dovecot-sieve upgrade overwrote configuration

[deb023@respiranto.de]

Hi,

I use unattended-upgrades in order to keep my Debian stretch server up
to date.

This morning, an update for dovecot-sieve was installed among others.
Previously, I had modified /etc/dovecot/conf.d/90-sieve.conf. During the
update, the file was simply overridden by the (probably new) default
configuration file, which I do not consider expected behaviour.

In the log, I got via mail, I see:
  Setting up dovecot-sieve (1:2.2.27-3+deb9u1) ...
  Replacing config file /etc/dovecot/conf.d/90-sieve.conf with new version

The respective file is not part of the package itself, rather it appears
to be installed as specified in
/var/lib/dpkg/info/dovecot-sieve.postinst, particularly by:
  ucf --three-way /usr/share/dovecot/$conffile /etc/dovecot/$conffile
  ucfr dovecot-sieve /etc/dovecot/$conffile
where $conffile equals to `conf.d/90-sieve.conf'.

Apparently, ucf is supposed to present some options to the user, none of
which will obviously be chosen by unattended-upgrades. If the upgrade
was to fail for this reason, unattended-upgrades would be supposed to
run `dpkg --force-confold --configure -a' due to the default setting
`Unattended-Upgrade::AutoFixInterruptedDpkg "true"'.

Does somebody have an explanation or a future-proof solution for the
depicted?

I should probably note, that a 90-sieve.conf.ucf-dist appears to have
been in place already before the recent upgrade.

I somewhere found the idea to add
  Dpkg::Options {
    "--force-confdef";
    "--force-confold";
  }
to 50-unattended-upgrades, however I doubt it applies to files not
listed as conffiles.


Greetings,
 respiranto