py_extension on Windows passes both CPython import libraries as one linker argument #4426

What happened

On Windows, py_extension fails to link when the Python toolchain's libpython holds both import libraries, which is the case for the hermetic interpreters from python.toolchain (python3.lib and python312.lib).

py_extension_macro.bzl passes the libs with

user_link_flags = ["$(locations :_<name>_win_libs)"]

cc_shared_library does not split that entry, so link.exe or lld-link get one argument with two paths:

lld-link: error: could not open 'external/.../libs/python3.lib external/.../libs/python312.lib': no such file or directory

Repro

Windows (x86_64 or arm64), rules_python 2.4.6, bzlmod with a registered python.toolchain(python_version = "3.12"):

load("@rules_python//python/cc:py_extension.bzl", "py_extension")

py_extension(
    name = "ext",
    srcs = ["ext.cc"],
)

bazel build //:ext fails at the link step as above. main and 2.5.0-rc0 have the same code.

Proposed fix

Link one import library: the stable ABI one (python3.lib, python3t.lib) when py_limited_api is set, else the versioned one. Pass py_limited_api from the macro to py_extension_libs and filter there:

abi3_files = [f for f in link_files if f.basename in ("python3.lib", "python3t.lib")]
if ctx.attr.py_limited_api:
    link_files = abi3_files or link_files
else:
    link_files = [f for f in link_files if f not in abi3_files] or link_files

We carry this as a patch downstream and can send a PR with an analysis test.

Assisted by Claude.