delocating wheels built with conda's clang

[anntzer]

Consider the following example:

foo.cpp (trivial module that links to libc++):

#include "Python.h"
#include <iostream>

static struct PyModuleDef moduledef = {
  PyModuleDef_HEAD_INIT,
  "foo", NULL, 0, NULL, NULL, NULL, NULL, NULL
};

PyMODINIT_FUNC PyInit_foo() {
  std::cout << "hello world\n";  // enforce that we'll link to libc++
  return PyModule_Create(&moduledef);
}

setup.py:

from setuptools import *
setup(name="foo",
      packages=["pkg"],
      ext_modules=[Extension("pkg.foo", ["foo.cpp"],
                             extra_compile_args=[
                                 "-mmacosx-version-min=10.9",
                                 "-stdlib=libc++"],
                             extra_link_args=["-lc++"])])

I explicitly link against libc++ because we want it to get packaged by delocate-wheel. The reason is that I actually rely on a very recent libc++ (for C++17 support in https://github.com/anntzer/mplcairo). The equivalent for a Linux build is to pass -static-libgcc -static-libstdc++ to gcc, but AFAICT libc++ cannot be statically linked.

Let's try to build this using conda's clang (I don't have the permission to use homebrew on that machine) in a clean environment:

conda create -yn tmp python=3.6 &&
    activate tmp &&
    conda install -yc anaconda clangxx_osx-64 &&
    pip install delocate

then python setup.py bdist_wheel and delocate-wheel dist/foo-*.whl.

This fails with

/Users/anntzer/miniconda3/envs/mplcairo-build/lib/python3.6/site-packages/delocate/libsana.py:101: UserWarning: Couldn't find @rpath/libc++abi.1.dylib on paths:
        /Users/anntzer/src/local/miniextension/@loader_path
  '\n\t'.join(realpath(path) for path in rpaths),
Copied to package .dylibs directory:
  /Users/anntzer/miniconda3/envs/tmp/lib/libc++.1.0.dylib

Note, interestingly, that if the extension module is at the toplevel (not placed in a pkg package), then delocate-wheel does not complain.