Upgrade series unit verification race

[Simon Richardson]

The verification logic for an upgrade-series doesn't take into account fully realised units (subordinate relations).

Considering the following:

    $ juju deploy ubuntu
    $ juju deploy nrpe
    # Wait until everything has settled
    $ juju add-relation nrpe ubuntu
    $ juju upgrade-series 0 prepare focal

If you time it correctly, the verification will only take into account the units deployed and not units that want to be deployed.

Essentially the verification code doesn't check if all the machine applications relations aren't fully realised (subordinates) as units. If we're missing units then we should error out asking the user to wait until running/idle state.

As we now have a validation state after a machine lock is created, we can ensure this happens correctly.

[Canonical Juju QA Bot (juju-qa-bot)]

This Medium-priority bug has not been updated in 60 days, so we're marking it Low importance. If you believe this is incorrect, please update the importance.