Fix race condition in Inspector shutdown

Summary:
The `state_` member variable was getting destroyed before the `executor_` (because `state_` is declared after `executor_` https://stackoverflow.com/questions/2254263/order-of-member-constructor-and-destructor-calls). This lead to a race condition where items still pending on the `executor_` thread could try to run and access `state_` after it had already been reset.

Changelog:
[General][Fixed] - Fix race condition in Debug Inspector shutdown

Reviewed By: mhorowitz

Differential Revision: D24705062

fbshipit-source-id: e575d66322ab980b1a8c3e6083a0b3d40b9c944b

Author: MartinSherburn

ReactCommon/hermes/inspector/Inspector.cpp

@@ -132,7 +132,6 @@ Inspector::Inspector(
 }
 
 Inspector::~Inspector() {
-  // TODO: think about expected detach flow
   debugger_.setEventObserver(nullptr);
 }
 

ReactCommon/hermes/inspector/Inspector.h

@@ -319,11 +319,6 @@ class Inspector : public facebook::hermes::debugger::EventObserver,
   facebook::hermes::debugger::Debugger &debugger_;
   InspectorObserver &observer_;
 
-  // All client methods (e.g. enable, setBreakpoint, resume, etc.) are executed
-  // on executor_ to prevent deadlocking on mutex_. See the implementation for
-  // more comments on the threading invariants used in this class.
-  std::unique_ptr<folly::Executor> executor_;
-
   // All of the following member variables are guarded by mutex_.
   std::mutex mutex_;
   std::unique_ptr<InspectorState> state_;
@@ -360,6 +355,12 @@ class Inspector : public facebook::hermes::debugger::EventObserver,
   // Are we currently waiting for a debugger to attach, because we
   // requested 'pauseOnFirstStatement'?
   bool awaitingDebuggerOnStart_;
+
+  // All client methods (e.g. enable, setBreakpoint, resume, etc.) are executed
+  // on executor_ to prevent deadlocking on mutex_. See the implementation for
+  // more comments on the threading invariants used in this class.
+  // NOTE: This needs to be declared LAST because it should be destroyed FIRST.
+  std::unique_ptr<folly::Executor> executor_;
 };
 
 } // namespace inspector