Hi Julian,

when using configuration management tools like ansible it's nice to have a
method to validate a templated sources.list/.sources file before writing it to
it's final location, e.g.

apt-validate /tmp/tmp.ptsTUMNbEH/kubernetes.sources

All the tool needs to do is return zero when it parses fine, and one when
there's a syntax error.

tools like systemd, borgmatic, sshd, icinga2, etc. have similar options.

This would allow to use following task in ansible:

- name: Template .sources file
  template:
    src:        sources.j2
    dest:       /etc/apt/sources.list.d/{{ name }}.sources
    owner:      root
    group:      root
    mode:       "0644"
    validate:   '/usr/sbin/apt-validate %s'
  notify: apt update

in the case of an error the destination file never gets written, preventing apt
from breaking and requiring manual intervention.

Hope I could explain why it would be nice to have, let me know if you have any
questions.

Greets,
Lee

I'm not sure what that even means, tbh. Our parser is so lax that it's
more or less meaningless; you'll only get errors at update time when
it actually tries to use the sources.