[Bug 1791931] Re: Update-manager crashes in _show_transaction due to packages being already removed

[Balint Reczey]

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

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https://bugs.launchpad.net/bugs/1791931

Title:
  Update-manager crashes in _show_transaction due to packages being
  already removed

Status in update-manager package in Ubuntu:
  Fix Released
Status in update-manager source package in Xenial:
  Fix Committed
Status in update-manager source package in Bionic:
  Fix Committed

Bug description:
  [Impact]

   * Update-manager crashes when it tries to remove a package that was
  already removed since update-manager refreshed its cache. A typical
  scenario is when the list of available packages is updated, then
  update-manager pops up to notify the user about the updates. The user
  is not in front of the screen and unattended-upgrades installs the
  security updates and removes unused kernels making the list of updates
  shown to the user outdated. User clicks "Install now" and update-
  manager crashes.

   * The fix obtains a fresh cache and skips package operations that
  would be obsolete and result an exception.

  [Test Case ]

   (with Bionic's package names)
   * Install older kernel packages, downgrade a few packages and mark the kernels autoremovable:
     $ sudo eatmydata apt install -y --allow-downgrades linux-headers-4.15.0-32 linux-headers-4.15.0-32-generic libpoppler73=0.62.0-2ubuntu2 poppler-utils=0.62.0-2ubuntu2 ; sudo /etc/kernel/postinst.d/apt-auto-removal ; sudo apt-mark auto linux-headers-4.15.0-32  linux-headers-4.15.0-32-generic

   * Start update-manager in a different terminal and _don't_ start the
  upgrade

   * Remove kernel packages behind u-m's back:
     $ sudo eatmydata apt remove -y linux-headers-4.15.0-32

   * Start the update on update-manager's GUI and observe it not
  crashing with the fixed version

  [Regression Potential]

   * Too eager filtering could let packages not updated/not removed, but the next run of update-manager would retry updating/removing them.
   * Obtaining a fresh cache takes a ~100 milliseconds and the GUI may freeze for that time. For this short time u-m could show itself busy but IMO it would be more disturbing than the potentially not noticeable freeze.

  [Original bug text]

  The Ubuntu Error Tracker has been receiving reports about a problem regarding update-manager.  This problem was most recently seen with package version 1:18.04.11.4, the problem page at https://errors.ubuntu.com/problem/c4f194c0df19fca32a25eff8c633ad69fe9b5abb contains more details, including versions of packages affected, stacktrace or traceback, and individual crash reports.
  If you do not have access to the Ubuntu Error Tracker and are a software developer, you can request it at http://forms.canonical.com/reports/.

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