Integrating IGP as a hook

solidity/contracts/Mailbox.sol

@@ -101,6 +101,7 @@ contract Mailbox is IMailbox, Versioned, Ownable {
                 _destinationDomain,
                 _recipientAddress,
                 _messageBody,
+                defaultHook,
                 bytes("")
             );
     }
@@ -134,8 +135,8 @@ contract Mailbox is IMailbox, Versioned, Ownable {
         bytes32 recipientAddress,
         bytes calldata messageBody,
         IPostDispatchHook hook,
-        bytes calldata metadata
-    ) public returns (bytes32) {
+        bytes memory metadata
+    ) public payable returns (bytes32) {
         // Format the message into packed bytes.
         bytes memory message = Message.formatMessage(
             VERSION,

solidity/contracts/client/MailboxClient.sol

@@ -8,11 +8,15 @@ import {IMailbox} from "../interfaces/IMailbox.sol";
 import {Address} from "@openzeppelin/contracts/utils/Address.sol";
 
 abstract contract MailboxClient {
+    /// @notice keeping mailbox immutable for maintaining sync
     IMailbox immutable mailbox;
+    /// @notice default hook for authorizing dispatch hook calls
+    address public defaultHook;
 
     constructor(address _mailbox) {
         require(Address.isContract(_mailbox), "!contract");
         mailbox = IMailbox(_mailbox);
+        // defaultHook = mailbox.defaultHook();
     }
 
     // ============ Modifiers ============

solidity/contracts/hooks/AbstractHook.sol

@@ -12,13 +12,13 @@ abstract contract AbstractHook is MailboxClient, IPostDispatchHook {
         payable
         override
         onlyMailbox
-        returns (IPostDispatchHook)
+        returns (address[] memory)
     {
         return _postDispatch(metadata, message);
     }
 
     function _postDispatch(bytes calldata metadata, bytes calldata message)
         internal
         virtual
-        returns (IPostDispatchHook);
+        returns (address[] memory);
 }

solidity/contracts/hooks/AbstractMessageIdAuthHook.sol

@@ -28,7 +28,7 @@ import {Address} from "@openzeppelin/contracts/utils/Address.sol";
 /**
  * @title AbstractMessageIdAuthHook
  * @notice Message hook to inform an Abstract Message ID ISM of messages published through
- * the native OPStack bridge.
+ * the third-party bridge.
  * @dev V3 WIP
  */
 abstract contract AbstractMessageIdAuthHook is AbstractHook {
@@ -66,7 +66,7 @@ abstract contract AbstractMessageIdAuthHook is AbstractHook {
     function _postDispatch(bytes calldata metadata, bytes calldata message)
         internal
         override
-        returns (IPostDispatchHook)
+        returns (address[] memory)
     {
         require(
             message.destination() == destinationDomain,
@@ -79,8 +79,7 @@ abstract contract AbstractMessageIdAuthHook is AbstractHook {
         );
         _sendMessageId(metadata, payload);
 
-        // no next post-dispatch hook
-        // TODO: consider configuring?
-        return IPostDispatchHook(address(0));
+        // leaf hook
+        return new address[](0);
     }
 }

solidity/contracts/hooks/DomainRoutingHook.sol

@@ -21,13 +21,13 @@ contract DomainRoutingHook is AbstractHook, Ownable {
         hooks[destination] = IPostDispatchHook(hook);
     }
 
-    function _postDispatch(bytes calldata metadata, bytes calldata message)
-        internal
-        override
-    {
-        hooks[message.destination()].postDispatch{value: msg.value}(
-            metadata,
-            message
-        );
+    function _postDispatch(
+        bytes calldata, /*metadata*/
+        bytes calldata message
+    ) internal view override returns (address[] memory) {
+        // check metadata
+        address[] memory result = new address[](1);
+        result[0] = address(hooks[message.destination()]);
+        return result;
     }
 }

solidity/contracts/hooks/MerkleTreeHook.sol

@@ -37,7 +37,9 @@ contract MerkleTreeHook is AbstractHook {
     function _postDispatch(
         bytes calldata, /*metadata*/
         bytes calldata message
-    ) internal override {
+    ) internal override returns (address[] memory) {
         _tree.insert(message.id());
+
+        return new address[](0);
     }
 }

solidity/contracts/hooks/PausableHook.sol

@@ -9,11 +9,12 @@ import {Pausable} from "@openzeppelin/contracts/security/Pausable.sol";
 contract PausableHook is AbstractHook, Ownable, Pausable {
     constructor(address _mailbox) AbstractHook(_mailbox) {}
 
-    function _postDispatch(bytes calldata metadata, bytes calldata message)
-        internal
-        override
-        whenNotPaused
-    {}
+    function _postDispatch(
+        bytes calldata, /*metadata*/
+        bytes calldata /*message*/
+    ) internal view override whenNotPaused returns (address[] memory) {
+        return new address[](0);
+    }
 
     function pause() external onlyOwner {
         _pause();

solidity/contracts/igps/InterchainGasPaymaster.sol

@@ -1,10 +1,27 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
 pragma solidity >=0.8.0;
 
