Nova clears Neutron port dns_name when shelving/unshelving instance

[Andrew Bonney]

As far as I can tell this is likely to be a Nova issue rather than Neutron, but as it relates to both I can't be certain.

Description
===========
When an instance is first created, the 'dns_name' for its network port(s) reflects the instance name. When the instance is then shelved (and unshelved), this 'dns_name' disappears, resulting in an inability to resolve this VM by name using the tenant network's internal DNS servers.

Steps to reproduce
==================
* Deploy an OpenStack instance including Neutron internal DNS integration as-per https://docs.openstack.org/neutron/latest/admin/config-dns-int.html
* Create an instance with any name and observe the matching DNS name associated with its network port.
Example: openstack server create --flavor tempest1 --image cirros --network public my-instance
* Shelve the instance and re-check the DNS name associated with the network port. Optionally unshelve to observe the issue persists.

Expected result
===============
The DNS name should always match the instance name, or at least should return to the correct value once the instance is unshelved.

Actual result
=============
The DNS name is removed once the instance is shelved, resulting in a generic FQDN like 'host-172-29-249-175.example.org.'

From the command line, after initial instance creation:

root@aio1-utility-container-cd7fbf05:~# openstack port show f6d4723a-af13-40e5-b94a-bba7fb1fb9fb
...
| dns_assignment | fqdn='my-instance.example.org.', hostname='my-instance', ip_address='172.29.249.175' |
| dns_domain | |
| dns_name | my-instance |
...

After a shelve operation (and optionally after an unshelve):

root@aio1-utility-container-cd7fbf05:~# openstack port show f6d4723a-af13-40e5-b94a-bba7fb1fb9fb
...
| dns_assignment | fqdn='host-172-29-249-175.example.org.', hostname='host-172-29-249-175', ip_address='172.29.249.175' |
| dns_domain | |
| dns_name | |
...

Environment
===========
Nova 2023.1 252e660
Neutron 2023.1 29cc1a6
Libvirt+KVM
Ceph Reef
Linux Bridge with VXLAN

Also tested against 2023.2 releases.

[Andrew Bonney (andrewbonney)]

https://review.opendev.org/c/openstack/nova/+/919961