hook failed: "install" when deployed to a non-k8s model

[George Kraft]

When deploying the coredns charm to a model backed by a non-k8s cloud (e.g. Azure, vSphere, etc), the charm fails during the install hook due to missing service account credentials.

The real problem here is that the charm doesn't support being deployed in such models, and the charm's metadata is missing the required bits to inform Juju that that is the case.

====== Traceback ======

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-coredns-0/charm/venv/lightkube/config/kubeconfig.py", line 175, in from_service_account
    token = account_dir.joinpath("token").read_text()
  File "/usr/lib/python3.10/pathlib.py", line 1134, in read_text
    with self.open(mode='r', encoding=encoding, errors=errors) as f:
  File "/usr/lib/python3.10/pathlib.py", line 1119, in open
    return self._accessor.open(self, mode, buffering, encoding, errors,
FileNotFoundError: [Errno 2] No such file or directory: '/var/run/secrets/kubernetes.io/serviceaccount/token'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-coredns-0/charm/venv/lightkube/config/kubeconfig.py", line 210, in from_env
    return KubeConfig.from_service_account(service_account=service_account)
  File "/var/lib/juju/agents/unit-coredns-0/charm/venv/lightkube/config/kubeconfig.py", line 178, in from_service_account
    raise exceptions.ConfigError(str(e))
lightkube.core.exceptions.ConfigError: [Errno 2] No such file or directory: '/var/run/secrets/kubernetes.io/serviceaccount/token'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-coredns-0/charm/./src/charm.py", line 182, in <module>
    main(CoreDNSCharm)
  File "/var/lib/juju/agents/unit-coredns-0/charm/venv/ops/main.py", line 429, in main
    charm = charm_class(framework)
  File "/var/lib/juju/agents/unit-coredns-0/charm/./src/charm.py", line 35, in __init__
    self.client = Client(field_manager=self.app.name, namespace=self.model.name)
  File "/var/lib/juju/agents/unit-coredns-0/charm/venv/lightkube/core/client.py", line 43, in __init__
    self._client = GenericSyncClient(config, namespace=namespace, timeout=timeout, lazy=lazy,
  File "/var/lib/juju/agents/unit-coredns-0/charm/venv/lightkube/core/generic_client.py", line 82, in __init__
    config = KubeConfig.from_env().get()
  File "/var/lib/juju/agents/unit-coredns-0/charm/venv/lightkube/config/kubeconfig.py", line 212, in from_env
    return KubeConfig.from_file(os.environ.get('KUBECONFIG', default_config))
  File "/var/lib/juju/agents/unit-coredns-0/charm/venv/lightkube/config/kubeconfig.py", line 142, in from_file
    raise exceptions.ConfigError(f"Configuration file {fname} not found")
lightkube.core.exceptions.ConfigError: Configuration file ~/.kube/config not found

[George Kraft (cynerva)]

PR: https://github.com/charmed-kubernetes/charm-coredns/pull/37

[George Kraft (cynerva)]

Cherry-pick to release_1.28: https://github.com/charmed-kubernetes/charm-coredns/commit/7be6ee5c20ec310e0c783ac253d438dca3589fab

[Adam Dyess (addyess)]

Thanks @cynerva for the cherry-pick. No other backport needed