[Bug 2099966] Re: Failure: The 3.0 solver produced a worse result: libcanberra-gtk3-0t64 fails to upgrade

[Julian Andres Klode]

# Problem statement

The problem here is that

* Installed gnome-settings-daemon Depends: libcannbera-gtk3-0
* Installed libcanberra-gtk3-0t64 (=  0.30-10ubuntu10) Provides: libcanberra-gtk3-0
* Upgrade   libcanberra-gtk3-0t64 (=  0.30-17ubuntu2) Depends: libcanberra-gtk3-0


i.e. we renamed libcanberra-gtk3-0 to libcanberra-gtk3-0t64 in noble and then back in plucky.

The solver now resolves that by expanding
  gnome-settings-daemon Depends: libcannbera-gtk3-0
to
  gnome-settings-daemon Depends: libcanberra-gtk3-0t64 (=  0.30-10ubuntu1) | libcannbera-gtk3-0 (= new version)

i.e. it prefers libcanberra-gtk3-0t64 because that is already installed.
But since the new version dropped the Provides, only the old version is
a valid solution.

Now when visiting the dependencies, it sees that dependency, and solves
it by picking the first choice, that is keeping the old version.

# Solutions

There are two solutions to this particular instance:

(1) The upgrade for libcanberra-gtk3-0t64 is Section: olddlibs. We
should not prefer oldlibs packages over other packages, that is, the
dependency should have expanded to libcannbera-gtk3-0 (= new version) |
libcanberra-gtk3-0t64 (=  0.30-10ubuntu1).

(2) The more complex general case is disappeared provides: If the candidate lost the Provides that satisfied the dependency before, we should consider the old version to be an "obsolete Provider", that is, 
libcanberra-gtk3-0t64 is obsolete wrt providing libcannbera-gtk3-0.

I have locally implemented solution (1), but it stands to reason that
(2) should be implemented too. A test case can be constructed for (2) by
taking the test case for (1) and changing the section from oldlibs to
something "not obsolete".

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https://bugs.launchpad.net/bugs/2099966

Title:
  Failure: The 3.0 solver produced a worse result: libcanberra-gtk3-0t64
  fails to upgrade

Status in apt package in Ubuntu:
  In Progress

Bug description:
  Desktop popup saying system problem.

  ProblemType: AptSolver
  DistroRelease: Ubuntu 25.04
  Package: apt 2.9.30ubuntu1
  ProcVersionSignature: Ubuntu 6.12.0-15.15-generic 6.12.11
  Uname: Linux 6.12.0-15-generic x86_64
  ApportVersion: 2.31.0+git20250220-0ubuntu2
  Architecture: amd64
  CasperMD5CheckResult: pass
  Date: Tue Feb 25 14:20:51 2025
  ErrorMessage:
   
  InstallationDate: Installed on 2024-04-03 (328 days ago)
  InstallationMedia: Ubuntu 22.04.4 LTS "Jammy Jellyfish" - Release amd64 (20240220)
  SourcePackage: apt
  Title: Failure: The 3.0 solver produced a worse result
  UpgradeStatus: No upgrade log present (probably fresh install)

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