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Re: dh_python3 sets shebang to Python 2 -- is this a bug?



Hello Christian,

On Mon, Jan 20, 2020 at 08:51:56AM +0100, Christian Kastner wrote:
> I have an issue with a package build, and I'm not sure whether this is a
> bug in dh-python, or whether I'm just misunderstanding something.
> 
> 
> A package that had recently been converted from Python 2 to Python3 was
> still generating dependencies for Python 2, see #936924.
> 
> Frederic-Emmanuel Picca correctly pointed me to the source of this
> issue, it was an unversioned shebang in some scripts installed into
> /usr/bin.
> 
> Here's the thing that puzzles me: dh_python3 identifies the unversioned
> shebang,
> 
>   | I: dh_python3 tools:114: replacing shebang in debian/libsvm-tools/usr/bin/svm-grid
>   | I: dh_python3 tools:114: replacing shebang in debian/libsvm-tools/usr/bin/svm-subset
>   | I: dh_python3 tools:114: replacing shebang in debian/libsvm-tools/usr/bin/svm-checkdata
>   | I: dh_python3 tools:114: replacing shebang in debian/libsvm-tools/usr/bin/svm-easy
> 
> but it replaces the shebang with the Python *2* versioned one.

I think you're hitting the following code paths:
https://sources.debian.org/src/dh-python/4.20191017/dhpython/tools.py/#L94
https://sources.debian.org/src/dh-python/4.20191017/dhpython/tools.py/#L100
https://sources.debian.org/src/dh-python/4.20191017/dhpython/interpreter.py/#L212
https://sources.debian.org/src/dh-python/4.20191017/dhpython/interpreter.py/#L196
https://sources.debian.org/src/dh-python/4.20191017/dhpython/tools.py/#L106
https://sources.debian.org/src/dh-python/4.20191017/dhpython/tools.py/#L114

I've personally seen some upstream use 'python' in shebang with the
intention of meaning 'works with either python2 or python3', but in
debian it seems people have always assumed unversioned
(/usr/bin/)python always means python *2*.
I guess the tools are written in the latter sense here, rewriting the
shebang from python -> python2 explicitly unless you override it....

You should note that if you know the code is actually python-version
agnostic you should be able to fix this by setting the shebang
explicitly by using:
https://sources.debian.org/src/dh-python/4.20191017/dh_python3/#L122

eg. in debian/rules use: export DH_OPTIONS=--shebang=/usr/bin/python3 

> 
> A Python 3 helper setting /usr/bin/python2 looks odd to me, but I have
> no in-depth knowledge of dh-python's design, or the corner cases it
> might need to address.
> 
> Is there a use case where dh_python3 (not dh_python2) doing this can
> make sense? Or can this be considered a bug?

I'm not really qualified to answer your questions here but my guess is
that in the past this made sense, but nowadays probably not so much...

> 
> Furthermore, the package does not list any Python 2 build dependencies,
> and all binary package dependencies are generated by the
> ${python3:Depends} substvar. Could this perhaps be worth an "if the
> package does not have Python 2 dependencies, a Python 2 shebang is a
> bug" lintian check?

Having some safety-net in place here would likely be a good idea because
I understand how this could easily be missed otherwise.

Regards,
Andreas Henriksson


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