remote unit data is inaccessible in relation-departed

[tonyandrewmeyer]

The docs say:

A unit’s relation settings persist beyond its own departure from the relation
[...]
During a relation-departed hook, relation settings can still be read (with relation-get) [...] All units will still be able to see all other units

However, it's not possible (without using a private attribute) to access remote data from ops.

This pair of example charms demonstrate the issue:

• 'provider-charm' puts some example data into the local unit relation databag on relation-joined and does nothing else.
• 'require-charm' logs that data on relation-changed (just to show it's accessible normally) and attempts to log that data on relation-departed. However, that will fail, unless we get the data with manually creating a RelationDataContent object, which requires using model._backend.

To use the charms:

1. charmcraft pack each charm, and juju deploy them into a model (I used k8s but I believe this will be the case for machine as well)
2. juju integrate the two charms together
3. Wait for everything to settle down (you'll see that the data is in the relation databag from the relation-changed debug-log output)
4. Use juju remove-relation to remove the integration between the applications
5. You should see departed: foo and departed (using private): foo in juju debug-log, but instead will see departed: COULD NOT GET IT. and departed (using private): foo.

The reason for this is that Relation.units is populated by running Juju's relation-list, which doesn't have the departing relation in the relation-departed event. Relation.data is a RelationData object, and the ._data attribute (which __getitem__ exposes) is populated with (a) data for the local unit, (b) data for the local app, and (c) data for each unit in the relation's .units. In (e.g.) relation-changed, this means that the remote unit's data will be there, but in relation-departed it will not.

Juju's relation-get will still provide access to the data, and this is meant to be available (according to the docs linked above and also the Juju team). However, the only method ops provides to use relation-get is via the RelationDataContent class (which needs the backend passed to instantiate an object) or RelationData (which needs the unit to be in relation-list as described above).

It seems like we should make this data available. It seems cleanest for it to be available in Relation.data like it would be outside of relation-departed - the departed unit is available in the environment and in the event object, so there would need to be a bit of work to get it into Relation.data without passing too much context around, but it's certainly feasible.