Add pause/resumeAcceptingNew

Summary:
The current `pauseAccepting` and `resumeAccepting` will direct operate on the underlying udp socket to pause/resume read any packet. However this makes it hard to make similar behavior as TCP, which stop/pause the listening socket but still serving the accepted socket.
As for UDP the bookkeeping is usually done at upper layer. this change will allowed the `AsyncUDPServerSocket` inform its callback that pause/resume new peer "connection".

Reviewed By: yfeldblum

Differential Revision: D24483859

fbshipit-source-id: ed10eaf3e9ef140112002303e37a73eff7d40943

folly/io/async/AsyncUDPServerSocket.h

@@ -66,6 +66,20 @@ class AsyncUDPServerSocket : private AsyncUDPSocket::ReadCallback,
      */
     virtual void onListenResumed() noexcept {}
 
+    /**
+     * Invoked when the server socket can still read but need to inform the
+     * callback object that it should not process read from new client address.
+     * It is invoked in each acceptors/listeners event base thread.
+     */
+    virtual void onAcceptNewPeerPaused() noexcept {}
+
+    /**
+     * Invoked when need to inform the callback object that it can resume
+     * process read from new client address. It is invoked in each
+     * acceptors/listeners event base thread.
+     */
+    virtual void onAcceptNewPeerResumed() noexcept {}
+
     /**
      * Invoked when a new packet is received
      */
@@ -185,6 +199,19 @@ class AsyncUDPServerSocket : private AsyncUDPSocket::ReadCallback,
     }
   }
 
+  /**
+   * Inform the callback object that it should not process read from new client
+   * address.
+   */
+  void pauseAcceptingNewPeer() {
+    for (auto& listener : listeners_) {
+      auto callback = listener.second;
+
+      listener.first->runInEventBaseThread(
+          [callback]() mutable { callback->onAcceptNewPeerPaused(); });
+    }
+  }
+
   /**
    * Starts accepting datagrams once again.
    */
@@ -198,6 +225,19 @@ class AsyncUDPServerSocket : private AsyncUDPSocket::ReadCallback,
     }
   }
 
+  /**
+   * Inform the callback object that it can process read from new client address
+   * now.
+   */
+  void resumeAcceptingNewPeer() {
+    for (auto& listener : listeners_) {
+      auto callback = listener.second;
+
+      listener.first->runInEventBaseThread(
+          [callback]() mutable { callback->onAcceptNewPeerResumed(); });
+    }
+  }
+
   void setEventCallback(EventRecvmsgCallback* cb) {
     eventCb_ = cb;
     applyEventCallback();