Never raise an exception in notify()

notify() is called from python-ovs code which is not built to
recover from an exception in this user-overriden code. If there
is an exception (e.g. the DB server is down when we process
the hash ring), this exception can cause an unrecoverable error
in processing OVSDB messages, rendering the neutron worker useless.

Change-Id: I5f703d82175d71a222c76df37a82b5ccad890d14

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

@@ -720,39 +720,43 @@ class OvnIdlDistributedLock(BaseOvnIdl):
                     self.driver.agent_chassis_table = 'Chassis_Private'
 
     def notify(self, event, row, updates=None):
-        self.handle_db_schema_changes(event, row)
-        self.notify_handler.notify(event, row, updates, global_=True)
         try:
-            target_node = self._hash_ring.get_node(str(row.uuid))
-        except exceptions.HashRingIsEmpty as e:
-            LOG.error('HashRing is empty, error: %s', e)
-            return
-        if target_node != self._node_uuid:
-            return
-
-        # If the worker hasn't been health checked by the maintenance
-        # thread (see bug #1834498), indicate that it's alive here
-        time_now = timeutils.utcnow()
-        touch_timeout = time_now - datetime.timedelta(
-            seconds=ovn_const.HASH_RING_TOUCH_INTERVAL)
-        if not self._last_touch or touch_timeout >= self._last_touch:
-            # NOTE(lucasagomes): Guard the db operation with an exception
-            # handler. If heartbeating fails for whatever reason, log
-            # the error and continue with processing the event
+            self.handle_db_schema_changes(event, row)
+            self.notify_handler.notify(event, row, updates, global_=True)
             try:
-                ctx = neutron_context.get_admin_context()
-                ovn_hash_ring_db.touch_node(ctx, self._node_uuid)
-                self._last_touch = time_now
-            except Exception:
-                LOG.exception('Hash Ring node %s failed to heartbeat',
-                              self._node_uuid)
-
-        LOG.debug('Hash Ring: Node %(node)s (host: %(hostname)s) '
-                  'handling event "%(event)s" for row %(row)s '
-                  '(table: %(table)s)',
-                  {'node': self._node_uuid, 'hostname': CONF.host,
-                   'event': event, 'row': row.uuid, 'table': row._table.name})
-        self.notify_handler.notify(event, row, updates)
+                target_node = self._hash_ring.get_node(str(row.uuid))
+            except exceptions.HashRingIsEmpty as e:
+                LOG.error('HashRing is empty, error: %s', e)
+                return
+            if target_node != self._node_uuid:
+                return
+
+            # If the worker hasn't been health checked by the maintenance
+            # thread (see bug #1834498), indicate that it's alive here
+            time_now = timeutils.utcnow()
+            touch_timeout = time_now - datetime.timedelta(
+                seconds=ovn_const.HASH_RING_TOUCH_INTERVAL)
+            if not self._last_touch or touch_timeout >= self._last_touch:
+                # NOTE(lucasagomes): Guard the db operation with an exception
+                # handler. If heartbeating fails for whatever reason, log
+                # the error and continue with processing the event
+                try:
+                    ctx = neutron_context.get_admin_context()
+                    ovn_hash_ring_db.touch_node(ctx, self._node_uuid)
+                    self._last_touch = time_now
+                except Exception:
+                    LOG.exception('Hash Ring node %s failed to heartbeat',
+                                  self._node_uuid)
+
+            LOG.debug('Hash Ring: Node %(node)s (host: %(hostname)s) '
+                      'handling event "%(event)s" for row %(row)s '
+                      '(table: %(table)s)',
+                      {'node': self._node_uuid, 'hostname': CONF.host,
+                       'event': event, 'row': row.uuid,
+                       'table': row._table.name})
+            self.notify_handler.notify(event, row, updates)
+        except Exception as e:
+            LOG.exception(e)
 
     @abc.abstractmethod
     def post_connect(self):