Importing created package fails with ImportError: DLL load failed (Fortran90)

[RubendeBruin]

I'm trying to migrate an existing project to meson.
It involves creating a python package from a single fortran function.

The package is here: https://github.com/RubendeBruin/specpart

It builds successfully on github actions on windows-latest. But when I download and test the created wheel it fails with ImportError: DLL load failed while importing specpart: The specified module could not be found.

Tried to solve this by enforcing a static build by including
project('specpart', 'c', default_options : ['default_library=static','default_library=static']) in the build file but the error remains.

details (copied from readme.md:

• generated wrapper using f2py and the pre-existing specpart.pyf file
• created meson build file:
• set meson-python as build backend
• used python -m build to build the package
• used Github Actions to perform the actual build on windows
• workflow runs fine
• created wheel (in the artifacts)
• download the wheel and install it using pip
• FAILS WITH:

(work11) C:\data\python>pip install specpart-0.0.6-cp311-cp311-win_amd64.whl --force-reinstall
Processing c:\data\python\specpart-0.0.6-cp311-cp311-win_amd64.whl
Installing collected packages: specpart
  Attempting uninstall: specpart
    Found existing installation: specpart 0.0.6
    Uninstalling specpart-0.0.6:
      Successfully uninstalled specpart-0.0.6
Successfully installed specpart-0.0.6

(work11) C:\data\python>python
Python 3.11.3 (tags/v3.11.3:f3909b8, Apr  4 2023, 23:49:59) [MSC v.1934 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import specpart
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: DLL load failed while importing specpart: The specified module could not be found.
>>>

[rgommers]

Hi @RubendeBruin, thanks for the nice reproducer and README questions. My immediate two thoughts:

1. If you want to distribute a wheel (i.e., run on a different machine than it was built, like you are doing here), you should run delvewheel over it first in CI. That will vendor the libgfortran.dll runtime you need and make the binary portable.
   • On Windows specifically, this may even be needed on the same machine, because Windows is missing RPATH functionality and hence pip install . is usually broken if a DLL from outside the package is needed and not on the library search path.
   • On Linux/macOS, the same holds for distribution - there the relevant tools are auditwheel/delocate. (or maybe repair-wheel for all platforms, I think it works fine but have no real experience with it)
2. Glancing at the SciPy meson.build files, the one difference is that it uses link_language: 'fortran' to build the extensions using Fortran. If auditwheel alone doesn't fix things for you, this may help.

[rgommers]

Sorry, edited my reply - it's delvewheel for Windows.

[dnicolodi]

@rgommers It may be appropriate to put a note about this in the f2py documentation when building with Meson is discussed.

Is there not a way to link libgfortran statically? I guess it is like linking libstdc++ statically, thus it may require some special treatment, but it should be possible.

[rgommers]

Yes, good point. We probably should also make it more prominent in our own docs how this work, because it's such a large footgun. We actually do mention auditwheel and co, but only on https://mesonbuild.com/meson-python/explanations/design-old.html?highlight=auditwheel (side note: that page is rendered after all)

Is there not a way to link libgfortran statically?

IIRC it's both hard technically and a license concern (GCC runtime exception: https://stackoverflow.com/questions/10130143/gpl-lgpl-and-static-linking)

[RubendeBruin]

Hi @rgommers , thanks for the reply!

adding only the link_language: 'fortran' did not do the trick.
Including delvewheel does work and includes the required .dll files in the wheel.

Now to figure out how to properly include it in the CI but that is another topic.