Improve nodescan worker cleanup

Rather than relying on __del__ to close the pipe fds, let's use
the join() method which we should be calling during our shutdown
procedure anyway.  This will ensure the pipes are closed even
if the object is not GC'd.

Also, remove the backref from the request to the worker to increase
the chances that the object can be GC'd.

Change-Id: I71eb34b2502adebe7b3bf32eefb0fa540535f8f7

zuul/launcher/server.py

@@ -327,6 +327,7 @@ class NodescanRequest:
                        self.key_connection_failures,
                        self.key_negotiation_failures,
                        dt)
+        self.worker = None
 
     def result(self):
         """Return the resulting keys, or raise an exception"""
@@ -553,8 +554,7 @@ class NodescanWorker:
     MAX_REQUESTS = 100
 
     def __init__(self):
-        # Remember to close all pipes on __del__ to prevent leaks in
-        # tests.
+        # Remember to close all pipes to prevent leaks in tests.
         self.wake_read, self.wake_write = os.pipe()
         fcntl.fcntl(self.wake_read, fcntl.F_SETFL, os.O_NONBLOCK)
         self._running = False
@@ -563,10 +563,6 @@ class NodescanWorker:
         self.poll = select.epoll()
         self.poll.register(self.wake_read, select.EPOLLIN)
 
-    def __del__(self):
-        os.close(self.wake_read)
-        os.close(self.wake_write)
-
     def start(self):
         self._running = True
         self.thread = threading.Thread(target=self.run, daemon=True)
@@ -578,6 +574,8 @@ class NodescanWorker:
 
     def join(self):
         self.thread.join()
+        os.close(self.wake_read)
+        os.close(self.wake_write)
 
     def addRequest(self, request):
         """Submit a nodescan request"""