Emscripten.cmake should set CMAKE_<LANG>_FLAGS_INIT rather than CMAKE_<LANG>_FLAGS

[tttapa]

From https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS_INIT.html:

CMAKE_<LANG>_FLAGS_INIT
Value used to initialize the CMAKE_<LANG>_FLAGS cache entry the first time a build tree is configured for language <LANG>. This variable is meant to be set by a toolchain file. CMake may prepend or append content to the value based on the environment and target platform. For example, the contents of a xxxFLAGS environment variable will be prepended, where xxx will be language-specific but not necessarily the same as <LANG>; (e.g. CXXFLAGS for CXX, FFLAGS for Fortran, and so on).

Setting CMAKE_<LANG>_FLAGS directly breaks compatibility with other toolchain files (such as the ones generated by Conan), and prevents users from appending their own flags to the default ones (https://discourse.cmake.org/t/strictly-appending-to-cmake-lang-flags/6478).

[ryanking13]

Thanks for the report. That is a good to know. Let me see if that change breaks any existing package.

[ryanking13]

There were a few test failures when I updated the toolchain file, so I guess this can be a breaking change for a few recipes. I'll take a look at each failures when I have a bandwidth.

Meanwhile, you can overwrite the toolchain file by passing CMAKE_TOOLCHAIN_FILE env variable when running pyodide build-recipes or pyodide build.

[tttapa]

Thanks a lot for the quick response!

Tests from two packages seem to be failing:

1. cpp-exceptions-test:
   There's a problem importing /usr/lib/cpp-exceptions-test-{catch,throw}.so, the result of pyodide._module.LDSO.loadedLibsByName["/usr/lib/cpp-exceptions-test-{catch,throw}.so"] is undefined:
   https://github.com/ryanking13/pyodide/blob/53fbc52ea3e9ae09395fdd1c672f3836eccc47d8/packages/cpp-exceptions-test/test_cpp_exceptions.py#L10
   https://github.com/ryanking13/pyodide/blob/53fbc52ea3e9ae09395fdd1c672f3836eccc47d8/src/tests/test_stack_switching.py#L264
   This error is the cause of all test failures in test_cpp_exceptions.py and test_stack_switching.py.
   The extension modules that are being loaded for these tests are built manually (https://github.com/ryanking13/pyodide/blob/53fbc52ea3e9ae09395fdd1c672f3836eccc47d8/packages/cpp-exceptions-test/meta.yaml#L14-L17), they do not (should not) interact with CMake in any way.
2. magic:
   python-magic/test_magic.py::magic fails on Chrome with the error
   ModuleNotFoundError: There was a problem with unpickling the return value/exception from your pyodide environment. This usually means the type of the return value does not exist in your host environment. The original message is: No module named 'magic'. I'm not sure what's going on here: magic itself is a pure-Python package, and its only dependency, libmagic, is built using Makefiles, so it shouldn't be affected by a CMake-only change either.

Finally, there's the build failure of SciPy's scipy/sparse/sparsetools/csr.cxx file in build-packages-numpy-dependents, which produces no actual error messages. SciPy uses Meson, not CMake.

---

The workaround I'm currently using is to have a simple wrapper around pyodide-build's Emscripten.cmake that sets CMAKE_<LANG>_FLAGS_INIT and unsets CMAKE_<LANG>_FLAGS:
https://github.com/tttapa/py-build-cmake-example/blob/45c0ee1f7849dc44f1c37ef64f6c71722085b261/scripts/ci/pyodide.toolchain.cmake
(pyodide.toolchain.cmake is included as a user_toolchain in the actual CMake toolchain file generated by Conan.)