feat/simulate message (#238)

* add function

* cleaning

* fix revert prefixing and add test revertmsg

* write SM

* debug: progress

* Remove unnecessary logs

* Fix typo

Typo
Co-authored-by: ben-chain <ben@pseudonym.party>

* Update contracts/optimistic-ethereum/OVM/execution/OVM_ExecutionManager.sol
Co-authored-by: ben-chain <ben@pseudonym.party>
Co-authored-by: Ben Jones <ben@pseudonym.party>
Co-authored-by: Kelvin Fichter <kelvinfichter@gmail.com>

packages/contracts/contracts/optimistic-ethereum/OVM/execution/OVM_ExecutionManager.sol

@@ -1805,4 +1805,38 @@ contract OVM_ExecutionManager is iOVM_ExecutionManager, Lib_AddressResolver {
         messageRecord.nuisanceGasLeft = 0;
         messageRecord.revertFlag = RevertFlag.DID_NOT_REVERT;
     }
+
+    /*****************************
+     * L2-only Helper Functions *
+     *****************************/
+
+    /**
+     * Unreachable helper function for simulating eth_calls with an OVM message context.
+     * This function will throw an exception in all cases other than when used as a custom entrypoint in L2 Geth to simulate eth_call.
+     * @param _transaction the message transaction to simulate.
+     * @param _from the OVM account the simulated call should be from.
+     */
+    function simulateMessage(
+        Lib_OVMCodec.Transaction memory _transaction,
+        address _from,
+        iOVM_StateManager _ovmStateManager
+    )
+        external
+        returns (
+            bool,
+            bytes memory
+        )
+    {
+        // Prevent this call from having any effect unless in a custom-set VM frame
+        require(msg.sender == address(0));
+
+        ovmStateManager = _ovmStateManager;
+        _initContext(_transaction);
+
+        messageRecord.nuisanceGasLeft = uint(-1);
+        messageContext.ovmADDRESS = _transaction.entrypoint;
+        messageContext.ovmCALLER = _from;
+
+        return _transaction.entrypoint.call{gas: _transaction.gasLimit}(_transaction.data);
+    }
 }