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

[Victor Stinner]

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.

I opened https://bugs.python.org/issue34814 to discuss how C
extensions of the standard library should be linked but I closed it
because we failed to find a consensus and the initial use case became
a non-issue. It seems like we should reopen the discussion :-)

Victor