[OVN] Initialize OVN agent in `start` method

The ``OVNAgentExtensionAPI`` and ``OVNAgentExtensionManager`` instances
create different threads instances. Now the OVN agent is using the
``oslo.service`` threading implementation (that uses ``cotyledon``).
This library creates a fork of the main process to execute the
``ServiceWrapper`` instance, where the agent is running. Any thread
should be created in the child process, at the
``oslo_service.service.Service.start`` method.

Closes-Bug: #2131540

Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
Change-Id: I97d52e8fd2236b7d9210174d05f01c81a2edcd0d

neutron/agent/ovn/agent/ovn_neutron_agent.py

@@ -103,9 +103,8 @@ class OVNNeutronAgent(service.Service):
         self._chassis = None
         self._chassis_id = None
         self._ovn_bridge = None
-        self.ext_manager_api = ext_mgr.OVNAgentExtensionAPI()
-        self.ext_manager = ext_mgr.OVNAgentExtensionManager(self._conf)
-        self.ext_manager.initialize(None, 'ovn', self)
+        self.ext_manager_api = None
+        self.ext_manager = None
 
     def __getitem__(self, name):
         """Return the named extension objet from ``self.ext_manager``"""
@@ -234,7 +233,22 @@ class OVNNeutronAgent(service.Service):
             'Chassis_Private', self.chassis,
             ('external_ids', external_ids)).execute(check_error=True)
 
+    def _initialize_ext_manager(self):
+        """Initialize the externsion manager and the extension manager API.
+
+        This method must be called once, outside the ``__init__`` method and
+        at the beginning of the ``start`` method.
+        """
+        if not self.ext_manager:
+            self.ext_manager_api = ext_mgr.OVNAgentExtensionAPI()
+            self.ext_manager = ext_mgr.OVNAgentExtensionManager(self._conf)
+            self.ext_manager.initialize(None, 'ovn', self)
+
     def start(self):
+        # This must be the first operation in the `start` method.
+        self._initialize_ext_manager()
+
+        # Extension manager configuration.
         self.ext_manager_api.ovs_idl = self._load_ovs_idl()
         self.load_config()
         # Before executing "_load_sb_idl", is is needed to execute

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

@@ -105,6 +105,7 @@ class TestOVNNeutronAgentBase(base.TestOVNFunctionalBase):
                                   'wait'), \
                 mock.patch.object(ovn_neutron_agent.OVNNeutronAgent,
                                   '_load_ovs_idl') as mock_load_ovs_idl:
+            agt._initialize_ext_manager()
             mock_load_ovs_idl.return_value = self._create_ovs_idl(agt)
             agt.start()
             external_ids = agt.sb_idl.db_get(