Thanks William, this LGTM now!

I've modified your debian/changelog and the XML/man pages a bit, to
account for the removed handling of floating point numbers. And uploaded
the package.

$ dput ubuntu ../shadow_4.8.1-1ubuntu9_source.changes
D: Setting host argument.
Checking signature on .changes
gpg: ../shadow_4.8.1-1ubuntu9_source.changes: Valid signature from 
5889C17AB1C8D890
Checking signature on .dsc
gpg: ../shadow_4.8.1-1ubuntu9.dsc: Valid signature from 5889C17AB1C8D890
Uploading to ubuntu (via sftp to upload.ubuntu.com):
  Uploading shadow_4.8.1-1ubuntu9.dsc: done.
  Uploading shadow_4.8.1-1ubuntu9.debian.tar.xz: done.  
  Uploading shadow_4.8.1-1ubuntu9_source.buildinfo: done.
  Uploading shadow_4.8.1-1ubuntu9_source.changes: done.
Successfully uploaded packages.

https://launchpad.net/ubuntu/+source/shadow/1:4.8.1-1ubuntu9

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You received this bug notification because you are a member of Ubuntu
Touch seeded packages, which is subscribed to shadow in Ubuntu.
https://bugs.launchpad.net/bugs/1927078

Title:
  Don't allow useradd to use fully numeric names

Status in shadow package in Ubuntu:
  New
Status in shadow source package in Focal:
  New
Status in shadow source package in Groovy:
  New
Status in shadow source package in Hirsute:
  New
Status in shadow source package in Impish:
  New

Bug description:
  [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

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