Traceback with `AttributeError: 'NoneType' object has no attribute 'data'` from interface_tls_certificates/requires.py

[George Kraft]

On new deploys, the charm logs a traceback:

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-kubeapi-load-balancer-0/charm/venv/charms/reconciler.py", line 34, in reconcile
    result = self.reconcile_function(event)
  File "/var/lib/juju/agents/unit-kubeapi-load-balancer-0/charm/./src/charm.py", line 329, in _reconcile
    self._write_certificates()
  File "/var/lib/juju/agents/unit-kubeapi-load-balancer-0/charm/./src/charm.py", line 381, in _write_certificates
    cert = self.certificates.server_certs_map.get(common_name)
  File "/var/lib/juju/agents/unit-kubeapi-load-balancer-0/charm/venv/ops/interface_tls_certificates/requires.py", line 178, in server_certs_map
    return {cert.common_name: cert for cert in self.server_certs}
  File "/var/lib/juju/agents/unit-kubeapi-load-balancer-0/charm/venv/ops/interface_tls_certificates/requires.py", line 153, in server_certs
    common_name = self.relation.data[self.model.unit].get("common_name")
AttributeError: 'NoneType' object has no attribute 'data'

This happens when the charm is first coming up, before the certificates relation has been established. The charm catches the error and enters Blocked status with the message: "Missing required certificates". The error resolves itself once the certificates relation is established.

[George Kraft (cynerva)]

Also affects kubernetes-control-plane from the `ops` branch. I recommend a fix in interface-tls-certificates to handle it more gracefully when `self.relation` is None.

[Adam Dyess (addyess)]

https://github.com/juju-solutions/interface-tls-certificates/pull/29