[Bug 1185896] Re: apt makes wrong resolver choice when foreign arch repo included

[Wookey]

script log of session demonstarting that it works until the arm64 repo
is added, then it doesn't until multiarch-support:amd64 is explicitly
specified

** Attachment added: "script of session demonstrating problem"
   https://bugs.launchpad.net/ubuntu/+source/apt/+bug/1185896/+attachment/3690715/+files/buglog

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Title:
  apt makes wrong resolver choice when foreign arch repo included

Status in “apt” package in Ubuntu:
  New

Bug description:
  In a fresh raring chroot adding foreign-arch armhf then apt-get installing crossbuild-essential-armhf works fine.
  But if a repo arch-qualified for another arch (arm64) is added, and apt-get update run, installing crossbuild-essential-armhf now fails.

  The reason for this is not clear. Here is a summary of what I've found so far.
  apt-get install crossbuild-essential-armhf
    crossbuild-essential-armhf : Depends: dpkg-cross but it is not going to be installed
  apt-get  install dpkg-cross
    dpkg-cross : Depends: libdebian-dpkgcross-perl (= 2.6.9ubuntu2) but it is not going to be installed
  apt-get  install libdebian-dpkgcross-perl 
    works, but tries to remove: libidn11 libssl1.0.0 libudev0 multiarch-support wget
  (and replace multiarch-support:amd64 with multiarch-support:armhf, which breaks things)

  doing apt-get install crossbuild-essential-armhf  libdebian-dpkgcross-
  perl multiarch-support:amd64 gets apt to do the right thing.

  So the question is why is apt prefering the solution of not installing
  crossbuild-essential-armhf over the correct solution of installing
  crossbuild-essential-armhf  libdebian-dpkgcross-perl multiarch-
  support:amd64

  I did 
  apt-get -oDebug::pkgProblemResolver=1 install crossbuild-essential-armhf
  and got the attached apt-debug file

  I've also attached a script log of the session demonstrating that
  crossbuild-essential-armhf installs as expected unti l the extra arm64
  repo is made available

  I think this can be reproduced by doing
  sbuild-createchroot for raring on amd64 box
  adding this repo to apt config:
  deb [arch=arm64] http://people.debian.org/~wookey/bootstrap/ubunturepo/ raring-bootstrap main universe
  deb-src http://people.debian.org/~wookey/bootstrap/ubunturepo/ raring-bootstrap main universe

  and doing the above tests, as documented in the script log attached

  the problem seems to revolve around apt starting by considering
  multiarch-support:arm64

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