Comment 5 for bug 1741723

openstack-dashboard is a bit unique as it needs to have many per-service policy files and they need to be in sync with individual policy files for other charms.

https://docs.openstack.org/horizon/latest/contributor/topics/policy.html
"The implementation in Horizon is based on copies of policy files found in the service’s source code."

Therefore, horizon charm will need to populate per-service policy dirs and this is something only available in Queens for Horizon project itself:

https://blueprints.launchpad.net/horizon/+spec/policy-dirs

So, for horizon, a single extra-policy option will need to have policy file templates for multiple services which makes it quite complex.

Maybe in case of horizon it would be necessary to get policy files via relation data but it is necessary to get 3 types of policy:

1. a render of policy-in-code for a given service (not applicable for older OpenStack releases);
2. policy files embedded into charms;
3. extra-policy configs from individual charms.