[Bug 1775660] Re: ubuntu-release-upgrader should purge obsolete packages, not just remove them

[Brian Murray]

I performed an upgrade of an Ubuntu 16.04 desktop to Ubuntu 18.04 today
and ran "dpkg -l | grep '^rc'" after purging packages and captured the
difference.

 $ diff --new-line-format="" --unchanged-line-format="" xenial-nopurge.txt xenial-purge.txt
rc  gnome-user-share                           3.18.3-3                                   amd64        User level public file sharing via WebDAV
rc  initscripts                                2.88dsf-59.3ubuntu2                        amd64        scripts for initializing and shutting down the system
rc  insserv                                    1.14.0-5ubuntu3                            amd64        boot sequence organizer using LSB init.d script dependency information
rc  libhybris                                  0.1.0+git20151016+6d424c9-0ubuntu7         amd64        Allows to run bionic-based HW adaptations in glibc systems - libs
rc  mountall                                   2.54ubuntu1                                amd64        filesystem mounting tool
rc  signond                                    8.59+17.10.20170606-0ubuntu1               amd64        Single Sign On framework
rc  snap-confine                               2.32.9+18.04                               amd64        Transitional package for snapd
rc  suru-icon-theme                            16.10+18.04.20180421.1-0ubuntu1            all          Ubuntu Suru Icon theme
rc  sysv-rc                                    2.88dsf-59.3ubuntu2                        all          System-V-like runlevel change mechanism
[  1:27PM 10767 ]  [ bdmurray at impulse:~/tmp/bug-1775660 ]
 $ wc -l xenial-nopurge.txt
51 xenial-nopurge.txt
[  1:29PM 10768 ]  [ bdmurray at impulse:~/tmp/bug-1775660 ]
 $ wc -l xenial-purge.txt
42 xenial-purge.txt

So this looks good to me for an upgrade from Ubuntu 16.04.

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https://bugs.launchpad.net/bugs/1775660

Title:
  ubuntu-release-upgrader should purge obsolete packages, not just
  remove them

Status in ubuntu-release-upgrader package in Ubuntu:
  Fix Released
Status in ubuntu-release-upgrader source package in Bionic:
  Fix Committed

Bug description:
  [Justification]
  When an obsolete package leaves conffiles behind, these conffiles can sometimes interact poorly with the system and cause bugs.  Since the obsoleted package no longer exists in the targeted release, this can be difficult to resolve by applying fixes to a package.  Furthermore, removing-but-not-purging packages on upgrade means that a system which is upgraded will as a rule be inconsistent with a newly-installed system, which should always be considered a bug.

  Wherever possible, we should strive to make an upgraded system look
  exactly like a newly installed system, to minimize the combinatorics
  of configurations that must be supported.

  Examples of cases where this has been a problem in practice:

  LP: #1770965
  LP: #1749199

  [Test case]
  1. Do an upgrade from xenial with the release ubuntu-release-upgrader.  Accept the option to remove obsolete packages.
  2. Capture the list of packages that are in state 'rc' afterwards.
  3. Do an equivalent upgrade with the -proposed ubuntu-release-upgrader, including removal of obsolete packages.
  4. Confirm that the upgrade completes successfully.
  5. Capture the list of packages that are in state 'rc' afterwards.
  6. Confirm that there are fewer packages in state 'rc' in the second upgrade than the first.
  7. Repeat all of the above with an upgrade from artful.

  [Regression potential]
  As a consequence of this, we will also be running a lot more postrm scripts with the 'purge' option on users desktops, so any bugs in those maintainer scripts will now be apparent as part of a release upgrade where before they were latent.

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