Comment 5 for bug 537978

The upstart task sets the system hostname from /etc/hostname (falling back to "localhost" if that file is not readable or is empty and no hostname has yet been set).

The idea here would be honor new_host_name for the same use cases (if the /etc/hostname file is non-empty). That would support both the first case (honor DHCP new_host_name if no hostname=localhost) and the case the debian bug tries to cover (honor further DHCP new_host_name if hostname was previously set by DHCP)

Something like:

if [ -n "$new_host_name" ]; then
  if [ ! -s /etc/hostname ]; then
     hostname "$new_host_name"
  fi
fi