Comment 3 for bug 1910276

Further evidence:

$ juju config nova-cloud-controller openstack-origin
cloud:bionic-train

juju config nova-cloud-controller action-managed-upgrade
false

On the nova-cloud-controller unit:

ubuntu@juju-330bd2-zaza-208432b92193-6:~$ apt search nova | grep installed

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

nova-api-os-compute/bionic-updates,now 2:19.3.1-0ubuntu1~cloud0 all [installed]
nova-common/bionic-updates,now 2:19.3.1-0ubuntu1~cloud0 all [installed,automatic]
nova-conductor/bionic-updates,now 2:19.3.1-0ubuntu1~cloud0 all [installed]
nova-placement-api/bionic-updates,now 2:19.3.1-0ubuntu1~cloud0 all [installed]
nova-scheduler/bionic-updates,now 2:19.3.1-0ubuntu1~cloud0 all [installed]
python3-cinderclient/bionic-updates,now 1:4.1.0-0ubuntu1~cloud0 all [installed,automatic]
python3-glanceclient/bionic-updates,now 1:2.16.0-0ubuntu1~cloud0 all [installed,automatic]
python3-neutronclient/bionic-updates,now 1:6.11.0-0ubuntu1~cloud0 all [installed,automatic]
python3-nova/bionic-updates,now 2:19.3.1-0ubuntu1~cloud0 all [installed]
python3-os-client-config/bionic,now 1.29.0-0ubuntu1 all [installed,automatic]
python3-os-win/bionic-updates,now 4.2.0-0ubuntu1~cloud0 all [installed,automatic]

(note that nova is on 19.3.1 and not 20.4.0).

ubuntu@juju-330bd2-zaza-208432b92193-6:/etc/apt/sources.list.d$ cat cloud-archive.list
# Ubuntu Cloud Archive
deb http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/stein main

So basically, the cloud archive has not be updated despite the config being set to bionic-train and action-managed-upgrade not being needed.

Essentially, the charm didn't upgrade to bionic-train.

Triggered the openstack upgrade by triggering a new config-changed (in this case setting debug=true)

The nova-compute units then require their nova-compute systemd's to be restarted with:

juju run --app nova-compute "systemctl restart nova-compute"

After this the cloud came back to normal.