Add a callback registry event for the init process - before spawning

The current PROCESS AFTER_INIT event (or AFTER_CREATE) are being called
multiple times, once for each process.
In case we want to perform an action only once after init, we would need a
callback that is called after init is done, but before spawning.

Usage examples can be:
- A plugin/driver needs to configure something at the backend after init is done
- A plugin/driver needs to create an object in one of the neutron tables
(add a network for example)

Change-Id: I249ba32dede16e61b1c757a8010dac610484da4f

neutron/callbacks/events.py

@@ -31,7 +31,8 @@ AFTER_REQUEST = 'after_request'
 
 # String literals representing events associated to process operations
 BEFORE_INIT = 'before_init'
-AFTER_INIT = 'after_init'
+BEFORE_SPAWN = 'before_spawn'  # sent per process
+AFTER_INIT = 'after_init'  # sent per worker
 
 # String literals representing events associated to error conditions
 ABORT_CREATE = 'abort_create'

neutron/service.py

@@ -27,6 +27,9 @@ from oslo_utils import excutils
 from oslo_utils import importutils
 
 from neutron._i18n import _LE, _LI
+from neutron.callbacks import events
+from neutron.callbacks import registry
+from neutron.callbacks import resources
 from neutron.common import config
 from neutron.common import profiler
 from neutron.common import rpc as n_rpc
@@ -87,6 +90,7 @@ def serve_wsgi(cls):
             LOG.exception(_LE('Unrecoverable error: please check log '
                               'for details.'))
 
+    registry.notify(resources.PROCESS, events.BEFORE_SPAWN, service)
     return service