[Bug 1928397] Re: ensure python is marked for removal when upgrading from 18.04 to 20.04

[Brian Murray]

Additionally, the 7 duplicates of the is bug were able to upgrade using
the version of the release upgrader from -proposed.

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https://bugs.launchpad.net/bugs/1928397

Title:
  ensure python is marked for removal when upgrading from 18.04 to 20.04

Status in ubuntu-release-upgrader package in Ubuntu:
  Invalid
Status in ubuntu-release-upgrader source package in Focal:
  Fix Committed

Bug description:
  There are still a lot of bug reports where users cannot upgrade from
  Ubuntu 18.04 LTS to Ubuntu 20.40 LTS due to python still being kept
  when the problem resolver runs. This is due to a bit of code being
  removed when 20.04.31 was uploaded[1]. That code should be restored as
  it will allow more users to upgrade to Ubuntu 20.04 LTS.

  [Test Cases]
  1) lxc launch ubuntu:18.04
  2) Restore the apt-clone file from LP: #1927268
  3) Run do-release-upgrade
  4) Observe a failure to calculate the upgrade to Focal

  With the version of the release upgrader from -proposed (do-release-
  upgrade -p) the upgrade to Focal will be calculated successfully.

  [Where Problems Could Occur]
  This code previously existed in the release upgrader and just ensures that we are are moving python from the system. However, there could also be typos in the restored code but that would cause a Traceback when running the test case so that would get caught.

  [Other Info]
  That being said it would be good to get more testing of this version of the release upgrader so I'll look at recent bug reports where python is marked for keeping (MarkKeep python:amd64 < 2.7.15~rc1-1 @ii gR > FU=0) and ask those users to test the release upgrader from -proposed.

  [1] http://launchpadlibrarian.net/531097604/ubuntu-release-
  upgrader_1%3A20.04.30_1%3A20.04.31.diff.gz

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