ams get hook failed: "registry-client-relation-changed" when deploying with aar

[Shunde Zhang]

A user deploys ams and aar in one juju deploy command, which means juju will deploy them at the same time. But ams runs into a hook error. aar is deployed properly though.

App Version Status Scale Charm Channel Rev Exposed Message
aar 1.25.1-0fe31... active 1 aar 1.25/stable 952 no
ams 1.25.1-0fe31... error 1 ams 1.25/edge 994 no hook failed: "registry-client-relation-changed"

aar/0* active idle 4 98.83.118.9 3000/tcp
ams/0* error idle 2 44.205.6.129 hook failed: "registry-client-relation-changed"

In ams juju log, it shows:

2025-04-07 00:58:35 INFO juju.worker.uniter resolver.go:180 awaiting error resolution for "relation-changed" hook
2025-04-07 00:58:35 WARNING unit.ams/0.juju-log server.go:326 registry-client:26: <class '__main__.AmsOperatorCharm'>.tracing_cert is None; sending traces over INSECURE connection.
2025-04-07 00:58:35 ERROR unit.ams/0.juju-log server.go:326 registry-client:26: Waiting for ams to start to share certificate
2025-04-07 00:58:35 ERROR unit.ams/0.juju-log server.go:326 registry-client:26: AMS not yet started
2025-04-07 00:58:35 ERROR unit.ams/0.juju-log server.go:326 registry-client:26: Uncaught exception while in charm code:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-ams-0/charm/./src/charm.py", line 682, in <module>
    main(AmsOperatorCharm)
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/ops/__init__.py", line 343, in __call__
    return _main.main(charm_class=charm_class, use_juju_for_storage=use_juju_for_storage)
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/ops/_main.py", line 543, in main
    manager.run()
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/ops/_main.py", line 529, in run
    self._emit()
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/ops/_main.py", line 518, in _emit
    _emit_charm_event(self.charm, self.dispatcher.event_name, self._juju_context)
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/ops/_main.py", line 134, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/ops/framework.py", line 347, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/ops/framework.py", line 857, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/ops/framework.py", line 947, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-ams-0/charm/lib/charms/tempo_coordinator_k8s/v0/charm_tracing.py", line 1064, in wrapped_function
    return callable(*args, **kwargs) # type: ignore
  File "/var/lib/juju/agents/unit-ams-0/charm/./src/charm.py", line 416, in _on_registry_client_changed
    self.ams.setup_aar(
  File "/var/lib/juju/agents/unit-ams-0/charm/src/ams.py", line 268, in setup_aar
    self.register_client(certificate)
  File "/var/lib/juju/agents/unit-ams-0/charm/src/ams.py", line 327, in register_client
    current_certs = self.get_registered_certificates()
  File "/var/lib/juju/agents/unit-ams-0/charm/src/ams.py", line 320, in get_registered_certificates
    return json.loads(result.stdout.decode())
  File "/usr/lib/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
2025-04-07 00:58:36 ERROR juju.worker.uniter.operation runhook.go:180 hook "registry-client-relation-changed" (via hook dispatching script: dispatch) failed: exit status 1
2025-04-07 00:58:36 INFO juju.worker.uniter resolver.go:180 awaiting error resolution for "relation-changed" hook

It looks the charm wants to get a token for aar but ams is not running yet. [1]
then set the unit state to error which stops ams configuration.
Probably should check if ams is running in [1] before attempting to configure aar.

A workaround is to deploy aar and ams without a relation between them. Then add the relation after they are up and running.

[1] https://github.com/canonical/anbox-cloud-charms/blob/stable-1.25/charms/ams/src/charm.py#L393