[Bug 1797636] [NEW] unattended do-release-upgrade to cosmic asks about /etc/gdm3/custom.conf

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I'm trying to do some automated testing that involved upgrading a system
from bionic to cosmic, so I need it to not ask for user input.

Before running do-release-upgrade, the system got a fresh dist-upgrade
and reboot.

To avoid interactive responses, I'm using:
$ sudo do-release-upgrade -d -f DistUpgradeViewNonInteractive

Part way through the upgrade, I do get prompted for something though:
Setting up gdm3 (3.30.1-1ubuntu1) ...

Configuration file '/etc/gdm3/custom.conf'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : start a shell to examine the situation
 The default action is to keep your current version.
*** custom.conf (Y/I/N/O/D/Z) [default=N] ?

I also found this in /var/log/dist-upgrade/main.log:
2018-10-12 16:36:44,910 WARNING got a conffile-prompt from dpkg for file: '/etc/gdm3/custom.conf'
2018-10-12 16:36:49,915 ERROR error 'a bytes-like object is required, not 'str'' when trying to write to the conffile

Shouldn't the '-f DistUpgradeViewNonInteractive' have prevented this
from getting asked?

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New

-- 
unattended do-release-upgrade to cosmic asks about /etc/gdm3/custom.conf
https://bugs.launchpad.net/bugs/1797636
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