[Python-modules-team] Bug#915856: sphinx: Failed cross building with Build-Depends on python3-sphinx

[Helmut Grohne]

Hi Dmitry,

On Sun, Jan 20, 2019 at 06:21:27PM +0300, Dmitry Shachnev wrote:
> Now there is no binary package named “sphinx”. The executable files are
> provided in python-sphinx and python3-sphinx binary packages, and are
> managed by alternatives system (Sphinx 2.0 will drop Python 2 support, so
> only python3-sphinx will remain). Are you suggesting to split these files
> into a separate binary package?

Upon closer inspection, I'm growing doubts. The autodoc extension is not
in some python3-sphinxcontrib.something package but in python3-sphinx
proper. Therefore you want a way of using a particular architecture's
sphinx anyway. Marking it M-A:foreign is likely going to cause trouble
later on. So if you split out this sphinx package, you'd likely have
python3-sphinx Depends: sphinx at least initially. Without that
dependency, lots of packages will FTBFS. Then sphinx would of course
Depends: python3-sphinx posing a circular dependency until we fix all
those FTBFS. Then any package that refers to the scripts and uses
autodoc must "Build-Depends: sphinx, python3-sphinx". This is confusing
at the very least.

Certainly not something we want to do before buster.

> > A "weaker" alternative already employed by some other python extensions
> > is "Multi-Arch: allowed". It's essentially the same thing just renaming
> > "sphinx" to "python3-sphinx:any". It'll look more ugly in dependencies,
> > but it crucially is the same thing.
> 
> Is it safe to add Multi-Arch: allowed to python-sphinx and python3-sphinx
> right now (and Multi-Arch: foreign to sphinx-common)?

I overlooked one major issue with Multi-Arch: allowed. The value is not
permitted on Architecture: all packages. So if you trying using it, you
end up switching python3-sphinx to Architecture: any.

Beyond that, the marking is certainly not doing any immediate harm.
Until some other package uses "python3-sphinx:any" literally nothing
changes. As soon as you get such a dependency however, reverting becomes
difficult. Removing "Multi-Arch: allowed" will make all
"python3-sphinx:any" dependencies immediately unsatisfiable (even
natively).

For Build-Depends, you can use "python3-sphinx:native" now. Until very
recently that wasn't possible as dpkg-checkbuilddeps would reject
:native annotations on Architecture: all packages, but dpkg has adjusted
behaviour to what apt and dose do now. Therefore such dependencies harm
stretch-backports. Still it might be the best thing we have at the
moment.

> If you need help with this (e.g. filing bugs/patches to split -doc packages),
> please let me know.

Well, we do have some data on which packages are affected. Carefully
open this huge html file: https://bootstrap.debian.net/cross_all.html.
Then search for "python3-sphinx" and you get an overview of which
packages are affected. It currently is the #10 cross unsatisfiable cause
with 146 affected packages. Searching again will reveal the actual
source packages. Note that the list already excludes Build-Depends-Indep
as well as source packages that only build architecture-independent
binary packages. Carefully pick valuable targets here. For instance
large documentation trees, long build time, high popcon.

There is a reason why I have - for a long time - not actively worked on
cross/sphinx: There is no obvious solution. I wish I could give more
encouraging answers.

Helmut