[Bug 1826044] Re: Packages unavailable in target release should be removed

[Tom Reynolds]

** Changed in: ubuntu-release-upgrader (Ubuntu)
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1826044

Title:
  Packages unavailable in target release should be removed

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

Bug description:
  Update: This bug report was the result of incomplete thoughts. I will
  replace this post within the next few hours, please hold back bug
  triage until then, or close it and i'll file a new one.

  ---

  
  Some of us had a discussion on what happens to systems running former Ubuntu flavours (which no longer exist) when they are upgraded. It was understood that the upgrade path will lead to (vanilla) Ubuntu. However, packages which used to be installed from the previous flavour, but which are no longer present in this targetted Ubuntu release, will remain installed.

  This could introduce some issues:
  * Since these packages themselves have no upgrade path, they will be subject to vulnerabilities which do not get fixed, and become a security threat (could even be services listening on the network)
  * These packages may interfere with the APT dependency resolver

  Example:
  A system running Mythbuntu 16.04 is to be upgraded. There is no Mythbuntu 18.04, nor are there any other Mythbuntu releases to which there is an upgrade path. The user would likely be presented the LTS upgrade path to Ubuntu 18.04. Some of the packages which existed in Mythbuntu 16.04 but do not exist in Ubuntu 18.04 LTS, will likely remain installed, have no upgrade path, may involve software started automatically or listening on the network, and put the system at risk.

  I think this is a problem the release upgrader needs to solve (I do
  not see how it could be solved anywhere else). Either by recommending
  to purge these packages (which obviously requires detecting them
  beforehand, as ubuntu-support-status is able to do) and prompting
  about it, or by just doing it.

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