policyd-override is not updated after the attach-resource (due to charm not implementing upgrade-charm hook)

[Giuseppe Petralia]

neutron-api units have already a policyd-override in place and I want to update it with a new one.
I create a zip file containing the new yaml and attach it with:

$ juju attach-resource neutron-api policyd-override=policyd-override.zip

But I can't see the new policy in the /etc/neutron/policy.d

If i list resources I can see that the units are not fetching the new revision of the resource from the controller and the revision they are using is not equal to the expected one:

$ juju resources neutron-api --details
Unit Resource Revision Expected
neutron-api/0 policyd-override 2021-11-02T11:01 2021-15-02T12:42
neutron-api/1 policyd-override 2021-11-02T11:01 2021-15-02T12:42
neutron-api/2 policyd-override 2021-11-02T11:01 2021-15-02T12:42

In the units logs I can see:
2021-02-15 12:48:21 INFO juju.worker.uniter resolver.go:147 found queued "upgrade-charm" hook
2021-02-15 12:48:22 INFO juju.worker.uniter.operation runhook.go:145 skipped "upgrade-charm" hook (missing)

Workaround
============
$ juju run -a neutron-api resource-get policyd-override

After that the revision is equal to the expected:

$ juju resources neutron-api --details
Unit Resource Revision Expected
neutron-api/0 policyd-override 2021-15-02T12:42 2021-15-02T12:42
neutron-api/1 policyd-override 2021-15-02T12:42 2021-15-02T12:42
neutron-api/2 policyd-override 2021-15-02T12:42 2021-15-02T12:42

and then I need to disable (false) and re-enable (true) policyd-override with:

$ juju config neutron-api use-policyd-override=false
$ juju config neutron-api use-policyd-override=true

Versions
===========
1. charm-neutron-api rev. 287
2. juju v2.8.7
3. cloud:bionic-ussuri

[Alex Kavanagh (ajkavanagh)]

The reason the policy-override is not working is due to the charm not having an upgrade-charm hook, which was a bit of a surprise.

Fix: add upgrade-charm hook to minimally process the policyd using `maybe_do_policyd_overrides()` helper function in charm-helpers.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-neutron-api/+/790372

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-neutron-api/+/790372
Committed: https://opendev.org/openstack/charm-neutron-api/commit/2c6e8255d12d0ba4a59c461ca45bc344cadc0379
Submitter: "Zuul (22348)"
Branch: master

commit 2c6e8255d12d0ba4a59c461ca45bc344cadc0379
Author: Cornellius Metto <email address hidden>
Date: Mon May 10 09:06:48 2021 +0300

    Add upgrade-charm symlink in hooks directory

    Closes-Bug: #1915724
    Change-Id: I8ca9e0c49d789921b2782ab95d0fc22483c09653