[Bug 2025462] Re: Apt deletes ubuntu-desktop during dist-upgrade

[Julian Andres Klode]

The regression fix causes an issue whereby any new dependencies or
recommends of updates that are phasing will be installed, we're
currently investigating how to solve this, I believe this is also a
problem in the apt upgrade code path (which can install new packages),
but not in apt-get upgrade.

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Title:
  Apt deletes ubuntu-desktop during dist-upgrade

Status in apt package in Ubuntu:
  In Progress
Status in apt source package in Jammy:
  New
Status in apt source package in Kinetic:
  New
Status in apt source package in Lunar:
  New
Status in apt source package in Mantic:
  In Progress

Bug description:
  [Impact]
  gnome-shell gets removed on upgrades of gnome-shell and mutter if due to phasing we can only upgrade src:gnome-shell.

  [Test plan]
  This adds a minimal test case to the test suite that reproduced the issue and verifies the fix, test plan consists of the autopkgtests.

  [Where problems could occur]
  We could see a resurgence of bugs like LP#1990586 where the solver failed to run at all because things got too complex, however this is a bit more unlikely as we now use the by-keep resolver to handle rolling back phased updates.

  [Other Info]
  gnome-shell Depends: gnome-shell-common (= ${binary-Version), mutter (>= matching)

  So when mutter cannot be updated due to phasing, gnome-shell becomes
  non-installable, but gnome-shell-common can be updated, so APT decided
  to remove gnome-shell and the meta packages in its infinite wisdom.

  The fix addresses this by resolving the dist-upgrade as if there were
  no phasing, and then retroactively marks phases for keep and then
  anything broken by that for keep.

  This required some restructuring because normally we'd also keep
  broken Recommends back, but here dist-upgrade may have decided that
  was ok, so we need to build an allowlist of where Recommends can be
  broken to avoid undoing unrelated changes.

  [Original bug report]

  This morning I got surprised by my laptop booting to a tty instead of
  a desktop environment. It turned out that the entire desktop
  environment was no longer present on my machine. Doing an apt install
  ubuntu-desktop-minimal resolved the issue.

  The machine had been running for a while. Looking at the apt logs, it
  looks like apt deleted ubuntu-desktop on its own during a dist-upgrade
  a couple of weeks back.

  Start-Date: 2023-06-08  14:20:46
  Commandline: apt dist-upgrade
  Requested-By: alex (1000)
  Upgrade: gnome-shell-common:amd64 (44.0-2ubuntu3, 44.1-0ubuntu1)
  Remove: gnome-shell:amd64 (44.0-2ubuntu3), ubuntu-desktop:amd64 (1.501), gdm3:amd64 (44.0-1ubuntu1), gnome-shell-extension-desktop-icons-ng:amd64 (46+really47.0.2-3), gnome-shell-extension-appindicator:amd64 (53-1), ubuntu-session:amd64 (44.0-1ubuntu1), gnome-shell-extension-manager:amd64 (0.4.0-1), gnome-shell-extension-ubuntu-dock:amd64 (79ubuntu2.23.04.1), ubuntu-desktop-minimal:amd64 (1.501)
  End-Date: 2023-06-08  14:20:48

  I'm using the following version of Ubuntu:
  Distributor ID:	Ubuntu
  Description:	Ubuntu 23.04
  Release:	23.04
  Codename:	lunar

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