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

[Brian Murray]

I tested an upgrade from Bionic to Focal with 'do-release-upgrade -p'
using the apt-clone file mentioned in the bug description and an upgrade
was successfully calculated.

ubuntu at grown-crow:~$ head /var/log/dist-upgrade/main.log
2021-05-17 21:02:59,243 INFO Using config files '['./DistUpgrade.cfg.bionic', '/etc/update-manager/release-upgrades.d/ubuntu-advantage-upgrades.cfg']'
2021-05-17 21:02:59,244 INFO uname information: 'Linux grown-crow 5.11.0-17-generic #18-Ubuntu SMP Thu May 6 20:10:11 UTC 2021 x86_64'
2021-05-17 21:02:59,495 INFO apt version: '1.6.13'
2021-05-17 21:02:59,495 INFO python version: '3.6.9 (default, Jan 26 2021, 15:33:00) 
[GCC 8.4.0]'
2021-05-17 21:02:59,498 INFO release-upgrader version '20.04.33' started
2021-05-17 21:02:59,502 INFO locale: 'en_US' 'UTF-8'
2021-05-17 21:02:59,566 WARNING estimate_kernel_initrd_size_in_boot() returned '0' for kernel?
2021-05-17 21:02:59,566 WARNING estimate_kernel_initrd_size_in_boot() returned '0' for initrd?
2021-05-17 21:02:59,579 DEBUG Using 'DistUpgradeViewText' view
ubuntu at grown-crow:~$ grep "Do you want to start" /var/log/dist-upgrade/screenlog.0
Do you want to start the upgrade?

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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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