Adding a user in postinstall

[Markus <m@tfiu.de>]

Dear Mentors,

While packaging a package for a Virtual Observatory server, I'm
encountering a postinst glitch in the CI that I don't quite
understand.

This is about  git@salsa.debian.org:debian-astro-team/gavodachs.git,
the gavo branch.

The idea is that the package creates a user that has all the
necessary permissions to configure the server, run debug copies, and
the like.  It is called dachsroot, and the postinst creates if if
necessary:

 	# create user "dachsroot" if (s)he is not already there
  if ! getent passwd dachsroot ; then
    adduser --disabled-password --gecos "" --ingroup gavo dachsroot
  fi

This has never resulted in headache over all the years that we have
been distributing the resulting package outside of Debian.  But the
CI on salsa gives:

  Adding user `dachsroot' ...
  Adding new user `dachsroot' (1011) with group `gavo' ...
  Creating home directory `/home/dachsroot' ...
  Stopped: Couldn't create home directory `/home/dachsroot': File exists.
  
  Removing directory `/home/dachsroot' ...
  Removing user `dachsroot' ...

after which the installation duly fails.

Now... I really cannot see where that home directory might come from
(the full log is on
https://salsa.debian.org/debian-astro-team/gavodachs/-/jobs/798533/raw),
which made me doubt whether a package is allowed to create users with
homes in the first place and what I'm seeing here really is some
adduser extra on the CI.

However, at least in 9.2 in the Policy I've not seen anything to that
effect.  Have I missed it there or anywhere else?

And if it's legal to create users like this: Why would the adduser in
the postinst fail?

Thanks,

          Markus