Doesn't report a useful error when distributions are priority pinned in apt preferences

[Sam Vilain]

Hi, every time I do a dist upgrade I run into this. The update manager knows how to change the package repositories, but it should also check that the repositories it needs end up with a suitable priority as seen in 'apt-cache policy' to allow the upgrade to happen. Otherwise, it fails and it doesn't know why.

This could be important if, say, some installation instructions give advice on how to pull in just a few packages from a newer release without upgrading wholesale by using release priority pinning. The user doing this may not understand what is happening when following such instructions, and then later get a cryptic error. As such, it might be useful for this check to be back-ported to the update mangers going back to one or more LTS releases.

Example /etc/apt/preferences:
# You will need to remove this (or change oneiric to psychedelic or
# whatever Ubuntu 12.04 gets named) when you want to dist-upgrade to
# oneiric; the do-release-upgrade script won't change it for you (or
# know what went wrong)
Explanation: Don't install oneiric packages by default
Package: *
Pin: release a=oneiric
Pin-Priority: 99

Explanation: Don't install natty-proposed packages by default
Package: *
Pin: release a=natty-proposed
Pin-Priority: 98

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: update-manager-core 1:0.150.2
Uname: Linux 3.0.0-rc7+ x86_64
Architecture: amd64
Date: Mon Jul 25 12:09:44 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 10.04 LTS "Lucid Lynx" - Release amd64 (20100429)
ProcEnviron:
 LANGUAGE=en_NZ:en
 PATH=(custom, user)
 LANG=en_NZ.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
UpgradeStatus: Upgraded to natty on 2011-07-23 (1 days ago)

[Robert Roth (evfool)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug #203659 so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Feel free to continue to report any other bugs you may find.
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