+/*@@@@@@@       @@@@@@@@@
+ @@@@@@@@@       @@@@@@@@@
+  @@@@@@@@@       @@@@@@@@@
+   @@@@@@@@@       @@@@@@@@@
+    @@@@@@@@@@@@@@@@@@@@@@@@@
+     @@@@@  HYPERLANE  @@@@@@@
+    @@@@@@@@@@@@@@@@@@@@@@@@@
+   @@@@@@@@@       @@@@@@@@@
+  @@@@@@@@@       @@@@@@@@@
+ @@@@@@@@@       @@@@@@@@@
+@@@@@@@@@       @@@@@@@@*/
+
 // ============ Internal Imports ============
+import {Message} from "../libs/Message.sol";
+import {IGPHookMetadata} from "../libs/hooks/IGPHookMetadata.sol";
 import {IGasOracle} from "../interfaces/IGasOracle.sol";
 import {IInterchainGasPaymaster} from "../interfaces/IInterchainGasPaymaster.sol";
+import {AbstractHook} from "../hooks/AbstractHook.sol";
+
 // ============ External Imports ============
+import {Address} from "@openzeppelin/contracts/utils/Address.sol";
 import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
 
 /**
@@ -15,12 +32,18 @@ import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/Own
 contract InterchainGasPaymaster is
     IInterchainGasPaymaster,
     IGasOracle,
+    AbstractHook,
     OwnableUpgradeable
 {
+    using Address for address;
+    using IGPHookMetadata for bytes;
+    using Message for bytes;
     // ============ Constants ============
 
     /// @notice The scale of gas oracle token exchange rates.
-    uint256 internal constant TOKEN_EXCHANGE_RATE_SCALE = 1e10;
+    uint256 internal constant DECIMALS = 1e10;
+    /// @notice default for user call if metadata not provided
+    uint256 internal constant DEFAULT_GAS_USAGE = 69_420;
 
     // ============ Public Storage ============
 
@@ -50,6 +73,10 @@ contract InterchainGasPaymaster is
         address gasOracle;
     }
 
+    // ============ Constructor ============
+
+    constructor(address _mailbox) AbstractHook(_mailbox) {}
+
     // ============ External Functions ============
 
     /**
@@ -80,19 +107,16 @@ contract InterchainGasPaymaster is
         uint32 _destinationDomain,
         uint256 _gasAmount,
         address _refundAddress
-    ) external payable override {
+    ) public payable override {
         uint256 _requiredPayment = quoteGasPayment(
             _destinationDomain,
             _gasAmount
         );
-        require(
-            msg.value >= _requiredPayment,
-            "insufficient interchain gas payment"
-        );
+        require(msg.value >= _requiredPayment, "IGP: insufficient gas payment");
         uint256 _overpayment = msg.value - _requiredPayment;
         if (_overpayment > 0) {
             (bool _success, ) = _refundAddress.call{value: _overpayment}("");
-            require(_success, "Interchain gas payment refund failed");
+            require(_success, "IGP: refund failed");
         }
 
         emit GasPayment(_messageId, _gasAmount, _requiredPayment);
@@ -155,9 +179,7 @@ contract InterchainGasPaymaster is
         uint256 _destinationGasCost = _gasAmount * uint256(_gasPrice);
 
         // Convert to the local native token.
-        return
-            (_destinationGasCost * _tokenExchangeRate) /
-            TOKEN_EXCHANGE_RATE_SCALE;
+        return (_destinationGasCost * _tokenExchangeRate) / DECIMALS;
     }
 
     /**
@@ -181,6 +203,33 @@ contract InterchainGasPaymaster is
 
     // ============ Internal Functions ============
 
+    function _postDispatch(bytes calldata _metadata, bytes calldata _message)
+        internal
+        override
+        returns (address[] memory)
+    {
+        if (_metadata.length == 0) {
+            payForGas(
+                _message.id(),
+                _message.destination(),
+                DEFAULT_GAS_USAGE,
+                _message.senderAddress()
+            );
+        } else {
+            address refundAddress = _metadata.refundAddress();
+            if (refundAddress != address(0))
+                refundAddress = _message.senderAddress();
+            payForGas(
+                _message.id(),
+                _message.destination(),
+                _metadata.gasLimit(),
+                refundAddress
+            );
+        }
+
+        return new address[](0);
+    }
+
     /**
      * @notice Sets the beneficiary.
      * @param _beneficiary The new beneficiary.

solidity/contracts/interfaces/hooks/IPostDispatchHook.sol

@@ -5,5 +5,5 @@ interface IPostDispatchHook {
     function postDispatch(bytes calldata metadata, bytes calldata message)
         external
         payable
-        returns (IPostDispatchHook next);
+        returns (address[] memory next);
 }