How come my hostname isn't set to the FQDN with cloud-init when using `prefer_fqdn_over_hostname`?

prefer_fqdn_over_hostname is documented as,

If both fqdn and hostname are set, it is distro dependent whether hostname or fqdn is used, unless the prefer_fqdn_over_hostname option is true and fqdn is set it will force the use of FQDN in all distros, and if false then it will force the hostname use.

In my cloud-init.cfg I have,

hostname: "host-10-2-65-89"
fqdn: "host-10-2-65-89.openstack.build."
prefer_fqdn_over_hostname: true

But when I run this my hostname is set to host-10-2-65-89. Here is the log, and you can see it explicitly says host-10-2-65-89.openstack.build.

249 - cc_set_hostname.py[DEBUG]: Setting the hostname to host-10-2-65-89.openstack.build. (host-10-2-65-89)
250 - util.py[DEBUG]: Reading from /etc/hostname (quiet=False)
250 - util.py[DEBUG]: Read 7 bytes from /etc/hostname
250 - util.py[DEBUG]: Writing to /etc/hostname - wb: [644] 16 bytes
250 - __init__.py[DEBUG]: Non-persistently setting the system hostname to host-10-2-65-89
250 - subp.py[DEBUG]: Running command ['hostname', 'host-10-2-65-89'] with allowed return codes [0] (shell=False, capture=True)
256 - atomic_helper.py[DEBUG]: Atomically writing to file /var/lib/cloud/data/set-hostname (via temporary file /var/lib/cloud/data/tmpaxqv667c) - w: [644] 80 bytes/chars

The contents of /var/lib/cloud/data/set-hostname
{
"fqdn": "host-10-2-65-89.openstack.build.",
"hostname": "host-10-2-65-89"
}


The contents of /var/lib/cloud/data/previous-hostname

host-10-2-65-89.openstack.build.

I'm using Cloud-init v. 21.4-0ubuntu1~20.04.1

I've tried deleting all but the FQDN and it still doesn't set the /etc/hostname to the fqdn.

This is a bug in cloud-init, I've filed it upstream as Bug #1966533

I was getting silent failures because the hostname was

host-10-2-65-89.openstack.build.

Instead of

host-10-2-65-89.openstack.build

What actually happens is,
$ sudo hostname host-10-2-65-89.openstack.build.
hostname: the specified hostname is invalid


Background, what I was actually doing was using Terraform,

fqdn: "${fqdn}"

And then setting the fqdn like this,

fqdn = openstack_networking_port_v2.port_company.dns_assignment[0].fqdn

But the problem is the terraform was return the actual FQDN,

host-10-2-65-89.openstack.build.

But you can't provide that to hostname. My solution was to remove the dot (.) with trimsuffix

fqdn = trimsuffix(
  openstack_networking_port_v2.port_company.dns_assignment[0].fqdn,
  "."
)

StackExchange DevOps Q#15678, answer score: 3