[Bug 2067071] Re: cannot upgrade from 23.10 to 24.04, error in _install_t64_replacement_packages

[Nick Rosbrook]

** Changed in: ubuntu-release-upgrader (Ubuntu Noble)
       Status: Triaged => In Progress

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Title:
  cannot upgrade from 23.10 to 24.04, error in
  _install_t64_replacement_packages

Status in ubuntu-release-upgrader package in Ubuntu:
  Invalid
Status in ubuntu-release-upgrader source package in Noble:
  In Progress

Bug description:
  [Impact]

  Upgrades to Noble may crash while searching for t64 replacement
  packages.

  [Test Plan]

  One way to reproduce this is by setting Pin-Priority: -1 for a t64
  replacement package so that `replacement` will have no candidate.

  $ apt install -y libts0
  $ cat > /etc/apt/preferences.d/lp2067071  << EOF
  Package: libts0t64
  Pin: release a=noble
  Pin-Priority: -1
  EOF
  $ do-release-upgrade

  Before the patch, that will lead to the same crash shown in this bug
  report.

  [Where problems could occur]

  The patch is confined to the t64 replacement quirk. Any other issues
  would be there. The fix is to simply make sure the replacement has a
  candidate (i.e. replacement.candidate is not None) before accessing
  more attributes.

  [Original Description]

  it crashes while calculating space - cannot proceed to package
  downloading.

  short from the logs:

  File "/tmp/ubuntu-release-upgrader-4lhpwkx7/DistUpgrade/DistUpgradeCache.py", line 497, in pre_upgrade_rule
      self.quirks.run("PreDistUpgradeCache")
    File "/tmp/ubuntu-release-upgrader-4lhpwkx7/DistUpgrade/DistUpgradeQuirks.py", line 95, in run
      func()
    File "/tmp/ubuntu-release-upgrader-4lhpwkx7/DistUpgrade/DistUpgradeQuirks.py", line 191, in PreDistUpgradeCache
      self._install_t64_replacement_packages()
    File "/tmp/ubuntu-release-upgrader-4lhpwkx7/DistUpgrade/DistUpgradeQuirks.py", line 1845, in _install_t64_replacement_packages
      if replacement.candidate._cand != ver:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  AttributeError: 'NoneType' object has no attribute '_cand'

  full logs attached as files.

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