Configuring APT

[Stefan Baums <baums@u.washington.edu>]

Dear list,

I would like to configure APT to do the following:

   - install requested and update installed packages from
     repository X (unstable or third-party) for as long as those
     packages are not available in repository Y (testing or
     stable)

   - automatically switch to updating those packages from
     repository Y instead of X as soon as they become available in
     Y, even if that requires a downgrade

Currently, my /etc/apt/preferences file contains

   Package: *
   Pin: release a=testing
   Pin-Priority: 900

   Package: *
   Pin: release a=stable
   Pin-Priority: 800

   Package: *
   Pin: release a=unstable
   Pin-Priority: 700

This does not do what I want because it will keep installing
packages from unstable once a version from unstable is on my
system, even after a testing version of that package becomes
available.

I do not have an /etc/apt/apt.conf file.  There is an
/etc/apt/apt.conf.d/70debconf file which contains

   // Pre-configure all packages with debconf before they are installed.
   // If you don't like it, comment it out.
   DPkg::Pre-Install-Pkgs {"/usr/sbin/dpkg-preconfigure --apt || true";};

Does anybody know how to do what I want?

Thanks in advance,
Stefan

-- 
Stefan Baums
Asian Languages and Literature
University of Washington