[Bug 2077358] Re: upgrades fail when package in ForcedObsoletes does not exist

[Nick Rosbrook]

I verified the fix using ubuntu-release-upgrader 1:24.04.22 from noble-
proposed:

nr at clean-jammy-amd64:~$ do-release-upgrade --proposed

[...SNIP...]

System upgrade is complete.

Restart required

To finish the upgrade, a restart is required.
If you select 'y' the system will be restarted.

Continue [yN] N
nr at clean-jammy-amd64:~$ grep forced_obsoletes /var/log/dist-upgrade/main.log
2024-08-22 13:00:06,173 DEBUG forced_obsoletes: ['desktop-effects', 'gnome-app-install', 'policykit-gnome', 'gnome-mount']

The non-existent packages in force_obsoletes did not cause a crash.


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

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https://bugs.launchpad.net/bugs/2077358

Title:
  upgrades fail when package in ForcedObsoletes does not exist

Status in ubuntu-release-upgrader package in Ubuntu:
  Triaged
Status in ubuntu-release-upgrader source package in Noble:
  Fix Committed

Bug description:
  [Impact]

  With ubuntu-release-upgrader in noble-proposed, I get the following
  error when upgrading Ubuntu Desktop:

  2024-08-19 14:48:13,087 ERROR not handled exception:
  Traceback (most recent call last):

    File "/usr/lib/python3/dist-packages/apt/cache.py", line 281, in __getitem__
      # It might be excluded due to not having a version or something

  KeyError: 'gnome-app-install'

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):

    File "/home/nr/24.04.22/./noble", line 8, in <module>
      sys.exit(main())

    File "/home/nr/24.04.22/DistUpgrade/DistUpgradeMain.py", line 241, in main
      if app.run():

    File "/home/nr/24.04.22/DistUpgrade/DistUpgradeController.py", line 2639, in run
      return self.fullUpgrade()

    File "/home/nr/24.04.22/DistUpgrade/DistUpgradeController.py", line 2615, in fullUpgrade
      self.doPostUpgrade()

    File "/home/nr/24.04.22/DistUpgrade/DistUpgradeController.py", line 2033, in doPostUpgrade
      if not self.cache[pkgname].marked_delete:

    File "/usr/lib/python3/dist-packages/apt/cache.py", line 283, in __getitem__
      raise KeyError("The cache has no package named %r" % key)

  KeyError: "The cache has no package named 'gnome-app-install'"

  This is because the new forced obsoletes code does not handle key
  errors with the cache.

  [Test Plan]

  The simplest way to test this is doing an upgrade of Ubuntu Desktop,
  because the DistUpgrade.cfg has a ForceObsoletes= section for `ubuntu-
  desktop` that contains at least one package that no longer exists in
  the archive.

  $ do-release-upgrade -d

  The upgrade should succeed past the "removing obsoletes" section.

  [Where problems could occur]

  The patch here is to check for the package name as a key in the cache
  before attempting to access it. Any more problems would be contained
  in the "searching/removing obsoletes" logic.

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