Expressly destroy a node's objects before the node. (ros2#456)

* Expressly destroy a node's objects before the node.

This seems to reduce hangs during test runs described in
ros2/build_farmer#248.

The handles corresponding to the destroyed objects *should* be getting
destroyed explicitly when self.handle.destroy() is called below. It
seems however that when running with Fast-RTPS it's possible to get into
a state where multiple threads are waiting on futexes and none can move
forward. The rclpy context of this apparent deadlock is while clearing
a node's list of publishers or services (possibly others, although
publishers and services were the only ones observed).

I consider this patch to be a workaround rather than a fix.
I think there may either be a race condition between the rcl/rmw layer
and the rmw implementation layer which is being tripped by the
haphazardness of Python's garbage collector or there is a logical
problem with the handle destruction ordering in rclpy that only
Fast-RTPS is sensitive to.

* Don't pre-emptively remove items from Node lists.

As pointed out by Shane, pop()ing each item from the list before
passing it to the .destroy_ITEM() method prevents it from being
destroyed as the individual methods first check that the item is present
in the list then remove it before continuing to destroy it.

Signed-off-by: Steven! Ragnarök <steven@nuclearsandwich.com>

rclpy/rclpy/node.py

@@ -1461,12 +1461,20 @@ def destroy_node(self) -> bool:
         # It will be destroyed with other publishers below.
         self._parameter_event_publisher = None
 
-        self.__publishers.clear()
-        self.__subscriptions.clear()
-        self.__clients.clear()
-        self.__services.clear()
-        self.__timers.clear()
-        self.__guards.clear()
+        # Destroy dependent items eagerly to work around a possible hang
+        # https://github.com/ros2/build_cop/issues/248
+        while self.__publishers:
+            self.destroy_publisher(self.__publishers[0])
+        while self.__subscriptions:
+            self.destroy_subscription(self.__subscriptions[0])
+        while self.__clients:
+            self.destroy_client(self.__clients[0])
+        while self.__services:
+            self.destroy_service(self.__services[0])
+        while self.__timers:
+            self.destroy_timer(self.__timers[0])
+        while self.__guards:
+            self.destroy_guard_condition(self.__guards[0])
         self.handle.destroy()
         self._wake_executor()