[Bug 716599] [NEW] apt can't open package cache under certain circumstances

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Binary package hint: apt

The problem in apt I'm reporting can be triggered through "Muon
updater". I originally reported the bug to KDE but the author of Muon
suggested me reporting the problem here, as it was an apt problem.

The following is the original bug report about Muon updater
(https://bugs.kde.org/show_bug.cgi?id=265741):

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Version:           0.2 (using KDE 4.6.0) 
OS:                Linux

Please bear with me as this is a rather difficult bug to explain and
reproduce.

The problem appears when there are pending updates after a repository refresh
(apt-get update), those updates are not applied and, later, Muon updater is
opened (it will show the updated packages), the repositories are updated again
and a newer version of one of the pending packages is found.

My personal theory is that Muon updater still tries to mark the old packages
which are now not present in the repositories, as a newer version of them was
released. Anyway, I don't know Muon internals, so I could be completely wrong.

Reproducible: Always

Steps to Reproduce:
1. Add a frequently update repository to "Software sources" (for example,
Chromium Daily PPA, https://launchpad.net/~chromium-daily/+archive/ppa).
2. Reload the repositories (apt-get update).
3. Hopefully, there will be pending updates. Muon will report them.
4. Don't reload the repositories until there are newer version of those
packages (Chromium Daily should publish new packages every day).
5. Without reloading, open Muon updater. It will show the same old pending
updates.
6. Refresh the repositories from Muon updater (Check for Updates).

Actual Results:  
Muon updater will show an error dialog (the following is an approximate
translation from the Spanish version):

"The package system couldn't be started. Your configuration may be defective.
Details:
A file for package $PACKAGE couldn't be located. This could mean that you will
have to manually fix that package."

("El sistema de paquetes no se ha podido iniciar. Su configuración puede estar
defectuosa.
Detalles:
No se pudo localizar un archivo para el paquete $PACKAGE. Esto puede significar
que necesita arreglar manualmente este paquete.")

Expected Results:  
Muon updater should have suggested the newer version of the package as an
update candidate without showing an error message as the update can be
performed the next time Muon is opened.

OS: Linux (x86_64) release 2.6.35-25-generic
Compiler: cc

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The following is the replly of the author of Muon
(https://bugs.kde.org/show_bug.cgi?id=265741#c1):

This is an issue with APT itself not being able to open the package cache. I
would recommend reporting this bug at Launchpad:
https://bugs.launchpad.net/ubuntu/+source/apt/

** Affects: qapt (Ubuntu)
     Importance: Undecided
         Status: Fix Released

-- 
apt can't open package cache under certain circumstances
https://bugs.launchpad.net/bugs/716599
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