hook "keystone-fid-service-provider-relation-departed" failed when destroying model

[Felipe Reyes]

This is happening when running destroy-model and keysone units left before keystone-kerberos completed all their hooks.

unit-keystone-kerberos-0: 16:54:16 INFO unit.keystone-kerberos/0.juju-log keystone-fid-service-provider:7: Reactive main running for hook keystone-fid-service-provider-relation-departed
unit-keystone-kerberos-0: 16:54:16 INFO unit.keystone-kerberos/0.juju-log keystone-fid-service-provider:7: Invoking reactive handler: reactive/keystone_kerberos_handlers.py:39:publish_sp_fid
unit-keystone-kerberos-0: 16:54:16 ERROR unit.keystone-kerberos/0.juju-log keystone-fid-service-provider:7: Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-keystone-kerberos-0/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-keystone-kerberos-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-keystone-kerberos-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-keystone-kerberos-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-keystone-kerberos-0/charm/reactive/keystone_kerberos_handlers.py", line 43, in publish_sp_fid
    charm_instance.options.kerberos_server)
  File "/var/lib/juju/agents/unit-keystone-kerberos-0/charm/hooks/relations/keystone-fid-service-provider/provides.py", line 78, in publish
    rel = self.relations[0]
  File "/var/lib/juju/agents/unit-keystone-kerberos-0/.venv/lib/python3.6/site-packages/charms/reactive/endpoints.py", line 582, in __getitem__
    return super().__getitem__(self._translate_key(key))
IndexError: list index out of range

unit-keystone-kerberos-0: 16:54:16 ERROR juju.worker.uniter.operation hook "keystone-fid-service-provider-relation-departed" (via explicit, bespoke hook script) failed: exit status 1
unit-keystone-kerberos-0: 16:54:16 INFO juju.worker.uniter awaiting error resolution for "relation-departed" hook

[workaround]

juju resolve --no-retry keystone-kerberos/N # until there no more hooks failing.

[Felipe Reyes (freyes)]

This situation will happen when the principal gets removed before the subordinate:

Unit Workload Agent Machine Public address Ports Message
keystone/0* terminated executing 1 192.168.11.152 5000/tcp (remove)
  keystone-kerberos/0* error idle 192.168.11.152 hook failed: "keystone-fid-service-provider-relation-departed"

[Felipe Reyes (freyes)]

Proposed patch https://review.opendev.org/#/c/751105/