During minor update from RHOSP 17.1.4 to 17.1.9, the update of Compute nodes fails with the following error on ALL Compute nodes:

FATAL | Create containers managed by Podman for /var/lib/tripleo-config/container-puppet-config/step_1 | host0002-cdm | error={"changed": false, "msg": "Failed containers: container-puppet-nova_libvirt"}

On the host i can see that the container container-puppet-nova_libvirt is in Exited (1) status, and in the logs i can see the following error:

<13>Sep 22 16:38:37 puppet-user: Error: Evaluation Error: Error while evaluating a Function Call, 'join' parameter 'arg' expects an Array value, got String (file: /etc/puppet/modules/nova/manifests/compute/libvirt.pp, line: 671, column: 43) on node

host0001-cdm.lab-1.sddc.bankit.it

+ rc=1
+ '[' false = false ']'
+ set +x

This cluster comes from an ffu of version 16.2, but it has already undergone a minor update in the past, and the templates have not been modified since then.

In the puppet module /etc/puppet/modules/nova/manifests/compute/libvirt.pp i find the following:

if size($disk_cachemodes_real) > 0 {
    nova_config {
      'libvirt/disk_cachemodes': value => join($disk_cachemodes_real, ',');
    }
  } else {
    nova_config {
      'libvirt/disk_cachemodes': ensure => absent;
    }
  }
}