[OVN] Add a start method to the OVN agent extensions

Each OVN agent extension can implement a start method. It is called
after the extension manager initialization, at the end of the
``OVNNeutronAgent.start`` call.

Related-Bug: #2017871
Change-Id: I334027f061fa963c1cf24540b27c1ff76145b977

neutron/agent/ovn/agent/ovn_neutron_agent.py

@@ -141,7 +141,8 @@ class OVNNeutronAgent(service.Service):
         # "load_config" to populate self.chassis.
         self.ext_manager_api.sb_idl = self._load_sb_idl()
         self.ext_manager_api.nb_idl = self._load_nb_idl()
-        LOG.info('Starting OVN Neutron Agent')
+        self.ext_manager.start()
+        LOG.info('OVN Neutron Agent started')
 
     def stop(self, graceful=True):
         LOG.info('Stopping OVN Neutron Agent')

neutron/agent/ovn/extensions/extension_manager.py

@@ -41,6 +41,11 @@ class OVNAgentExtensionManager(agent_ext_mgr.AgentExtensionsManager):
                         '"OVNAgentExtension"')
                 raise ConfigException(description=desc)
 
+    def start(self):
+        """Start the extensions, once the OVN agent has been initialized."""
+        for ext in self:
+            ext.obj.start()
+
 
 class OVNAgentExtension(extension.AgentExtension, metaclass=abc.ABCMeta):
 
@@ -65,6 +70,14 @@ class OVNAgentExtension(extension.AgentExtension, metaclass=abc.ABCMeta):
         """
         self.agent_api = agent_api
 
+    def start(self):
+        """Start the extension, once the OVN agent has been initialized.
+
+        This method executes any action needed after the initialization of the
+        OVN agent and the extension manager API. It is executed at the end of
+        the OVN agent ``start`` method.
+        """
+
     @property
     @abc.abstractmethod
     def ovs_idl_events(self):

neutron/tests/functional/agent/ovn/agent/fake_ovn_agent_extension.py

@@ -77,3 +77,6 @@ class FakeOVNAgentExtension(ext_mgr.OVNAgentExtension):
     @property
     def sb_idl_events(self):
         return [OVNSBChassisEvent]
+
+    def start(self):
+        self._is_ext_started = True

neutron/tests/functional/agent/ovn/agent/test_ovn_neutron_agent.py

@@ -39,9 +39,10 @@ class TestOVNNeutronAgent(base.TestOVNFunctionalBase):
             agent_ovsdb, 'get_own_chassis_name').start()
         self.ovn_agent = self._start_ovn_neutron_agent()
 
-    def _check_loaded_extensions(self, ovn_agent):
+    def _check_loaded_and_started_extensions(self, ovn_agent):
         loaded_ext = ovn_agent[TEST_EXTENSION]
         self.assertEqual('Fake OVN agent extension', loaded_ext.name)
+        self.assertTrue(loaded_ext._is_ext_started)
 
     def _start_ovn_neutron_agent(self):
         conf = self.useFixture(fixture_config.Config()).conf
@@ -59,7 +60,7 @@ class TestOVNNeutronAgent(base.TestOVNFunctionalBase):
         agt.test_ovn_sb_idl = []
         agt.test_ovn_nb_idl = []
         agt.start()
-        self._check_loaded_extensions(agt)
+        self._check_loaded_and_started_extensions(agt)
 
         self.add_fake_chassis(self.FAKE_CHASSIS_HOST, name=self.chassis_name)