Public bug reported:

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

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: rls-oo-incoming

** Tags added: rls-oo-incoming

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https://bugs.launchpad.net/bugs/2078436

Title:
  Reject upgrades if packages are installed in a higher version than the
  target release

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