Fix handling the restart of ovn-controllers

The previous `getattr(old, 'nb_cfg', False)` would evaluate to `False`
if the `old` row either did not contain a `nb_cfg` value or if the value
was 0.

As 0 is the value set on startup of the ovn-controller this causes the
neutron-api to ignore any event a ovn-controller directly sends after
startup. In turn this causes us to miss the information that the agent
is synchronized, causing the agent to appear as down, until something
bumps the `nb_cfg` value globally.

Closes-Bug: #1997982

Change-Id: Icec8fee93e64b871999f38674e305238e9705fd4
(cherry picked from commit 4cc611d)

Author: felixhuettner

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovsdb_monitor.py

@@ -336,7 +336,7 @@ def match_fn(self, event, row, old=None):
         # don't update the AgentCache. We use chassis_private.chassis to return
         # data about the agent.
         return event == self.ROW_CREATE or (
-            getattr(old, 'nb_cfg', False) and not
+            hasattr(old, 'nb_cfg') and not
             (self.table == 'Chassis_Private' and not row.chassis))
 
     def run(self, event, row, old):

neutron/tests/functional/plugins/ml2/drivers/ovn/mech_driver/ovsdb/test_ovsdb_monitor.py

@@ -480,6 +480,66 @@ def check_agent_ts():
             self.fail('Chassis timestamp: %s, agent updated_at: %s' %
                       (chassis_ts, str(agent.updated_at)))
 
+    def test_agent_restart(self):
+        def check_agent_up():
+            agent = neutron_agent.AgentCache()[self.chassis_name]
+            return agent.alive
+
+        def check_agent_down():
+            return not check_agent_up()
+
+        def check_nb_cfg_timestamp_is_not_null():
+            agent = neutron_agent.AgentCache()[self.chassis_name]
+            return agent.updated_at != 0
+
+        if not self.sb_api.is_table_present('Chassis_Private'):
+            self.skipTest('Ovn sb not support Chassis_Private')
+
+        # Set nb_cfg to some realistic value, so that the alive check can
+        # actually work
+        self.nb_api.db_set(
+            'NB_Global', '.', ('nb_cfg', 1337)).execute(check_error=True)
+        self.sb_api.db_set(
+            'Chassis_Private', self.chassis_name, ('nb_cfg', 1337)
+        ).execute(check_error=True)
+
+        chassis_uuid = self.sb_api.db_get(
+            'Chassis', self.chassis_name, 'uuid').execute(check_error=True)
+
+        self.assertTrue(check_agent_up())
+        n_utils.wait_until_true(check_nb_cfg_timestamp_is_not_null, timeout=5)
+
+        # Lets start by shutting down the ovn-controller
+        # (where it will remove the Chassis_Private table entry)
+        self.sb_api.db_destroy(
+            'Chassis_Private', self.chassis_name).execute(check_error=True)
+        try:
+            n_utils.wait_until_true(check_agent_down, timeout=5)
+        except n_utils.WaitTimeout:
+            self.fail('Agent did not go down after Chassis_Private removal')
+
+        # Now the ovn-controller starts up again and has not yet synced with
+        # the southbound database
+        self.sb_api.db_create(
+            'Chassis_Private', name=self.chassis_name,
+            external_ids={}, chassis=chassis_uuid,
+            nb_cfg_timestamp=0, nb_cfg=0
+        ).execute(check_error=True)
+        self.assertTrue(check_agent_down())
+
+        # Now the ovn-controller has synced with the southbound database
+        nb_cfg_timestamp = timeutils.utcnow_ts() * 1000
+        with self.sb_api.transaction() as txn:
+            txn.add(self.sb_api.db_set('Chassis_Private', self.chassis_name,
+                                       ('nb_cfg_timestamp', nb_cfg_timestamp)))
+            txn.add(self.sb_api.db_set('Chassis_Private', self.chassis_name,
+                                       ('nb_cfg', 1337)))
+        try:
+            n_utils.wait_until_true(check_agent_up, timeout=5)
+        except n_utils.WaitTimeout:
+            self.fail('Agent did not go up after sync is done')
+        self.assertTrue(check_nb_cfg_timestamp_is_not_null())
+
 
 class TestOvnIdlProbeInterval(base.TestOVNFunctionalBase):
     def setUp(self):