Comment 9 for bug 1806111

Drew, thank you for bringing this problem to our attention.

This is a problem we would see in multiple principal-subordinate plugin type relations, cinder storage backends comes to mind as another area where this might be a problem.

It is particularly visible in the python2 - python3 migration, but I would not be surprised if we hit breaking upgrade changes at other stages in the future too.

What we would like to do is implement general helpers that would allow subordinates to register a map of OpenStack release: install/purge packages pairs with their principal charm (thanks to Liam Young for that idea). With this in place the principal charm can perform the necessary package actions in-place at upgrade time, without having to risk charm relation RPC type processing while in this critical step of an upgrade process.

After the package upgrade the principal will put a notification on the subordinate relation allowing the subordinate charm to perform any necessary post-upgrade tasks.