The bpo-21536 modified how C extensions are built: they are no longer linked to libpython. The problem is that when an application embeds Python: the application wants to be linked to libpython.

commit 8c3ecc6bacc8d0cd534f2b5b53ed962dd1368c7b (HEAD -> master, upstream/master)
Author: Victor Stinner <vstinner@redhat.com>
Date:   Thu Apr 25 20:13:10 2019 +0200

    bpo-21536: C extensions are no longer linked to libpython (GH-12946)
    
    On Unix, C extensions are no longer linked to libpython.
    
    It is now possible to load a C extension built using a shared library
    Python with a statically linked Python.
    
    When Python is embedded, libpython must not be loaded with
    RTLD_LOCAL, but RTLD_GLOBAL instead. Previously, using RTLD_LOCAL, it
    was already not possible to load C extensions which were not linked
    to libpython, like C extensions of the standard library built by the
    "*shared*" section of Modules/Setup.
    
    distutils, python-config and python-config.py have been modified.

I chose to modify distutils, python-config (shell) and python-config.py (Python), but *not* Misc/python.pc (pkg-config).

Previously, we had:

$ pkg-config python-3.7 --libs
-lpython3.7m 
$ python3.7-config --libs
-lpython3.7m -lcrypt -lpthread -ldl -lutil -lm 
$ python3.7-config.py --libs
-lpython3.7m -lcrypt -lpthread -ldl -lutil -lm

Now, we get:

$ pkg-config python-3.8 --libs
-lpython3.8
$ python3.8-config --libs
-lcrypt -lpthread -ldl -lutil -lm -lm 
$ python-config.py --libs
-lcrypt -lpthread -ldl -lutil -lm -lm

python-config and python-config.py can now be used to build C extensions, but not to build an application embedding Python.

pkg-config should not be used to build a C extenstion since it links to libpython, but we don't want to do that (see bpo-21536).

I'm not sure that different tools should return different results.

I propose:

* Add a new command "pkg-config python-3.8-embed"
* Add a new "--embed" option to python3.8-config and python3.8-config.py commands
* Remove "-lpython@VERSION@@ABIFLAGS@" from "Libs: -L${libdir} -lpython@VERSION@@ABIFLAGS@" of Misc/python.pc.in

On Windows, we already provide different binaries for embedded Python with "-embed" suffix:

* Download Windows x86-64 embeddable zip file: python-3.7.3-embed-amd64.zip
* Download Windows x86-64 executable installer: python-3.7.3-amd64.exe

https://www.python.org/downloads/windows/

I don't understand the need for this.  AFAICT, the purpose of python-config is to provide configuration info for embedding Python (Issue1161914 and https://docs.python.org/3/extending/embedding.html#compiling-and-linking-under-unix-like-systems).  At some point, a pkg-config template was added (in Issue3585) but it seems to duplicate the purpose of python-config and thus is also intended for use in embedding Python (although we don't seem to document it, it is familiar to users of automake). I don't know what other purposes either python-config or the pkg-config template serve other than for embedding. AFAIK, they should not be used for building C extension modules. But admittedly this area is not well-documented.  In any case, I think pkg-config and python-config should return the same values for similar parameters.  Anyone else have an opinion?

As a note, waf seems to use python3-config for both embedded and extension modules. Currently, embedded is broken.

See for example https://bugzilla.redhat.com/show_bug.cgi?id=1711638

I mark this issue as a release blocker: bpo-21536 basically broke the compilation of all applications which embed Python. IMHO this issue is the best solution to fix it.


> I don't understand the need for this.

Oh. Let me explain this issue differently. There are two use cases for libpython:

* Build a C extension and load it in Python: use case called "pyext" by waf
* Embed Python into an application: use case called "pyembed" by waf

My bpo-21536 broke waf "pyembed" which fails to detect Python. waf "pyembed" creates a C program which calls Py_Initialize(), but the linker fails to locate Py_Initialize() symbol. To embed Python into an application, we really need to link the application to libpython: we need -lpython3.8.

In Python 3.7, "pyext" and "pyembed" use cases were covered both by default "python3.7-config --libs" and "pkg-config python3.7 --libs" configuration.

In Python 3.8, we now have to distinguish both use cases and provide *different* configuration depending if binary must be linked to libpython or not.

More info about waf breakage in my waf bug report:
https://gitlab.com/ita1024/waf/issues/2239

FYI the waf breakage was discovered by trying to build libtdb on Fedora Rawhide with Python 3.8a4, it's a dependency of Samba which embeds Python:
https://bugzilla.redhat.com/show_bug.cgi?id=1711638

"AFAICT, the purpose of python-config is to provide configuration info for embedding Python"

If that's the intention, then at least it's not used as such.  It's also used to build/configure extensions using automake/cmake based build systems.  There is one tool, and two different use cases.  I think Victor's suggestion is appropriate

New changeset 0a8e57248b913851640c64375600f05157c997df by Victor Stinner in branch 'master':
bpo-36721: Add --embed option to python-config (GH-13500)
https://github.com/python/cpython/commit/0a8e57248b913851640c64375600f05157c997df

Even if I'm not confident in my change (add --embed option), I chose to merge it anyway since at least "waf" build system is broken by my other changes (no longer link C extensions to libpython). I would like to get this change into Python 3.8 beta1 to attempt to fix most applications embedding Python.

