Comment 0 for bug 1831550

The kubernetes master and worker charm both have an import error bug that impacts charm upgrades.
# contents of reactive/kubernetes_master_worker_base.py

@when_any('kubernetes-master.snaps.installed',
          'kubernetes-worker.snaps.installed')
@when('snap.refresh.set')
@when('leadership.is_leader')
def process_snapd_timer():
    ''' Set the snapd refresh timer on the leader so all cluster members
    (present and future) will refresh near the same time. '''
    # Get the current snapd refresh timer; we know layer-snap has set this
    # when the 'snap.refresh.set' flag is present.
    timer = snap.get(
        snapname='core', key='refresh.timer').decode('utf-8').strip()
    if not timer:
        # The core snap timer is empty. This likely means a subordinate timer
        # reset ours. Try to set it back to a previously leader-set value,
        # falling back to config if needed. Luckily, this should only happen
        # during subordinate install, so this should remain stable afterward.
        timer = leader_get('snapd_refresh') or hookenv.config('snapd_refresh')
        snap.set_refresh_timer(timer)

        # Ensure we have the timer known by snapd (it may differ from config).
        timer = snap.get(
            snapname='core', key='refresh.timer').decode('utf-8').strip()

    # The first time through, data_changed will be true. Subsequent calls
    # should only update leader data if something changed.
    if data_changed('snapd_refresh', timer):
        hookenv.log('setting leader snapd_refresh timer to: {}'.format(timer))
        leadership.leader_set({'snapd_refresh': timer})