[DPE-4998] Workaround empty event.relation.app

[Gu1nness]

Issue

Some tests randmoly fail with this stacktrace:

unit-shard-0: 03:11:01 ERROR unit.shard/0.juju-log cluster:17: Uncaught exception while in charm code:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-shard-0/charm/./src/charm.py", line 1644, in <module>
    main(MongodbOperatorCharm)
  File "/var/lib/juju/agents/unit-shard-0/charm/venv/ops/main.py", line 551, in main
    manager.run()
  File "/var/lib/juju/agents/unit-shard-0/charm/venv/ops/main.py", line 530, in run
    self._emit()
  File "/var/lib/juju/agents/unit-shard-0/charm/venv/ops/main.py", line 516, in _emit
    self.framework.reemit()
  File "/var/lib/juju/agents/unit-shard-0/charm/venv/ops/framework.py", line 870, in reemit
    self._reemit()
  File "/var/lib/juju/agents/unit-shard-0/charm/venv/ops/framework.py", line 950, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-shard-0/charm/lib/charms/mongodb/v1/shards_interface.py", line 703, in _on_relation_changed
    if not self.pass_hook_checks(event):
  File "/var/lib/juju/agents/unit-shard-0/charm/lib/charms/mongodb/v1/shards_interface.py", line 759, in pass_hook_checks
    mongos_hosts = event.relation.data[event.relation.app].get(HOSTS_KEY)
  File "/var/lib/juju/agents/unit-shard-0/charm/venv/ops/model.py", line 1671, in __getitem__
    return self._data[key]
KeyError: None
unit-shard-0: 03:11:02 ERROR juju.worker.uniter.operation hook "cluster-relation-created" (via hook dispatching script: dispatch) failed: exit status 1

Solution

While this is a bug to be fixed on ops side (see https://github.com/canonical/operator/issues/1306), this is a temporary workaround to be removed when we have a new version of ops.

[Gu1nness]

Mind creating a GitHub issue on the repo so we can remember to remove it later on?

Sure, on my way to do it