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

[Łukasz Zemczak]

Hello Steve, or anyone else affected,

Accepted ubuntu-release-upgrader into bionic-proposed. The package will
build now and be available at https://launchpad.net/ubuntu/+source
/ubuntu-release-upgrader/1:18.04.21 in a few hours, and then in the
-proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested and change the tag from
verification-needed-bionic to verification-done-bionic. If it does not
fix the bug for you, please add a comment stating that, and change the
tag to verification-failed-bionic. In either case, without details of
your testing we will not be able to proceed.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance!

** Tags removed: verification-done verification-done-bionic
** Tags added: verification-needed verification-needed-bionic

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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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