Skip OVNL3 post-fork init in non-API workers

The ``OVNL3RouterPlugin`` registers OVN OVSDB monitor events during
``_post_fork_initialize``. That callback is also invoked from
neutron-periodic-workers and other non-API processes, where the OVN IDL
connections are not initialized. Checking OVN readiness before filtering
by worker type raised ``MechanismDriverOVNNotReady`` and blocked worker
startup.

Return early when the trigger is not a Neutron API ``WorkerService``, and
only then validate OVN readiness and register monitor events.

Conflicts:
  neutron/services/ovn_l3/plugin.py

Related-Bug: #2154192
Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
Change-Id: I9e43fdb06ce7868e5fa699657d8da9a4aa4d02fb
(cherry picked from commit b6d260e)

Author: ralonsoh

neutron/services/ovn_l3/plugin.py

@@ -183,18 +183,20 @@ def subscribe(self):
                            cancellable=True)
 
     def _post_fork_initialize(self, resource, event, trigger, payload=None):
+        # TODO(ralonsoh): once [1] is released and required in Neutron, it
+        # won't be needed the ``get_method_class`` method.
+        # [1] https://review.opendev.org/c/openstack/neutron-lib/+/988563
+        if utils.get_method_class(trigger) != wsgi.WorkerService:
+            return
+
         if not self._nb_ovn or not self._sb_ovn:
             raise ovn_l3_exc.MechanismDriverOVNNotReady()
 
         # Register needed events, only for the Neutron API workers.
-        # TODO(ralonsoh): once [1] is released and required in Neutron, it
-        # won't be needed the ``get_method_class`` method.
-        # [1] https://review.opendev.org/c/openstack/neutron-lib/+/988563
-        if utils.get_method_class(trigger) == wsgi.WorkerService:
-            self._nb_ovn.idl.notify_handler.watch_events([
-                ovsdb_monitor.LogicalRouterPortEvent(self),
-                ovsdb_monitor.LogicalRouterPortGatewayChassisEvent(self),
-            ])
+        self._nb_ovn.idl.notify_handler.watch_events([
+            ovsdb_monitor.LogicalRouterPortEvent(self),
+            ovsdb_monitor.LogicalRouterPortGatewayChassisEvent(self),
+        ])
 
     def _add_neutron_router_interface(self, context, router_id,
                                       interface_info):

releasenotes/notes/ovnl3-post-fork-init-skip-non-api-workers-b2c3d4e5f6a78901.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    The ``OVNL3RouterPlugin._post_fork_initialize`` method now returns early
+    when the worker is not a Neutron API ``WorkerService`` instance. This
+    prevents ``MechanismDriverOVNNotReady`` errors in the
+    ``neutron-periodic-workers`` and other non-API processes, where the OVN
+    IDL connections are not initialized.