Anyway, if something goes wrong, we still have plenty of time to decide what to do before 3.8.0 final, scheduled for 2019-10-21: https://www.python.org/dev/peps/pep-0569/

--

Since I merged my change, I reset the priority from Release Blocker to normal.

Miro commented my PR:
https://github.com/python/cpython/pull/13500#issuecomment-495510268

"""
This is what it gave me in Fedora, feels a bit inconsistent:

   /usr/lib64/pkgconfig/python-3.8-embed.pc
   /usr/lib64/pkgconfig/python-embed-3.8d.pc
   /usr/lib64/pkgconfig/python3-embed.pc
"""

I installed Python in release mode:

$ ./configure --enable-shared --prefix=/opt/py38 CFLAGS="-O0" && make && make install

It gives me:

vstinner@apu$ ls -l /opt/py38/lib/pkgconfig/
-rw-r--r--. 1 vstinner vstinner 312 24 mai   17:39 python-3.8-embed.pc
-rw-r--r--. 1 vstinner vstinner 286 24 mai   17:39 python-3.8.pc
lrwxrwxrwx. 1 vstinner vstinner  19 24 mai   17:39 python3-embed.pc -> python-3.8-embed.pc
lrwxrwxrwx. 1 vstinner vstinner  13 24 mai   17:39 python3.pc -> python-3.8.pc

Debug build:

$ ./configure --enable-shared --prefix=/opt/py38dbg --with-pydebug CFLAGS="-O0" && make && make install

It gives me:

vstinner@apu$ ls -l /opt/py38dbg/lib/pkgconfig/
lrwxrwxrwx. 1 vstinner vstinner  13 24 mai   17:43 python-3.8d.pc -> python-3.8.pc
-rw-r--r--. 1 vstinner vstinner 317 24 mai   17:43 python-3.8-embed.pc
-rw-r--r--. 1 vstinner vstinner 290 24 mai   17:43 python-3.8.pc
lrwxrwxrwx. 1 vstinner vstinner  19 24 mai   17:43 python3-embed.pc -> python-3.8-embed.pc
lrwxrwxrwx. 1 vstinner vstinner  13 24 mai   17:43 python3.pc -> python-3.8.pc
lrwxrwxrwx. 1 vstinner vstinner  19 24 mai   17:43 python-embed-3.8d.pc -> python-embed-3.8.pc

Oh! "python-embed-3.8d.pc" is a broken symbolic link to "python-embed-3.8.pc" (which doesn't exist).


Ok, now I get it: I messed up in names of symbolic links :-) I wrote PR 13551 which gives me the following files for a debug build:

vstinner@apu$ ls -l /opt/py38dbg/lib/pkgconfig/*.pc
lrwxrwxrwx. 1 vstinner vstinner  19 24 mai   17:51 /opt/py38dbg/lib/pkgconfig/python-3.8d-embed.pc -> python-3.8-embed.pc
lrwxrwxrwx. 1 vstinner vstinner  13 24 mai   17:51 /opt/py38dbg/lib/pkgconfig/python-3.8d.pc -> python-3.8.pc
-rw-r--r--. 1 vstinner vstinner 317 24 mai   17:51 /opt/py38dbg/lib/pkgconfig/python-3.8-embed.pc
-rw-r--r--. 1 vstinner vstinner 290 24 mai   17:51 /opt/py38dbg/lib/pkgconfig/python-3.8.pc
lrwxrwxrwx. 1 vstinner vstinner  19 24 mai   17:51 /opt/py38dbg/lib/pkgconfig/python3-embed.pc -> python-3.8-embed.pc
lrwxrwxrwx. 1 vstinner vstinner  13 24 mai   17:51 /opt/py38dbg/lib/pkgconfig/python3.pc -> python-3.8.pc


The format is now: "python" "version" "embed suffix".

By the way, I'm not sure that the current layout of .pc files. The name "module" give a different configuration. Is that correct?

$ grep ^Libs: /opt/py38/lib/pkgconfig/python-3.8-embed.pc
Libs: -L${libdir} -lpython3.8

$ grep ^Libs: /opt/py38dbg/lib/pkgconfig/python-3.8-embed.pc
Libs: -L${libdir} -lpython3.8d

New changeset bc66faccb8a6140e7e07b5e67843b7f21152c144 by Victor Stinner in branch 'master':
bpo-36721: Fix pkg-config symbolic links on "make install" (GH-13551)
https://github.com/python/cpython/commit/bc66faccb8a6140e7e07b5e67843b7f21152c144

The initial issue has been fixed. As far as I know, all known issues have been fixed, so I close the issue. Thanks Miro for the help in reviews and tests!

FYI waf is already fixed for the future Python 3.8 beta1! It now attempts to use --embed for its "pyembed" config ;-) Miro wrote a fix:

https://gitlab.com/ita1024/waf/merge_requests/2236
https://gitlab.com/ita1024/waf/issues/2239