
     IInterchainSecurityModule public interchainSecurityModule;
 
+    uint256[48] private __GAP; // gap for upgrade safety
+
     // ============ Modifiers ============
     modifier onlyContract(address _contract) {
         require(