Description of problem:
While performing ovs2ovn migration the following exception occurred, from neutron/neutron-ovn-db-sync-util.log.1 on controller-0

2022-05-17 17:20:18.999 37 INFO networking_ovn.cmd.neutron_ovn_db_sync_util [req-dfcbb272-a1d7-434c-87b0-96d0eafd0a58 - - - - -] Migrating Neutron database from OVS to OVN
2022-05-17 17:20:19.363 37 CRITICAL neutron_ovn_db_sync_util [req-dfcbb272-a1d7-434c-87b0-96d0eafd0a58 - - - - -] Unhandled error: sqlalchemy.exc.InvalidRequestError: This session is in 'inactive' state, due to the SQL transaction being rolled back; no further SQL can be emitted within this transaction.
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util Traceback (most recent call last):
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/bin/neutron-ovn-db-sync-util", line 10, in <module>
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     sys.exit(main())
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/networking_ovn/cmd/neutron_ovn_db_sync_util.py", line 235, in main
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     db_migration.migrate_neutron_database_to_ovn(core_plugin)
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/networking_ovn/ml2/db_migration.py", line 98, in migrate_neutron_database_to_ovn
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     pb_missed.add(port_id)
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib64/python3.6/contextlib.py", line 88, in __exit__
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     next(self.gen)
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/oslo_db/sqlalchemy/enginefacade.py", line 1065, in _transaction_scope
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     yield resource
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib64/python3.6/contextlib.py", line 88, in __exit__
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     next(self.gen)
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/oslo_db/sqlalchemy/enginefacade.py", line 667, in _session
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     self.session.rollback()
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     self.force_reraise()
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     six.reraise(self.type_, self.value, self.tb)
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/six.py", line 693, in reraise
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     raise value
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/oslo_db/sqlalchemy/enginefacade.py", line 664, in _session
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     self._end_session_transaction(self.session)
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib/python3.6/site-packages/oslo_db/sqlalchemy/enginefacade.py", line 692, in _end_session_transaction
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     session.commit()
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib64/python3.6/site-packages/sqlalchemy/orm/session.py", line 1026, in commit
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     self.transaction.commit()
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib64/python3.6/site-packages/sqlalchemy/orm/session.py", line 491, in commit
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     self._assert_active(prepared_ok=True)
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util   File "/usr/lib64/python3.6/site-packages/sqlalchemy/orm/session.py", line 298, in _assert_active
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util     "This session is in 'inactive' state, due to the "
2022-05-17 17:20:19.363 37 ERROR neutron_ovn_db_sync_util sqlalchemy.exc.InvalidRequestError: This session is in 'inactive' state, due to the SQL transaction being rolled back; no further SQL can be emitted within this transaction.



Version-Release number of selected component (if applicable):
RHOS-16.2-RHEL-8-20220513.n.2
python3-networking-ovn-7.4.2-2.20220409154848.el8ost.noarch
python3-networking-ovn-migration-tool-7.4.2-2.20220409154848.el8ost.noarch
ovn-2021-21.12.0-42.el8fdp.x86_64

How reproducible:
Reproduced once for now.

Steps to Reproduce:
1. Deploy HA ML2OVS environment (3 controllers + 2 compute nodes), in my case DVR was not enabled
2. Create a workload, i.e. router, internal network, connect the network via router to external network, then spawn 2 VMs, one on each compute node
3. Apply changes from https://review.opendev.org/openstack/neutron refs/changes/38/835638/8 . This is a WIP patch intended to support backup and restore after OVN migration. It will add a backup control plane stage before starting the migration.
4. Follow the official documentation page for migrating to OVN, use no-DVR to no-DVR (centralized routing to centralized routing) scenario.

Actual results:
OVN migration script failed on TASK [migration : Sync neutron db with OVN db (container) - Run 1

Expected results:
OVN migration completed successfully

Additional info: