Minor AtomicNotificationQueue cleanup

Summary: Following up on some comments on the initial diff.

Differential Revision: D24489715

fbshipit-source-id: baea66c7b811ebabf4476d7fd6a1136ab9ec2282

folly/io/async/AtomicNotificationQueue.cpp

@@ -22,6 +22,17 @@
 
 namespace folly {
 
+void AtomicNotificationQueue::Task::execute() && noexcept {
+  RequestContextScopeGuard rctx(std::move(rctx_));
+  try {
+    func_();
+    func_ = nullptr;
+  } catch (...) {
+    LOG(FATAL) << "Exception thrown by a task in AtomicNotificationQueue: "
+               << exceptionStr(std::current_exception());
+  }
+}
+
 AtomicNotificationQueue::Queue::Queue(Queue&& other) noexcept
     : head_(std::exchange(other.head_, nullptr)),
       size_(std::exchange(other.size_, 0)) {}
@@ -273,13 +284,7 @@ bool AtomicNotificationQueue::drive() {
       }
     }
     queueSize_.fetch_sub(1, std::memory_order_relaxed);
-    RequestContextScopeGuard rctx(std::move(queue_.front().second));
-    try {
-      queue_.front().first();
-    } catch (...) {
-      LOG(FATAL) << "Exception thrown by a task in AtomicNotificationQueue: "
-                 << exceptionStr(std::current_exception());
-    }
+    std::move(queue_.front()).execute();
     queue_.pop();
   }
   return i > 0;

folly/io/async/AtomicNotificationQueue.h

@@ -33,11 +33,21 @@ namespace folly {
  * A producer-consumer queue for passing tasks to EventBase thread.
  *
  * Tasks can be added to the queue from any thread. A single EventBase
- * thread can be listening to the queue. Tasks are processed in the FIFO order.
+ * thread can be listening to the queue. Tasks are processed in FIFO order.
  */
 class AtomicNotificationQueue : private EventBase::LoopCallback,
                                 private EventHandler {
-  using Task = std::pair<Func, std::shared_ptr<RequestContext>>;
+  class Task {
+   public:
+    Task(Func&& func, std::shared_ptr<RequestContext> rctx)
+        : func_(std::move(func)), rctx_(std::move(rctx)) {}
+
+    void execute() && noexcept;
+
+   private:
+    Func func_;
+    std::shared_ptr<RequestContext> rctx_;
+  };
   class AtomicQueue;
   class Queue {
    public: