Extend FuncRunner to include ExecutionObserver list callbacks

Summary:
I'm trying to land this again. I had to revert because 100+ tests failed in Payments side: https://www.internalfb.com/phabricator/paste/view/P1189678203

I had to revert previous diff due to T180498582.

The trigger for those tests was `Func&& func` making func rvalue.

# Updated:

std::exchange(func, {})(); fixes the issue,

Reviewed By: ot, dmm-fb

Differential Revision: D54236364

fbshipit-source-id: d6051c37c433c8b323d7b96c71aa5c48482d4897

folly/io/async/EventBase.cpp

@@ -184,7 +184,15 @@ namespace folly {
 
 class EventBase::FuncRunner {
  public:
-  void operator()(Func func) noexcept { func(); }
+  explicit FuncRunner(EventBase& eventBase) : eventBase_(eventBase) {}
+  void operator()(Func&& func) noexcept {
+    ExecutionObserverScopeGuard guard(
+        &eventBase_.getExecutionObserverList(), &func);
+    std::exchange(func, {})();
+  }
+
+ private:
+  EventBase& eventBase_;
 };
 
 class EventBase::ThreadIdCollector : public WorkerProvider {
@@ -592,7 +600,6 @@ EventBase::LoopStatus EventBase::loopMain(int flags, LoopOptions options) {
     // loop callback scheduled by execute(), so if there is an enqueue after the
     // empty check here the queue's event will eventually be active.
     if (res != 0 && !queue_->empty()) {
-      ExecutionObserverScopeGuard guard(&executionObserverList_, queue_.get());
       queue_->execute();
     }
 
@@ -947,8 +954,8 @@ bool EventBase::runLoopCallbacks() {
 
 void EventBase::initNotificationQueue() {
   // Infinite size queue
-  queue_ =
-      std::make_unique<EventBaseAtomicNotificationQueue<Func, FuncRunner>>();
+  queue_ = std::make_unique<EventBaseAtomicNotificationQueue<Func, FuncRunner>>(
+      FuncRunner{*this});
 
   // Mark this as an internal event, so event_base_loop() will return if
   // there are no other events besides this one installed.