[Bug 2078436] [NEW] Reject upgrades if packages are installed in a higher version than the target release

[Launchpad Bug Tracker]

You have been subscribed to a public bug by Dan Bungert (dbungert):

PPAs shipping a newer version of key system packages is a significant
problem and leads to long and complex to identify bugs such as

https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/2078376
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/2078418
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/2078415
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/2078391
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/2078442
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/2078445
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/2078454

We should not upgrade systems that have replaced system packages (or
added packages that become core system packages) with PPAs.

My proposal basically entails the following:

* Identify seeded packages as "in main or has the Task field set". This
is a bit lossy as not all seeded packages have the Task field set, but
it is better than nothing. Alternatively we can embed seeds?

* After we have rewritten the sources and disabled third-party
repositories, bail out on upgrades if any of the identified seeded
packages have a higher installed version than is available in the
repository; this can be done by

for package in cache:
  if not package.current_ver: continue
  if not <is seeded>: continue
  cand = get candidate
  if not cand.is_downloadable:
    raise SomeError(f"The seeded package {package.name} is installed from a third-party repository or in an inconsistent state)

(Relying on the properties that (a) if the installed version is higher
than the target release, it is the candidate and (b) if we disable the
third-party repositories, it is no longer downloadable)

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: foundations-todo
-- 
Reject upgrades if packages are installed in a higher version than the target release
https://bugs.launchpad.net/bugs/2078436
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