[Bug 1764172] Re: Unable to change hostname from the one specified during Bionic server installation

[Nathan Stratton Treadway]

When installing Ubuntu Server using Subiquity (i.e. using the
ubuntu-18.04-live-server-amd64.iso installer image), the user is
prompted to enter a hostname for the new installation (along with
username/password info, etc.)

At the very end of the Subquity run, it writes this info into
var/lib/cloud/seed/nocloud-net/user-data within the installation target
filesystem (e.g.

======
root at bionic:/var/lib/cloud# head -6 seed/nocloud-net/user-data 
#cloud-config
growpart: {mode: 'off'}
hostname: bionic
locale: en_US.UTF-8
resize_rootfs: false
users:
======
)

When the newly-installed system is booted for the first time, this file
causes cloud-init finish up the configuration of the new instance --
include setting the new system's hostname to the specified value as part
of that initial boot.

However, cloud-init on Bionic (e.g. 19.2-36-g059d049c-0ubuntu2~18.04.1)
calls the cc_set_hostname module upon every boot (from within the
cmd/main.py:main_init() function, thus overriding the normal "per-
instance" frequency for the module)...

...and because the "hostname" cloud-config parameter is found to be set,
the cc_set_hostname module will always re-write /etc/hostname back to
the originally-entered hostname at that point --  unexpectedly undoing
the user's manual change to the file.

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https://bugs.launchpad.net/bugs/1764172

Title:
  Unable to change hostname from the one specified during Bionic server
  installation

Status in Release Notes for Ubuntu:
  New
Status in hostname package in Ubuntu:
  Confirmed
Status in subiquity package in Ubuntu:
  New

Bug description:
  In Bionic Beta 2 I am not able on a host or in a guest VM to change
  the hostname of a Bionic Beta 2 installation by changing
  /etc/hostname. The name in the file can be changed but it always
  reverts to the hostname configured during installation.

  In the following thread on the topic it is suggested to remove a
  number of cloud packages. When doing this in a VM install I could
  change the hostname again via /etc/hostname.

  https://ubuntuforums.org/showthread.php?t=2389098&page=2

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