removing relation results in hook error

[James Page]

We've also noticed that removing the mellon relations puts the charm in error state with this error logged:

2019-06-26 10:56:35 DEBUG worker.uniter.jujuc server.go:182 running hook tool "juju-log"
2019-06-26 10:56:35 INFO juju-log keystone-fid-service-provider:121: Invoking reactive handler: reactive/keystone_saml_mellon_handlers.py:39:publish_sp_fid
2019-06-26 10:56:35 DEBUG worker.uniter.jujuc server.go:182 running hook tool "juju-log"
2019-06-26 10:56:35 ERROR juju-log keystone-fid-service-provider:121: Hook error:
Traceback (most recent call last):
File "/var/lib/juju/agents/unit-keystone-saml-mellon-0/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 73, in main
bus.dispatch(restricted=restricted_mode)
File "/var/lib/juju/agents/unit-keystone-saml-mellon-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-saml-mellon-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 359, in _invoke
handler.invoke()
File "/var/lib/juju/agents/unit-keystone-saml-mellon-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-saml-mellon-0/charm/reactive/keystone_saml_mellon_handlers.py", line 43, in publish_sp_fid
charm_instance.options.remote_id_attribute)
File "/var/lib/juju/agents/unit-keystone-saml-mellon-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-saml-mellon-0/.venv/lib/python3.6/site-packages/charms/reactive/endpoints.py", line 569, in __getitem__
return super().__getitem__(self._translate_key(key))
IndexError: list index out of range

[Alex Kavanagh (ajkavanagh)]

The bug still exists in the charm-interface-keystone-fid-service-provider interface in provides.py at line 78:

    def publish(self, protocol_name, remote_id_attribute):
        # get the first relation object as we only have one relation
        # to a primary as a subordinate
        rel = self.relations[0]
        # can have multiple dashboard charms in general, therefore,
        # all relation objects and all units must be handled
        # to_publish/relation_set work on a per-relation basis
        rel.to_publish['protocol-name'] = protocol_name
        rel.to_publish['remote-id-attribute'] = remote_id_attribute

Easiest fix is just to check that self.relations has at least one element or wrap it in a try: except: