[OVN] Protect the AgentCache iteration by copying the local cache

During the execution of ``AgentCache.__iter__``, the content of
"self.agents" can change; the Neutron server can attend to an event
to add or delete an OVN agent. If that happens, the iteration will
fail with the error:
  "RuntimeError: dictionary changed size during iteration"

To avoid this, before iterating the list of agents, the cache
singleton creates a copy of the local cache. The aim of making this
copy is to avoid using locks between threads.

Closes-Bug: #1976292
Change-Id: Icf92685579409282bad0a80ba42531c93738e0b1

neutron/plugins/ml2/drivers/ovn/agent/neutron_agent.py

@@ -13,6 +13,7 @@
 #
 
 import abc
+import copy
 import datetime
 
 from oslo_config import cfg
@@ -229,7 +230,10 @@ class AgentCache:
         self.driver = driver
 
     def __iter__(self):
-        return iter(self.agents.values())
+        # Copying self.agents will avoid any issue during the iteration if an
+        # agent is added or deleted.
+        _agents = copy.copy(self.agents)
+        return iter(_agents.values())
 
     def __getitem__(self, key):
         return self.agents[key]
@@ -253,12 +257,12 @@ class AgentCache:
         agent_ids = {cls.id_from_chassis_private(chassis_private)
                      for cls in NeutronAgent.types.values()}
         # Return the cached agents of agent_ids whose keys are in the cache
-        return (self.agents[id_] for id_ in agent_ids & self.agents.keys())
+        return (agent for agent in self if agent.agent_id in agent_ids)
 
     def get_agents(self, filters=None):
         filters = filters or {}
         agent_list = []
-        for agent in self.agents.values():
+        for agent in self:
             agent_dict = agent.as_dict()
             if all(agent_dict[k] in v for k, v in filters.items()):
                 agent_list.append(agent)

neutron/tests/unit/plugins/ml2/drivers/ovn/agent/test_neutron_agent.py

@@ -0,0 +1,69 @@
+# Copyright 2022 Red Hat, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from unittest import mock
+
+import eventlet
+
+from neutron.common.ovn import constants as ovn_const
+from neutron.plugins.ml2.drivers.ovn.agent import neutron_agent
+from neutron.tests import base
+from neutron.tests.unit import fake_resources as fakes
+
+
+class AgentCacheTestCase(base.BaseTestCase):
+
+    def setUp(self):
+        super().setUp()
+        self.agent_cache = neutron_agent.AgentCache(driver=mock.ANY)
+        self.addCleanup(self._clean_agent_cache)
+        self.names_ref = []
+        for i in range(10):  # Add 10 agents.
+            chassis_private = fakes.FakeOvsdbRow.create_one_ovsdb_row(
+                attrs={'name': 'chassis' + str(i)})
+            self.agent_cache.update(ovn_const.OVN_CONTROLLER_AGENT,
+                                    chassis_private)
+            self.names_ref.append('chassis' + str(i))
+
+    def _clean_agent_cache(self):
+        self.agent_cache.agents = {}
+
+    def _list_agents(self):
+        self.names_read = []
+        for idx, agent in enumerate(self.agent_cache):
+            self.names_read.append(agent.agent_id)
+            if idx == 5:  # Swap to "_add_and_delete_agents" thread.
+                eventlet.sleep(0)
+
+    def _add_and_delete_agents(self):
+        del self.agent_cache['chassis8']
+        chassis_private = fakes.FakeOvsdbRow.create_one_ovsdb_row(
+            attrs={'name': 'chassis10'})
+        self.agent_cache.update(ovn_const.OVN_CONTROLLER_AGENT,
+                                chassis_private)
+
+    def test_update_while_iterating_agents(self):
+        pool = eventlet.GreenPool(2)
+        pool.spawn(self._list_agents)
+        pool.spawn(self._add_and_delete_agents)
+        pool.waitall()
+        self.assertEqual(self.names_ref, self.names_read)
+
+    def test_agents_by_chassis_private(self):
+        chassis_private = fakes.FakeOvsdbRow.create_one_ovsdb_row(
+            attrs={'name': 'chassis5'})
+        agents = self.agent_cache.agents_by_chassis_private(chassis_private)
+        agents = list(agents)
+        self.assertEqual(1, len(agents))
+        self.assertEqual('chassis5', agents[0].agent_id)