Multiarch upgrades to precise

[Colin Watson]

I spent a chunk of today looking into
https://bugs.launchpad.net/bugs/924079, which was an attempted upgrade
of an amd64 system with ia32-libs installed from oneiric to precise.  It
fails because libgcc1:i386 is not marked for unpack sufficiently early
so it is unable to do immediate configuration on the :i386 dependency
chain.  This looks like it will affect all upgrades of amd64 systems
that have any packages installed that need to become
foreign-architecture packages in precise, which is going to be a lot of
desktop systems.

I've determined that it's enough to backport the GSoC work on improved
unpack/configure ordering, which applies unmodified to the version of
apt in oneiric:

  http://paste.ubuntu.com/825133/

The question then seems to be how we can best arrange for oneiric users
(at least those on amd64) to have this when they try to upgrade.  We
could publish a release-upgrader-apt package as we do for lucid, or we
could try to SRU this into oneiric's apt directly.  The former would
only work for users of update-manager (this bug affects 'apt-get
dist-upgrade' too), so I think we should aim for the latter if possible.
Do people think this is plausible?  It would probably be a moderate pain
to validate, although I think it would be doable given the existence of
automatic upgrade tests.

As it stands, this patch breaks apt's ABI.  I would strongly prefer not
to do that in an SRU.  Michael said he thought it might be possible to
do this in a way that doesn't break ABI, which would be great.

If we take the SRU route, we also need to ensure that update-manager
installs the new apt from oneiric-updates before starting the upgrade.
That looks reasonably straightforward, but I mention it for
completeness.

Thanks,

-- 
Colin Watson                                       [cjwatson at ubuntu.com]