Distuitls generates "export symbols" for extension modules to help ensure that they have have the correct linkage on Windows.

https://github.com/python/cpython/blob/0d30ae1a03102de07758650af9243fd31211325a/Lib/distutils/command/build_ext.py#L692

It generates the correct symbol in most causes, but if the filename contains unicode characters then it creates the wrong symbol, causing linkage errors.

The behaviour should be updated to reflect PEP-489: https://www.python.org/dev/peps/pep-0489/#export-hook-name

New changeset 9538bc9185e934bee2bd5ae2cda2b2e92a61906d by Stefan Behnel in branch 'master':
bpo-39432: Implement PEP-489 algorithm for non-ascii "PyInit_*" symbol names in distutils (GH-18150)
https://github.com/python/cpython/commit/9538bc9185e934bee2bd5ae2cda2b2e92a61906d

Ok, this is merged into 3.9. To which versions should we backport it?
Definitely 3.8, definitely not 3.5, probably not 3.6 (since it's not a security issue). Ned, what about 3.7?

The test fails on Windows. Example on AMD64 Windows8.1 Refleaks 3.x:
https://buildbot.python.org/all/#/builders/157/builds/76

======================================================================
FAIL: test_unicode_module_names (distutils.tests.test_build_ext.BuildExtTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\buildarea\3.x.ware-win81-release.refleak\build\lib\distutils\tests\test_build_ext.py", line 315, in test_unicode_module_names
    self.assertRegex(cmd.get_ext_filename(modules[0].name), r'foo\..*')
AssertionError: Regex didn't match: 'foo\\..*' not found in 'foo_d.cp39-win_amd64.pyd'

======================================================================
FAIL: test_unicode_module_names (distutils.tests.test_build_ext.ParallelBuildExtTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\buildarea\3.x.ware-win81-release.refleak\build\lib\distutils\tests\test_build_ext.py", line 315, in test_unicode_module_names
    self.assertRegex(cmd.get_ext_filename(modules[0].name), r'foo\..*')
AssertionError: Regex didn't match: 'foo\\..*' not found in 'foo_d.cp39-win_amd64.pyd'

Python 3.6 doesn't accept bugfixes anymore:
https://devguide.python.org/#status-of-python-branches

The bugfix can go into 3.7 and 3.8.

I put a breakpoint before the error:

test_unicode_module_names (distutils.tests.test_build_ext.BuildExtTestCase) ... > c:\vstinner\python\master\lib\distutils\tests
\test_build_ext.py(316)test_unicode_module_names()
-> self.assertRegex(cmd.get_ext_filename(modules[0].name), r'foo\..*')
(Pdb) p modules
[<distutils.extension.Extension('foo') at 0x274003035f0>, <distutils.extension.Extension('föö') at 0x27400303730>]
(Pdb) p modules[0].name
'foo'
(Pdb) p modules[1].name 
'föö'

On Windows, names get a "_d" suffix for debug. Extract of build_ext.py:

    def get_libraries(self, ext):
        """Return the list of libraries to link against when building a
        shared extension.  On most platforms, this is just 'ext.libraries';
        on Windows, we add the Python library (eg. python20.dll).
        """
        # The python library is always needed on Windows.  For MSVC, this
        # is redundant, since the library is mentioned in a pragma in
        # pyconfig.h that MSVC groks.  The other Windows compilers all seem
        # to need it mentioned explicitly, though, so that's what we do.
        # Append '_d' to the python import library on debug builds.
        if sys.platform == "win32":
            from distutils._msvccompiler import MSVCCompiler
            if not isinstance(self.compiler, MSVCCompiler):
                template = "python%d%d"
                if self.debug:
                    template = template + '_d'
(...)

issue39555 and PR 18357 have the fix for the buildbot.

We should not be changing Distutils 3.7 behavior at this late point in its life cycle, particularly since AFAIK this issue has not come up before.  Let's see what Łukasz thinks for 3.8.

I'm with Stefan on "Definitely 3.8". It's a bug fix (for a rarely used feature).

New changeset 5bf58cef151249f1cca92166d1b70693348da9d8 by Miss Islington (bot) in branch '3.8':
bpo-39432: Implement PEP-489 algorithm for non-ascii "PyInit_*" symbol names in distutils (GH-18150) (GH-18546)
https://github.com/python/cpython/commit/5bf58cef151249f1cca92166d1b70693348da9d8

It looks like this wasn't quite fixed by the patch: the patch encoded `_<module_name>` when it should have encoded `<module_name>`.

I've submitted a revised version to setuptools https://github.com/pypa/setuptools/pull/2646. My impression is that distutils is no longer updated and so there's no value in submitting this patch to Python too. However, I can do so if it would be used.