Do not start conntrack worker thread from __init__

Instead, start it when the first entry is being added to
the queue.  Also, log any exceptions just in case get()
throws something so we can do further debugging.

Changed class from Queue to LightQueue was done after going
through the eventlet.queue code looking at usage, since
it's a little smaller and should be faster.

Change-Id: Ie84be88382f327ebe312bf17ec2dc5c80a8de35f
Closes-bug: 1750777

neutron/agent/linux/ip_conntrack.py

@@ -17,7 +17,6 @@ import eventlet
 import netaddr
 from oslo_concurrency import lockutils
 from oslo_log import log as logging
-from six.moves import queue as Queue
 
 from neutron.agent.linux import utils as linux_utils
 from neutron.common import constants as n_const
@@ -44,7 +43,7 @@ class IpConntrackUpdate(object):
 class IpConntrackProcessingQueue(object):
     """Manager of the queue of conntrack updates to process."""
     def __init__(self):
-        self._queue = Queue.Queue()
+        self._queue = eventlet.queue.LightQueue()
 
     def add(self, update):
         self._queue.put(update)
@@ -52,8 +51,15 @@ class IpConntrackProcessingQueue(object):
     def updates(self):
         """Grabs the next conntrack update from the queue and processes."""
         while not self._queue.empty():
+            try:
                 update = self._queue.get()
                 yield update
+            except IndexError:
+                # queue was empty, another worker stole our entry
+                continue
+            except Exception as e:
+                LOG.error("Failed to yield ip_conntrack process queue "
+                          "entry: %s", e)
 
 
 @lockutils.synchronized('conntrack')
@@ -83,9 +89,11 @@ class IpConntrackManager(object):
         self.zone_per_port = zone_per_port  # zone per port vs per network
         self._populate_initial_zone_map()
         self._queue = IpConntrackProcessingQueue()
-        self.start_process_queue()
+        # Don't start the queue processing thread here, do later when
+        # the first entry is added to the queue.
+        self._process_queue_started = False
 
-    def start_process_queue(self):
+    def _start_process_queue(self):
         eventlet.spawn_n(self._process_queue_loop)
 
     def _process_queue_loop(self):
@@ -105,6 +113,9 @@ class IpConntrackManager(object):
                                              update.rule)
 
     def _process(self, device_info_list, rule, remote_ips=None):
+        if not self._process_queue_started:
+            self._process_queue_started = True
+            self._start_process_queue()
         # queue the update to allow the caller to resume its work
         update = IpConntrackUpdate(device_info_list, rule, remote_ips)
         self._queue.add(update)