Make NeutronOvsdbIdl singleton

"NeutronOvsdbIdl" should be a singleton class; only one instance
should be shared everywhere is called. Currently the connection
and the "OvsIdlMonitor" instances are global; those instances are
used to declare the "NeutronOvsdbIdl" instance.

As commented in the related bug, this singleton will ensure that
the OvsIdl indexes are created just once.

Change-Id: I639cf673a983b7b1be810495d8a8c2d89919a9b6
Related-Bug: #1881424

neutron/agent/ovsdb/impl_idl.py

@@ -22,6 +22,7 @@ from ovsdbapp.backend.ovs_idl import vlog
 from ovsdbapp.schema.open_vswitch import impl_idl
 
 from neutron.agent.ovsdb.native import connection as n_connection
+from neutron.common import utils
 from neutron.conf.agent import ovs_conf
 from neutron.plugins.ml2.drivers.openvswitch.agent.common import constants
 
@@ -83,12 +84,13 @@ class OvsCleanup(command.BaseCommand):
         return True
 
 
+@utils.SingletonDecorator
 class NeutronOvsdbIdl(impl_idl.OvsdbIdl):
     def __init__(self, connection, idl_monitor):
         max_level = None if cfg.CONF.OVS.ovsdb_debug else vlog.INFO
         vlog.use_python_logger(max_level=max_level)
         self.idl_monitor = idl_monitor
-        super(NeutronOvsdbIdl, self).__init__(connection)
+        super().__init__(connection)
 
     def ovs_cleanup(self, bridges, all_ports=False):
         return OvsCleanup(self, bridges, all_ports)

neutron/common/utils.py

@@ -987,3 +987,15 @@ def timecost(f):
         ret = f(*args, **kwargs)
         return ret
     return wrapper
+
+
+class SingletonDecorator:
+
+    def __init__(self, klass):
+        self._klass = klass
+        self._instance = None
+
+    def __call__(self, *args, **kwargs):
+        if self._instance is None:
+            self._instance = self._klass(*args, **kwargs)
+        return self._instance

neutron/tests/unit/common/test_utils.py

@@ -624,3 +624,21 @@ class SpawnWithOrWithoutProfilerTestCase(
 
     def test_spawn_without_profiler(self):
         self._compare_profilers_in_parent_and_in_child(init_profiler=False)
+
+
+@utils.SingletonDecorator
+class _TestSingletonClass(object):
+
+    def __init__(self):
+        self.variable = None
+
+
+class SingletonDecoratorTestCase(base.BaseTestCase):
+
+    def test_singleton_instance_class(self):
+        instance_1 = _TestSingletonClass()
+        instance_1.variable = 'value1'
+
+        instance_2 = _TestSingletonClass()
+        self.assertEqual(instance_1.__hash__(), instance_2.__hash__())
+        self.assertEqual('value1', instance_2.variable)