[Python-modules-team] Bug#818115: turn python-sphinx into an arch:any package

[Helmut Grohne]

Package: python-sphinx
Version: 1.3.6-2
Tags: patch
User: helmutg at debian.org
Usertags: rebootstrap
Control: block #807848 by -1

Hi,

I have a request that may sound strange: Please turn python-sphinx and
python3-sphinx from Architecture: all into Architecture: any. On first
hearing this, it may make no sense and indeed the rationale is
elaborate.

You may have heard about the "multiarch interpreter problem".
Unfortunately, sphinx is a victim of it in a strange way. When other
packages Build-Depends: python-sphinx, they ask for the host
architecture version. Since python-sphinx is Architecture: all, it is
treated as native architecture and commonly the build architecture is
the native architecture. During cross builds, the host and build
architectures differ and thus there is no way to satisfy such a
dependency (from a resolver pov).

>From a practical pov, we do want the build architecture python for
running sphinx, because the host architecture python cannot be executed.
So maybe we can say that architectures don't matter for sphinx by
marking it Multi-Arch: foreign? Unfortunately, no. python-sphinx
transitively depends on python-markupsafe (via python-jinja2) and thus
exposes it. Since python-markupsafe is an architecture dependent Python
extension, python-sphinx exposes the architecture. This is the gist of
the multiarch interpreter problem.

Bummer, so maybe we can tell the dependency to not care about the
architecture? The multiarch specification considered that problem and
therefore you can annotate Build-Depends with :native to select the
build architecture for a dependency. Unfortunately, this is only allowed
for Architecture: any packages. So we cannot use this to fix
dependencies either, unless we turn python-sphinx (and python3-sphinx)
into Architecture: any packages.

We thought long and hard about better solutions[1], but ultimately the
we figured there are none in practical reach. At DC15, I sat down with
Guillem Jover and Ian Jackson and after a long discussion, we concluded
that we will have to employ this workaround to move forward, because any
change to the dependency model (and thus dpkg) would be too involved.
sphinx is an early target, because it affects[2] lots of reverse
dependencies (>= 100).  python-sphinx will certainly not be the last[3]
package that requires this weired change.

So here is a patch:

    sed -i -e '/Package: python3\?-sphinx/,+1s/\(Architecture: a\)ll/\1ny/' debian/control

Please apply it, so we can start adding the :native annotation to
reverse dependencies such as jansson #807848.

If you have any questions about the reasoning, don't hesitate to ask and
Cc debian-cross at lists.debian.org.

Helmut

[1] https://wiki.debian.org/HelmutGrohne/MultiarchSpecChanges
[2] http://bootstrap.debian.net/cross_all.html
[3] http://bootstrap.debian.net/ma_interpreter.html