[OVN] Add a post fork initialization method in OVN L3

This is the base implementation of a post fork initialization, happening
after the initialization of the "process" resource. This event happens
after the WSGI workers are already setup.

Related-Bug: #2125553
Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
Change-Id: If708b969d86940dbec9956c58a4b22271aa9a198

Author: ralonsoh

neutron/services/ovn_l3/exceptions.py

@@ -19,3 +19,8 @@
 class MechanismDriverNotFound(n_exc.NotFound):
     message = _("None of the supported mechanism drivers found: "
                 "%(mechanism_drivers)s. Check your configuration.")
+
+
+class MechanismDriverOVNNotReady(n_exc.ServiceUnavailable):
+    message = _('Mechanism driver OVN connection not ready. This service '
+                'plugin must be initialized after the mechanism driver.')

neutron/services/ovn_l3/plugin.py

@@ -19,6 +19,7 @@
 from neutron_lib.api.definitions import qos_fip as qos_fip_apidef
 from neutron_lib.api.definitions import qos_gateway_ip as qos_gateway_ip_apidef
 from neutron_lib.callbacks import events
+from neutron_lib.callbacks import priority_group
 from neutron_lib.callbacks import registry
 from neutron_lib.callbacks import resources
 from neutron_lib import constants as n_const
@@ -94,6 +95,7 @@ def __init__(self):
         self.scheduler = l3_ovn_scheduler.get_scheduler()
         self.port_forwarding = port_forwarding.OVNPortForwarding(self)
         self.l3_driver_controller = driver_controller.DriverController(self)
+        self.subscribe()
 
     @staticmethod
     def _disable_qos_extensions_by_extension_drivers(aliases):
@@ -161,6 +163,18 @@ def get_plugin_description(self):
         return ("L3 Router Service Plugin for basic L3 forwarding"
                 " using OVN")
 
+    def subscribe(self):
+        # By default, the post fork initialization must be done first in the
+        # ML2 plugin (the lower the priority number, the sooner is attended).
+        registry.subscribe(self._post_fork_initialize,
+                           resources.PROCESS, events.AFTER_INIT,
+                           priority=priority_group.PRIORITY_DEFAULT + 1,
+                           cancellable=True)
+
+    def _post_fork_initialize(self, resource, event, trigger, payload=None):
+        if not self._nb_ovn or not self._sb_ovn:
+            raise ovn_l3_exc.MechanismDriverOVNNotReady()
+
     def _add_neutron_router_interface(self, context, router_id,
                                       interface_info):
         try: