Comment 0 for bug 1927078

[Description]

Fully numeric names support in Ubuntu is inconsistent in Focal onwards because systemd does not like them[1] but are still allowed by default by useradd, leaving the session behavior in hands of the running applications. Two examples:

1. After creating a user named "0", the user can log in via ssh or console but loginctl won't create a session for it:

root@focal:/home/ubuntu# useradd -m 0
root@focal:/home/ubuntu# id 0
uid=1005(0) gid=1005(0) groups=1005(0)

..

0@192.168.122.6's password:
Welcome to Ubuntu 20.04.2 LTS (GNU/Linux 5.8.0-48-generic x86_64)

Last login: Thu Apr 8 16:17:06 2021 from 192.168.122.1
$ loginctl
No sessions.
$ w
 16:20:09 up 4 min, 1 user, load average: 0.03, 0.14, 0.08
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
0 pts/0 192.168.122.1 16:17 0.00s 0.00s 0.00s w

And pam-systemd shows the following message:

Apr 08 16:17:06 focal sshd[1584]: pam_unix(sshd:session): session opened for user 0 by (uid=0)
Apr 08 16:17:06 focal sshd[1584]: pam_systemd(sshd:session): pam-systemd initializing
Apr 08 16:17:06 focal sshd[1584]: pam_systemd(sshd:session): Failed to get user record: Invalid argument

2. With that same username, every successful authentication in gdm will loop back to gdm again instead of starting gnome, making the user unable to login.

Making useradd fail (unless --badnames is set) when a fully numeric name is used will make the default OS behavior consistent.

[Other info]

- Upstream does not support fully numeric usernames
- useradd has a --badnames parameter that would still allow the use of these type of names