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

[David Kalnischkies]

[@wookey: Your last attachment is the same as the first one – I at least
see nothing working in it ;) ]

mhh, I had a little stare-down contest with the resolver in the train now and it looks like the resolver is trying to satisfy the dependencies of crossbuild-essential-armhf via arm64 indicated by trying to install file:arm64 for libdebian-dpkgcross-perl:all.
Couldn't reproduce it in a selfbuilt testcase myself though, so I will try it later with a reliable internet connection.


The other interesting bit is that multiarch-support:arm64 isn't "providing" multiarch-support:amd64, which is actually kinda nice, but the reverse that :amd64 isn't providing  :arm64 is wrong (as we don't have a distinction between "known to dpkg/configured in APT::Architectures" and "included in the cache via sources.list [arch=]" both is wrong).

Which leads me to wonder if multiarch-support is actually correct in being M-A:foreign. Given that it is supposed to ensure that the libc6 used is new enough it feels strange that at least in theory on i386 system I could install multiarch-support:armel and make the dependencies happy while the ld.so I am using is potentially still not new enough.
(A bit late/academic to wonder about that now, but yeah…)

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https://bugs.launchpad.net/bugs/1185896

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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