[Bug 1874272] Re: Stage "searching for obsolete software" takes a very long time (30 minutes)

[Julian Andres Klode]

So at the moment this loops over all packages that seem obsolete, and
tries to remove them and then undoes that if it removes other packages,
using one actiongroup per package. This is horrible.

What we should do is use the ProblemResolver's resolve_by_keep something
like this

with an actiongroup:
   problemsolver = apt_pkg.ProblemResolver(depcache)
   for package in obsolete:
       depcache.mark_delete(package)
   for package in not_obsolete:
       depcache.mark_keep(package)
       problemsolver.protect(package)
   problemsolver.resolve_by_keep()

Caveat is that this may also affect any other actions currently queued,
but I think at this point there aren't any others.

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Title:
  Stage "searching for obsolete software" takes a very long time (30
  minutes)

Status in ubuntu-release-upgrader package in Ubuntu:
  Confirmed
Status in ubuntu-release-upgrader source package in Focal:
  Confirmed

Bug description:
  Test Case:
  1. Fresh Bionic installation from 18.04.4 + all updates applied
  2. Run: update-manager -d
  3. Proceed with the upgrade.

  Actual Result:
  Everything goes well but the stage "Searching for obsolete software" takes a very long time.

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: ubuntu-release-upgrader-core 1:20.04.18
  ProcVersionSignature: Ubuntu 5.4.0-26.30-generic 5.4.30
  Uname: Linux 5.4.0-26-generic x86_64
  ApportVersion: 2.20.11-0ubuntu27
  Architecture: amd64
  CasperMD5CheckResult: skip
  CrashDB: ubuntu
  CurrentDesktop: ubuntu:GNOME
  Date: Wed Apr 22 10:34:19 2020
  InstallationDate: Installed on 2020-04-20 (2 days ago)
  InstallationMedia: Ubuntu 18.04.4 LTS "Bionic Beaver" - Release amd64 (20200203.1)
  PackageArchitecture: all
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: ubuntu-release-upgrader
  Symptom: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to focal on 2020-04-22 (0 days ago)
  VarLogDistupgradeTermlog:

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