fixes for rmw callbacks in qos_event class (ros2#2102)

Signed-off-by: Alberto Soragna <[email protected]>

Author: alsora

rclcpp/src/rclcpp/qos_event.cpp

@@ -35,6 +35,11 @@ UnsupportedEventTypeException::UnsupportedEventTypeException(
 
 QOSEventHandlerBase::~QOSEventHandlerBase()
 {
+  // Since the rmw event listener holds a reference to
+  // this callback, we need to clear it on destruction of this class.
+  // This clearing is not needed for other rclcpp entities like pub/subs, since
+  // they do own the underlying rmw entities, which are destroyed
+  // on their rclcpp destructors, thus no risk of dangling pointers.
   clear_on_ready_callback();
 
   if (rcl_event_fini(&event_handle_) != RCL_RET_OK) {