Avoid race condition when processing RowEvents

In https://review.opendev.org/#/c/753824/ ovsdbapp adds the ability
to pass a "frozen" object to the RowEventHandler so that if a
transaction is started from the main thread that changes the row,
it won't step on the values that the Event is trying to process.

This patch backports the needed ovsdbapp changes for
RowEventHandler which converts the row to a frozen_row.

Change-Id: I87489596e2ff224431f7e83f43a1725172ee0953
Related-Bug: #1896816
(cherry picked from commit ab6d4afd95)

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

@@ -0,0 +1,36 @@
+# Copyright 2021 Red Hat, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+#
+# We don't technically require ovsdbapp that has these fixes so
+# just include them here for stable releases
+try:
+    from ovsdbapp.backend.ovs_idl import idlutils
+    frozen_row = idlutils.frozen_row
+except AttributeError:
+    def frozen_row(row):
+        return row._table.rows.IndexEntry(
+            uuid=row.uuid,
+            **{col: getattr(row, col)
+                for col in row._table.columns if hasattr(row, col)})
+
+try:
+    from ovsdbapp.backend.ovs_idl import event as row_event
+    from ovsdbapp import event as ovsdb_event
+
+    RowEventHandler = row_event.RowEventHandler
+except AttributeError:
+    class RowEventHandler(ovsdb_event.RowEventHandler):
+        def notify(self, event, row, updates=None):
+            row = frozen_row(row)
+            super().notify(event, row, updates)

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

@@ -27,7 +27,6 @@ from ovs.stream import Stream
 from ovsdbapp.backend.ovs_idl import connection
 from ovsdbapp.backend.ovs_idl import event as row_event
 from ovsdbapp.backend.ovs_idl import idlutils
-from ovsdbapp import event
 
 from neutron.common.ovn import constants as ovn_const
 from neutron.common.ovn import exceptions
@@ -35,6 +34,7 @@ from neutron.common.ovn import hash_ring_manager
 from neutron.common.ovn import utils
 from neutron.conf.plugins.ml2.drivers.ovn import ovn_conf
 from neutron.db import ovn_hash_ring_db
+from neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb import backports
 
 
 CONF = cfg.CONF
@@ -358,7 +358,7 @@ class NeutronPgDropPortGroupCreated(row_event.WaitEvent):
         self.event_name = 'PortGroupCreated'
 
 
-class OvnDbNotifyHandler(event.RowEventHandler):
+class OvnDbNotifyHandler(backports.RowEventHandler):
     def __init__(self, driver):
         super(OvnDbNotifyHandler, self).__init__()
         self.driver = driver
@@ -374,7 +374,7 @@ class Ml2OvnIdlBase(connection.OvsdbIdl):
 
 class BaseOvnIdl(Ml2OvnIdlBase):
     def __init__(self, remote, schema):
-        self.notify_handler = event.RowEventHandler()
+        self.notify_handler = backports.RowEventHandler()
         super(BaseOvnIdl, self).__init__(remote, schema)
 
     @classmethod