cert_handler lib assumes peer relation named `peers` exists for upgrade_charm event handler

[shayancanonical]

Bug Description

The cert_handler.py charm lib assumes here that a peer relation named peers exists in the charm that uses it. This assumption is not correct in database charms (as it is named database-peers in the mysql charms). This leads to an error in Juju 2.9.X where the peers relation is not found in the upgrade_charm event handler

To Reproduce

1. clone the mysql vm charm
2. git checkout 6817e0a
3. pack charm and deploy it
4. juju refresh mysql --path ./path-to-packed-charm.charm

Environment

ubuntu: 22.04.4 juju: 2.9.46 lxd: 5.21.1-2d13beb

Relevant log output

unit-mysql-1: 19:53:41 ERROR unit.mysql/1.juju-log Uncaught exception while in charm code:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-mysql-1/charm/./src/charm.py", line 898, in <module>
    main(MySQLOperatorCharm)
  File "/var/lib/juju/agents/unit-mysql-1/charm/venv/ops/main.py", line 436, in main
    _emit_charm_event(charm, dispatcher.event_name)
  File "/var/lib/juju/agents/unit-mysql-1/charm/venv/ops/main.py", line 144, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-mysql-1/charm/venv/ops/framework.py", line 351, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-mysql-1/charm/venv/ops/framework.py", line 853, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-mysql-1/charm/venv/ops/framework.py", line 942, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-mysql-1/charm/lib/charms/observability_libs/v1/cert_handler.py", line 352, in _on_upgrade_charm
    has_privkey = self.vault.get_value("private-key")
  File "/var/lib/juju/agents/unit-mysql-1/charm/lib/charms/observability_libs/v1/cert_handler.py", line 255, in get_value
    return self._backend.get_value(key)
  File "/var/lib/juju/agents/unit-mysql-1/charm/lib/charms/observability_libs/v1/cert_handler.py", line 179, in get_value
    return self._read().get(key)
  File "/var/lib/juju/agents/unit-mysql-1/charm/lib/charms/observability_libs/v1/cert_handler.py", line 155, in _read
    value = self._databag.get(self._NEST_UNDER)
  File "/var/lib/juju/agents/unit-mysql-1/charm/lib/charms/observability_libs/v1/cert_handler.py", line 150, in _databag
    self._check_ready()
  File "/var/lib/juju/agents/unit-mysql-1/charm/lib/charms/observability_libs/v1/cert_handler.py", line 136, in _check_ready
    relation = self.charm.model.get_relation(self.relation_name)
  File "/var/lib/juju/agents/unit-mysql-1/charm/venv/ops/model.py", line 237, in get_relation
    return self.relations._get_unique(relation_name, relation_id)
  File "/var/lib/juju/agents/unit-mysql-1/charm/venv/ops/model.py", line 853, in _get_unique
    relations = self[relation_name]
  File "/var/lib/juju/agents/unit-mysql-1/charm/venv/ops/model.py", line 821, in __getitem__
    relation_list: Optional[List[Relation]] = self._data[relation_name]
KeyError: 'peers'

Additional context

No response

[PietroPasotti]

the "peers" relation name is hardcoded in the certhandler, we can fix that easily