[Bug 1924604] Re: do-release-upgrade crashed with KeyError in __getitem__(): "The cache has no package named 'python-is-python2'"

[Brian Murray]

Per the where problems could occur section I also ran a release upgrade
using the GUI view w/o universe disabled and the succeeded.

bdmurray at clean-bionic-amd64:~$ head /var/log/dist-upgrade/main.log 
2021-07-02 12:59:55,150 INFO Using config files '['./DistUpgrade.cfg.bionic', '/etc/update-manager/release-upgrades.d/ubuntu-advantage-upgra
des.cfg']'
2021-07-02 12:59:55,150 INFO uname information: 'Linux clean-bionic-amd64 5.4.0-77-generic #86~18.04.1-Ubuntu SMP Fri Jun 18 01:23:22 UTC 20
21 x86_64'
2021-07-02 12:59:55,540 INFO apt version: '1.6.14'
2021-07-02 12:59:55,541 INFO python version: '3.6.9 (default, Jan 26 2021, 15:33:00) 
[GCC 8.4.0]'
2021-07-02 12:59:55,543 INFO release-upgrader version '20.04.34' started
2021-07-02 12:59:55,598 INFO locale: 'en_US' 'UTF-8'
2021-07-02 12:59:55,798 DEBUG Using 'DistUpgradeViewGtk3' view
2021-07-02 12:59:55,881 DEBUG enable dpkg --force-overwrite
2021-07-02 12:59:55,912 DEBUG creating statefile: '/var/log/dist-upgrade/apt-clone_system_state.tar.gz'
bdmurray at clean-bionic-amd64:~$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.2 LTS"


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

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https://bugs.launchpad.net/bugs/1924604

Title:
  do-release-upgrade crashed with KeyError in __getitem__(): "The cache
  has no package named 'python-is-python2'"

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

Bug description:
  [Impact]
  Users with universe not enabled and python2 packages installed are unable to upgrade from Ubuntu 18.04 to Ubuntu 20.04.

  [Test Plan]
  -----------
  On an Ubuntu 18.04 system with universe and multiverse disabled
  1) apt-get install duplicity (it depends on python2.7)
  2) run do-release-upgrade

  Observe the upgrader hang at Calculating the changes (in text mode)
  kill it and observe a Traceback in /var/log/dist-upgrade/main.log.

  ...
    File "/usr/lib/python3/dist-packages/apt/cache.py", line 303, in __getitem__
      raise KeyError('The cache has no package named %r' % key)
  KeyError: "The cache has no package named 'python-is-python2'"

  With the version of the release-upgrader from -proposed (do-release-
  upgrade -p) you'll instead receive a message regarding universe not
  being enabled and the need to either remove python2 packages or enable
  universe.

  [Where problems could occur]
  The switch to using the finally clause in DistUpgradeCache.py is a significant (albeit safe) change but it is necessary for the error message to actually be displayed. So it'd make sense to run through a whole release upgrade to ensure that the gui behaves normally.

  [Original Description]
  This is because python-minimal was installed and universe was not enabled on the system being upgraded.

  Error Tracker buckets:
  https://errors.ubuntu.com/problem/f92900f3c057db7600d88857e7c05434b69d2496
  https://errors.ubuntu.com/problem/23e6edb38993cb74331d30beeadccdfffe1d557a

  ProblemType: CrashDistroRelease: Ubuntu 20.04
  Package: ubuntu-release-upgrader-core 1:20.04.31
  ProcVersionSignature: Ubuntu 5.4.0-71.79~18.04.1-generic 5.4.101
  Uname: Linux 5.4.0-71-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.23
  Architecture: amd64
  CrashDB: ubuntu
  Date: Thu Apr 15 10:59:53 2021
  ExecutablePath: /usr/bin/do-release-upgrade
  ExecutableTimestamp: 1606763386
  InstallationDate: Installed on 2021-04-13 (1 days ago)
  InstallationMedia: Ubuntu 18.04.5 LTS "Bionic Beaver" - Release amd64 (20200806.1)
  InterpreterPath: /usr/bin/python3.6
  PackageArchitecture: all
  ProcCmdline: /usr/bin/python3 -s ./focal --frontend DistUpgradeViewText
  ProcCwd: /tmp/ubuntu-release-upgrader-e397n4sn
  ProcEnviron:
   TERM=screen
   SHELL=/bin/bash
   LANG=en_US.UTF-8
   LC_CTYPE=en_US.UTF-8
   PATH=(custom, no user)
  Python3Details: /usr/bin/python3.6, Python 3.6.9, python3-minimal, 3.6.7-1~18.04
  PythonArgs: ['/usr/bin/do-release-upgrade', '--frontend', 'DistUpgradeViewText']
  PythonDetails: /usr/bin/python2.7, Python 2.7.17, python-minimal, 2.7.15~rc1-1SourcePackage: ubuntu-release-upgrader
  Title: do-release-upgrade crashed with KeyError in __getitem__(): "The cache has no package named 'python-is-python2'"
  UpgradeStatus: Upgraded to bionic on 2021-04-15 (0 days ago)

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