Deprecated conffile change causes release upgrade failure

[Nigel Pegram]

Everything seemed to be going as expected on upgrade of server. Crash report a surprise.

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: exim4-config 4.90.1-1ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-135.161-generic 4.4.140
Uname: Linux 4.4.0-135-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.3
Architecture: amd64
Date: Sun Sep 23 12:24:16 2018
ErrorMessage: installed exim4-config package post-installation script subprocess returned error exit status 1
InstallationDate: Installed on 2015-10-11 (1077 days ago)
InstallationMedia: Ubuntu-Server 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
PackageArchitecture: all
Python3Details: /usr/bin/python3.6, Python 3.6.6, python3-minimal, 3.6.5-3ubuntu1
PythonDetails: /usr/bin/python2.7, Python 2.7.15rc1, python-minimal, 2.7.15~rc1-1
RelatedPackageVersions:
 dpkg 1.19.0.5ubuntu2
 apt 1.6.3ubuntu0.1
SourcePackage: exim4
Title: package exim4-config 4.90.1-1ubuntu1 failed to install/upgrade: installed exim4-config package post-installation script subprocess returned error exit status 1
UpgradeStatus: Upgraded to bionic on 2018-09-23 (0 days ago)

[Nigel Pegram (ndpegram)]

Further investigation showed error was due to deprecated TLS options in settings files.

[Robie Basak (racb)]

Thank you for taking the time to report this bug and helping to make Ubuntu better.

It look like an exim4 maintainer script experienced a failure on release upgrade from Xenial to Bionic following a local configuration customisation in the Xenial timeframe that Bionic's exim4 does not understand, and on declining (or accepting the default or by non-interaction) to update the conffile during upgrade. Is this accurate?

I understand how this can be a problem. I don't think it should break the upgrade like this. Unfortunately it's an instance of the general class of problem for which we don't currently have a good solution. I've heard some developers say that the upgrade is correctly failing because exim4 can't continue and not failing would be falsely suggesting that the upgrade was successful when it was not (specifically that exim4 failed to upgrade and is no longer running).

Since there is a workaround - to update your configuration customisation to fit the latest exim4 and restart the upgrade ("apt -f install, etc) - I'm marking this bug as Medium. I'm also not sure that there's any clear consensus on what to do about it, unfortunately, so please don't expect any movement soon.