upgrade-charm hook from neutron-api-plugin-ovn should restart neutron-server service

[DUFOUR Olivier]

When upgrading the charm, it may modify some configuration files, in particular "/etc/neutron/plugins/ml2/ml2_conf.ini", to apply some new configuration options for OVN.
While the files are properly generated with the new configuration, the changes are not applied unless neutron-server.service is restarted on all neutron-api units manually.

It would be great to have neutron-api-plugin-ovn charm to trigger a necessary restart to "neutron-server.service" to have the service working with the intended settings.
This was discovered when testing a fix for LP#1947391

[Alex Kavanagh (ajkavanagh)]

So the facility does exist in the neutron-api charm for a subordinate to get a restart over the neutron-plugin-api-subordianates relation using the "restart_trigger" key. However, it doesn't look like the neutron-api-plugin-ovn charm uses the key sadly.

One place to start might be the configure_neutron() function which sets up the data to be sent to the neutron-api principal charm; it would just need to add a 'restart_trigger' using the default str(uuid.uuid4()) assignment to ensure that neutron-api causes a restart.

[Dmytro Kazantsev (dkazants)]

Based on troubleshooting steps performed I can say that in current version of charms whenever any configuration in neutron-api-plugin-ovn is changed and "neutron-api" reads it, it triggers neutron-server restart. Also if any configuration is changed at the time of upgrade-charm actions (f.e. juju refresh ...) it also triggers restart of neutron server. Would you mind to provide some specific examples where ml2_conf.ini was changed and no restart was triggered?

[Alex Kavanagh (ajkavanagh)]

Relevant review: https://review.opendev.org/c/openstack/charm-neutron-api-plugin-ovn/+/887258