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

[林博仁]

Public bug reported:

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 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

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

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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:
  New

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 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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