[Python-Dev] Use C extensions compiled in release mode on a Python compiled in debug mode

[Matthias Klose]

On 24.04.19 18:02, Victor Stinner wrote:
> Hum, I found issues with libpython: C extensions are explicitly linked
> to libpython built in release mode. So a debug python loading a C
> extension may load libpython in release mode, whereas libpython in
> debug mode is already loaded.
> 
> When Python is built with --enable-shared, the python3.7 program is
> linked to libpython3.7m.so.1.0 on Linux. C extensions are explicitly
> linked to libpython3.7m as well:
> 
> $ python3.7-config --ldflags
> ... -lpython3.7m ...
> 
> Example with numpy:
> 
> $ ldd /usr/lib64/python3.7/site-packages/numpy/core/umath.cpython-37m-x86_64-linux-gnu.so
>     ...
>     libpython3.7m.so.1.0 => /lib64/libpython3.7m.so.1.0 (...)
>     ...
> 
> When Python 3.7 is compiled in debug mode, libpython gets a "d" flag
> for debug: libpython3.7dm.so.1.0.
> 
> I see 2 solutions:
> 
> (1) Use a different directory. If "libpython" gets the same filename
> in release and debug mode, at least, they must be installed in
> different directories. If libpython build in debug mode is installed
> in /usr/lib64/python3.7-dbg/ for example, python3.7-dbg should be
> compiled with -rpath /usr/lib64/python3.7-dbg/ to get the debug
> libpython.
> 
> (2) If "libpython" gets a different filename in debug mode, C
> extensions should not be linked to libpython explicitly but
> *implicitly* to avoid picking the wrong libpython. For example, remove
> "-lpython3.7m" from "python3.7-config --ldflags" output.
> 
> The option (1) rely on rpath which is discouraged by Linux vendors and
> may not be supported by all operating systems.
> 
> The option (2) is simpler and likely more portable.
> 
> Currently, C extensions of the standard library may or may not be
> linked to libpython depending on they are built. In practice, both
> work since python3.7 is already linked to libpython: so libpython is
> already loaded in memory before C extensions are loaded.

the purpose of python-config here is not clear. Whether it's intended to be used
for linking extensions, or embedded interpreters. Currently you are using the
same for both use cases.