[Bug 2061175] [NEW] Handle (t64) package replacements

[Julian Andres Klode]

Public bug reported:

We should do something like:

for package in cache:
  if package.is_installed and <candidate version is obsolete>:
     replacement = <find a package in the archive that Conflicts/Breaks + Replaces + Provides it>
     replacement.mark_install(auto_fix=False, auto_inst=False)
     replacement.mark_remove(auto_fix=False)

To hint the solver to remove the installed pre-t64 packages and install
packages replacing it. We gotta do some auto_fix, auto_inst False stuff
as we do not want to resolve the dependencies at this point, let the
upgrader work on this.

I do not believe we have to protect our choice to remove the packages
here like we did for gpg-wks-server as the packages are not updatable,
but I can't say I've checked. Potentially maybe just the mark_install
are enough since they will inevitably result in removals anyway, but
this would be easier if the removal sticks around.

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


** Tags: foundations-todo

** Tags added: foundations-todo

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https://bugs.launchpad.net/bugs/2061175

Title:
  Handle (t64) package replacements

Status in ubuntu-release-upgrader package in Ubuntu:
  New

Bug description:
  We should do something like:

  for package in cache:
    if package.is_installed and <candidate version is obsolete>:
       replacement = <find a package in the archive that Conflicts/Breaks + Replaces + Provides it>
       replacement.mark_install(auto_fix=False, auto_inst=False)
       replacement.mark_remove(auto_fix=False)

  To hint the solver to remove the installed pre-t64 packages and
  install packages replacing it. We gotta do some auto_fix, auto_inst
  False stuff as we do not want to resolve the dependencies at this
  point, let the upgrader work on this.

  I do not believe we have to protect our choice to remove the packages
  here like we did for gpg-wks-server as the packages are not updatable,
  but I can't say I've checked. Potentially maybe just the mark_install
  are enough since they will inevitably result in removals anyway, but
  this would be easier if the removal sticks around.

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