dashboards-relation-changed failed, if remove_unit is None

[Robert Gildein]

When `dashboards-relation-{joined,changed}` hook is triggered and the
remote_unit is None, this hook fails and the unit enters an error state.

The solution would be to move this condition [1] to this line [2].

---
[1]: https://git.launchpad.net/charm-grafana/tree/src/reactive/grafana.py#n1282
[2]: https://git.launchpad.net/charm-grafana/tree/src/reactive/grafana.py#n1277

[Robert Gildein (rgildein)]

This error was noticed when I tried to update the relation data via
`event.relation.data[self.app].update(...)` in the charm based on the
operator framework.

I think it makes more sense when data is part of an application
relation data, which is not supported now. Solution for that will
be using `relation_id=None` if remote_unit is None.

[Alvaro Uria (aluria)]

The charm runs:
"""
remote_unit = os.environ.get("JUJU_REMOTE_UNIT", None)
relation_id = hookenv.relation_id("dashboards", remote_unit)
"""

However, an envvar exists with the relation_id [1]. Since this handler is only run when the dashboards-relation-{joined,changed} are triggered, I assume the relation ids are of type "dashboards:<ID>", which is the value that would be returned by "hookenv.relation_id".

It is still unclear to me what are the envvars available when "event.relation.data[self.app].update" is run at the other side of the relation, and how charm-grafana should react to it. Can "hookenv.relation_get" be run when data is shared at the application level rather than at the unit level? Please clarify.

1. https://juju.is/docs/olm/environment-variables#heading--juju_relation_id

[Robert Gildein (rgildein)]

I will describe this with an example that is able to accept the use of
`self.unit` as well as` self.app`.

Here is how I modified the `dashboards_joined` [1] function:
```python
@hook("dashboards-relation-joined", "dashboards-relation-changed")
def dashboards_joined(dashboards):
    """Process dashboards from related charms."""
    hookenv.log("hook %s triggered" % hookenv.hook_name(), level="CRITICAL")

    remote_unit = os.environ.get("JUJU_REMOTE_UNIT", None)
    hookenv.log("remote_unit: `%s`" % remote_unit, level="CRITICAL")
    hookenv.log("environ: %s" % {key: value for key, value in os.environ.items() if key.startswith("JUJU_REMOTE")}, level="CRITICAL")

    if not remote_unit:
        hookenv.log("remote_unit is None, return", "WARNING")
        relation_id = None
    else:
        relation_id = hookenv.relation_id("dashboards", remote_unit)
        hookenv.log(
            "remote_unit {}, relation_id {}".format(remote_unit, relation_id), "INFO"
        )
```

And here's an example when I used it
`event.relation.data[self.unit].update(...)`:

```bash
unit-grafana-0: 13:09:11 CRITICAL unit.grafana/0.juju-log dashboards:67: hook dashboards-relation-joined triggered
unit-grafana-0: 13:09:11 CRITICAL unit.grafana/0.juju-log dashboards:67: remote_unit: `prometheus-bind-exporter/1`
unit-grafana-0: 13:09:11 CRITICAL unit.grafana/0.juju-log dashboards:67: environ: {'JUJU_REMOTE_UNIT': 'prometheus-bind-exporter/1', 'JUJU_REMOTE_APP': 'prometheus-bind-exporter'}
unit-grafana-0: 13:09:13 CRITICAL unit.grafana/0.juju-log dashboards:67: hook dashboards-relation-changed triggered
unit-grafana-0: 13:09:13 CRITICAL unit.grafana/0.juju-log dashboards:67: remote_unit: `prometheus-bind-exporter/1`
unit-grafana-0: 13:09:13 CRITICAL unit.grafana/0.juju-log dashboards:67: environ: {'JUJU_REMOTE_UNIT': 'prometheus-bind-exporter/1', 'JUJU_REMOTE_APP': 'prometheus-bind-exporter'}
```

And here's an example when I used it
`event.relation.data[self.app].update(...)`:

```bash
unit-grafana-0: 13:13:15 CRITICAL unit.grafana/0.juju-log dashboards:68: hook dashboards-relation-joined triggered
unit-grafana-0: 13:13:15 CRITICAL unit.grafana/0.juju-log dashboards:68: remote_unit: `prometheus-bind-exporter/1`
unit-grafana-0: 13:13:15 CRITICAL unit.grafana/0.juju-log dashboards:68: environ: {'JUJU_REMOTE_UNIT': 'prometheus-bind-exporter/1', 'JUJU_REMOTE_APP': 'prometheus-bind-exporter'}
unit-grafana-0: 13:13:17 CRITICAL unit.grafana/0.juju-log dashboards:68: hook dashboards-relation-changed triggered
unit-grafana-0: 13:13:17 CRITICAL unit.grafana/0.juju-log dashboards:68: remote_unit: `prometheus-bind-exporter/1`
unit-grafana-0: 13:13:17 CRITICAL unit.grafana/0.juju-log dashboards:68: environ: {'JUJU_REMOTE_UNIT': 'prometheus-bind-exporter/1', 'JUJU_REMOTE_APP': 'prometheus-bind-exporter'}
unit-grafana-0: 13:13:19 CRITICAL unit.grafana/0.juju-log dashboards:68: hook dashboards-relation-changed triggered
unit-grafana-0: 13:13:19 CRITICAL unit.grafana/0.juju-log dashboards:68: remote_unit: ``
unit-grafana-0: 13:13:19 CRITICAL unit.grafana/0.juju-log dashboards:68: environ: {'JUJU_REMOTE_UNIT': '', 'JUJU_REMOTE_APP': 'prometheus-bind-exporter'}
```

As you can see, whe...