Fix data race in EventHandlerBase (#2349)

Both the EventHandler and its associated pubs/subs share
the same underlying rmw event listener.
When a pub/sub is destroyed, the listener is destroyed.
There is a data race when the ~EventHandlerBase wants
to access the listener after it has been destroyed.

The EventHandler stores a shared_ptr of its associated pub/sub.
But since we were clearing the listener event callbacks on the
base class destructor ~EventHandlerBase, the pub/sub was
already destroyed, which means the rmw event listener was also
destroyed, thus causing a segfault when trying to obtain it
to clear the callbacks.

Clearing the callbacks on ~EventHandler instead of
~EventHandlerBase avoids the race, since the pub/sub are still valid.

Signed-off-by: Mauro Passerino <mpasserino@irobot.com>
(cherry picked from commit 411dbe8)

rclcpp/include/rclcpp/event_handler.hpp

@@ -260,6 +260,16 @@ class EventHandler : public EventHandlerBase
     }
   }
 
+  ~EventHandler()
+  {
+    // Since the rmw event listener holds a reference to the
+    // "on ready" 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();
+  }
+
   /// Take data so that the callback cannot be scheduled again
   std::shared_ptr<void>
   take_data() override

rclcpp/src/rclcpp/event_handler.cpp

@@ -39,13 +39,6 @@ UnsupportedEventTypeException::UnsupportedEventTypeException(
 
 EventHandlerBase::~EventHandlerBase()
 {
-  // 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) {
     RCUTILS_LOG_ERROR_NAMED(
       "rclcpp",