[issue30069] External library behave differently when loaded by Python (maybe thread issue)

[sylgar]

New submission from sylgar:

Hello, 

I have libsylvain.so which has just one function (sylvain_display()) which displays an hello world windows through QT libraries.

I have this one program `sylvain' which is a single line program which calls sylvain_display from libsylvain.so.

When I run `sylvain', I have some debug output from QT on the console, and the hello world window is displayed on my TV (I am running this on a Raspberry Pi with FreeBSD).

Now, when I run python2.7 or python3.6, and load libsylvain.so either through the ctypes module or through a C extension module, and call the sylvain_display() function from python, I have the console log output from QT as with the `sylvain' program, but I don't have the window on my TV!

I guess the rendering to the TV is done on a separate thread and this one is not called or locked?

I came up with this simple test program to isolate the issue because both PyQT and PySide now don't work as expected, I have console log but no actual TV output.

This used to work one year ago, I don't know what has changed in Python. 

Hope you guys will figure this out.

libsylvain.cpp: http://dev.sylvaingarrigues.com/libsylvain.cpp
sylvain.cpp (test program for libsylvain): http://dev.sylvaingarrigues.com/sylvain.cpp 
sylvain-ctypes.py (load libsylvain): http://dev.sylvaingarrigues.com/sylvain-ctypes.py

Also tried with a C extension to rule out ctypes bug, same issue:
sylvainmodule.cpp (python module wrapper): http://dev.sylvaingarrigues.com/sylvainmodule.cpp
sylvain-next.pu (load sylvainmodule and sylvain_display): http://dev.sylvaingarrigues.com/sylvain-cext.py

build script: http://dev.sylvaingarrigues.com/build.sh

----------
components: Extension Modules, FreeBSD, ctypes
messages: 291644
nosy: koobs, sylgar
priority: normal
severity: normal
status: open
title: External library behave differently when loaded by Python (maybe thread issue)
versions: Python 2.7, Python 3.3, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30069>
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