Opening a new bug to track the current state of the charm for the purpose of fixing the original issue [1].
Basically when upgrading from an older version of the charm (such as charm store revision 6) that does not include commit [2], which updates the /etc/systemd/system/{}-mysql-router.service file definition and adds the PID file path to the config file, the systemd definition is updated only when upgrade_charm method finishes running, as the template is re-rendered.
After the many recently-landed patches up to [3], what happens now is that the config_changed hook is invoked at [4], before upgrade-charm is run, and then it writes a new config file and attempts to restart at [5], which does not succeed because upgrade-charm hasn't run and the systemd definition was not updated yet.
What needs to happen is that the restart [5] is skipped, so upgrade_charm runs first, then [5] runs. Aside from the PID change, the currently flow is also not correct as [6] makes adjustments to the config and needs to be run before, and it is currently running after [5].
[1] https://bugs.launchpad.net/charm-mysql-router/+bug/1927981
[2] https://github.com/openstack/charm-mysql-router/commit/72aca90ea3551a95ecf0bf301fd626bfb8b25a97
[3] https://review.opendev.org/c/openstack/charm-mysql-router/+/841708
[4] https://github.com/openstack/charm-mysql-router/blob/a49fc1e693f7b53526e9286e4d2d289f7e635d4e/src/reactive/mysql_router_handlers.py#L102
[5] https://github.com/openstack/charm-mysql-router/blob/a49fc1e693f7b53526e9286e4d2d289f7e635d4e/src/lib/charm/openstack/mysql_router.py#L781
[6] https://github.com/openstack/charm-mysql-router/blob/a49fc1e693f7b53526e9286e4d2d289f7e635d4e/src/lib/charm/openstack/mysql_router.py#L337
Fix proposed to branch: master
Review: https:/