Unregister event handler fd before closing the fd in EventBaseAtomicNotificationQueue.

Summary:
Thread 1:
  - Creates an eventfd X
  - Registers X with an event base
  - Closes X
  - Unregisters X from the event base

Thread 2:
  - Creates some other file descriptor (eventfd or socket or whatever); the operating system reuses file descriptor #X.

The unregistering of X from the event base in thread 1 races with the opening of reused X in thread 2.

The fix is to unregister X before closing X.

Reviewed By: yfeldblum

Differential Revision: D27962155

fbshipit-source-id: 1bcb62c7ebe0297ab9687e1a0edf37d319637fda

folly/io/async/EventBaseAtomicNotificationQueue-inl.h

@@ -78,6 +78,11 @@ EventBaseAtomicNotificationQueue<Task, Consumer>::
       [](Task&&) { return AtomicNotificationQueueTaskStatus::DISCARD; })) {
   }
 
+  // We must unregister before closing the fd. Otherwise the base class
+  // would unregister the fd after it's already closed, which is invalid
+  // (some other thread could've opened something that reused the fd).
+  unregisterHandler();
+
   // Don't drain fd in the child process.
   if (pid_ == get_cached_pid()) {
     // Wait till we observe all the writes before closing fds