[Bug 1569324] Re: Update Manager shouldn't suggest 14.04 -> 15.10 unsupported upgrade path

[林博仁]

Hi Brian, I believe that the new upgrade behaviour/practice should also
be documented/updated in the [General Upgrade Information
section](https://help.ubuntu.com/community/UpgradeNotes) in the Ubuntu
Wiki?

> To avoid damaging your running system, upgrading should only be done from one release
>  to the next release (e.g. Ubuntu 12.04 to Ubuntu 12.10) or from one LTS release to the 
> next (e.g. Ubuntu 10.04 LTS to Ubuntu 12.04 LTS)

Also I would like to re-confirm that the 14.04 -> 15.10 skip-release
upgrade isn't (well, in best effort) considered to have migration
issues(e.g. the Upstart -> Systemd conversion happens between 15.04 ->
15.10)?  Like I previously mentioned above I did experience problems
while attempting to perform the upgrade on a complete clean install[1].

[1] https://www.youtube.com/watch?v=p6f78HLaHmQ

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https://bugs.launchpad.net/bugs/1569324

Title:
  Update Manager shouldn't suggest 14.04 -> 15.10 unsupported upgrade
  path

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

Bug description:
  As far as I can see Ubuntu only supports 2 release upgrade paths[1]:

      1. ${release} upgrade to ${next-release} (e.g. for 14.04 -> 15.10, you'll have to 14.04 -> 14.10 -> 15.04 -> 15.10, 3 upgrades in a row)
      2. ${lts-release} upgrade to ${next-lts-release} (e.g. for 12.04 -> 14.04, you can do a direct upgrade)

  However currently on a ubuntu 14.04 if user chooses to "show any new
  Ubuntu versions" Update Manager(and do-release-upgrade) will suggest
  to directly upgrade to Ubuntu 15.10, this upgrade path has been
  verified[2] to cause entire system hang during the upgrade process and
  even if you do a hard reboot you'll end in a console-only 15.10
  environment(UPDATE: still fixable with a `# dpkg --configure -a` under
  console).

  Update Manager/do-release-upgrade should instead follow the 14.04 ->
  14.10 -> 15.04 -> 15.10 upgrade path, while using old-
  releases.ubuntu.com[3] software source during the interim EOL'd ubuntu
  release upgrade

  ## References ##
  [1]upgrade - Can I skip over releases when upgrading? - Ask Ubuntu
       http://askubuntu.com/questions/34430/can-i-skip-over-releases-when-upgrading
  [2]Ubuntu 14.04→15.10 upgrade path testing - YouTube
       https://www.youtube.com/watch?v=p6f78HLaHmQ
  [3]EOLUpgrades - Community Help Wiki
        https://help.ubuntu.com/community/EOLUpgrades

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