dismiss_changed can break under certain conditions

[cmars]

I'm developing an opinionated zookeeper charm based on the bigtop charm, but not dependent on bigtop's puppet-based deployment. Newer version of Zookeeper installed from a snap, comes with openjdk installed from ubuntu main archive, puppet replaced with charmhelpers jinja templating.

In testing my charm, I discovered an edge case where the dismiss_changed method on the peer object from this interface can break.

Steps that led to this issue, which might be reproducible:

1. Deploy zookeeper
2. Add 4 units
3. Remove 2 units
4. Leave it running for a while, go get some lunch

I think it might have something to do with the fact that the units were removed, so maybe some of the relation data was missing in a relation broken hook firing.

As a workaround, I'm just going to clear the flag in my zookeeper charm. That's all that the dismiss_changed method seems to do.

2019-03-19 21:58:22 DEBUG update-status Traceback (most recent call last):
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/charm/hooks/update-status", line 22, in <module>
2019-03-19 21:58:22 DEBUG update-status     main()
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 73, in main
2019-03-19 21:58:22 DEBUG update-status     bus.dispatch(restricted=restricted_mode)
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 390, in dispatch
2019-03-19 21:58:22 DEBUG update-status     _invoke(other_handlers)
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 359, in _invoke
2019-03-19 21:58:22 DEBUG update-status     handler.invoke()
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 181, in invoke
2019-03-19 21:58:22 DEBUG update-status     self._action(*args)
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/charm/reactive/zookeeper.py", line 230, in check_cluster_changed
2019-03-19 21:58:22 DEBUG update-status     zkpeer.dismiss_changed()
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/charm/hooks/relations/zookeeper-quorum/peers.py", line 48, in dismiss_changed
2019-03-19 21:58:22 DEBUG update-status     conv.remove_state('{relation_name}.changed')
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/.venv/lib/python3.6/site-packages/charms/reactive/relations.py", line 706, in remove_state
2019-03-19 21:58:22 DEBUG update-status     state = state.format(relation_name=self.relation_name)
2019-03-19 21:58:22 DEBUG update-status   File "/var/lib/juju/agents/unit-zookeeper-4/.venv/lib/python3.6/site-packages/charms/reactive/relations.py", line 577, in relation_name
2019-03-19 21:58:22 DEBUG update-status     return self.namespace.split(':')[0]
2019-03-19 21:58:22 DEBUG update-status AttributeError: 'NoneType' object has no attribute 'split'