Allow for calling rd_kafka_queue_io_event_enable() from the C++ world

src-cpp/QueueImpl.cpp

@@ -64,3 +64,8 @@ RdKafka::Message *RdKafka::QueueImpl::consume (int timeout_ms) {
 int RdKafka::QueueImpl::poll (int timeout_ms) {
         return rd_kafka_queue_poll_callback(queue_, timeout_ms);
 }
+
+void RdKafka::QueueImpl::io_event_enable (int fd, const void *payload,
+                                          size_t size) {
+        rd_kafka_queue_io_event_enable(queue_, fd, payload, size);
+}

src-cpp/rdkafkacpp.h

@@ -1429,6 +1429,23 @@ class RD_EXPORT Queue {
   virtual int poll (int timeout_ms) = 0;
 
   virtual ~Queue () = 0;
+
+  /**
+   * @brief Enable IO event triggering for queue.
+   *
+   * To ease integration with IO based polling loops this API
+   * allows an application to create a separate file-descriptor
+   * that librdkafka will write \p payload (of size \p size) to
+   * whenever a new element is enqueued on a previously empty queue.
+   *
+   * To remove event triggering call with \p fd = -1.
+   *
+   * librdkafka will maintain a copy of the \p payload.
+   *
+   * @remark When using forwarded queues the IO event must only be enabled
+   *         on the final forwarded-to (destination) queue.
+   */
+  virtual void io_event_enable (int fd, const void *payload, size_t size) = 0;
 };
 
 /**@}*/

src-cpp/rdkafkacpp_int.h

@@ -824,6 +824,7 @@ class QueueImpl : virtual public Queue {
   ErrorCode forward (Queue *queue);
   Message *consume (int timeout_ms);
   int poll (int timeout_ms);
+  void io_event_enable(int fd, const void *payload, size_t size);
 
   rd_kafka_queue_t *queue_;
 };