fix: Add libs/python3.lib to libpython target for SABI builds on Windows (bazel-contrib#1820)

When targeting the Python Stable ABI on Windows (by setting the
Py_LIMITED_API macro to a Python minimum version hex), the unversioned
python3.lib needs to be linked instead of the versioned one (e.g.
python38.lib for Python 3.8).

Python's own config sets the library to link by default in a header
called pyconfig.h
(https://github.com/python/cpython/blob/9cc9e277254023c0ca08e1a9e379fd89475ca9c2/PC/pyconfig.h#L270),
which prompts the linker to search for python3.lib if a stable ABI
extension is built using `@rules_python` toolchains.

Since this library is not exported on Windows in the
`python_repository()` rule, building Python C++ extensions with
rules_python toolchains fails in the linking step, because the library
is never copied. Consequently, it is added now to allow Python SABI
extensions to be built (and linked) on Windows with `@rules_python`.

Since Python takes responsibility for linking the correct lib on
Windows, and never both at the same time, no other changes are made.

---------

Co-authored-by: Richard Levasseur <[email protected]>
Co-authored-by: Richard Levasseur <[email protected]>

Author: nicholasjng

CHANGELOG.md

@@ -95,6 +95,9 @@ Other changes:
   ([2169](https://github.com/bazelbuild/rules_python/issues/2169)).
 * (workspace) Corrected protobuf's name to com_google_protobuf, the name is
   hardcoded in Bazel, WORKSPACE mode.
+* (repositories): Add libs/python3.lib and pythonXY.dll to the `libpython` target
+  defined by a repository template. This enables stable ABI builds of Python extensions
+  on Windows (by defining Py_LIMITED_API).
 
 {#v0-0-0-added}
 ### Added

python/private/hermetic_runtime_repo_setup.bzl

@@ -89,6 +89,14 @@ def define_hermetic_runtime_toolchain_impl(
         }),
         system_provided = True,
     )
+    cc_import(
+        name = "abi3_interface",
+        interface_library = select({
+            _IS_FREETHREADED: "libs/python3t.lib",
+            "//conditions:default": "libs/python3.lib",
+        }),
+        system_provided = True,
+    )
 
     native.filegroup(
         name = "includes",
@@ -97,7 +105,7 @@ def define_hermetic_runtime_toolchain_impl(
     cc_library(
         name = "python_headers",
         deps = select({
-            "@bazel_tools//src/conditions:windows": [":interface"],
+            "@bazel_tools//src/conditions:windows": [":interface", ":abi3_interface"],
             "//conditions:default": None,
         }),
         hdrs = [":includes"],
@@ -156,15 +164,22 @@ def define_hermetic_runtime_toolchain_impl(
                 "lib/libpython{major}.{minor}t.dylib".format(**version_dict),
             ],
             ":is_freethreaded_windows": [
-                "python3.dll",
+                "python3t.dll",
+                "python{major}{minor}t.dll".format(**version_dict),
                 "libs/python{major}{minor}t.lib".format(**version_dict),
+                "libs/python3t.lib",
             ],
             "@platforms//os:linux": [
                 "lib/libpython{major}.{minor}.so".format(**version_dict),
                 "lib/libpython{major}.{minor}.so.1.0".format(**version_dict),
             ],
             "@platforms//os:macos": ["lib/libpython{major}.{minor}.dylib".format(**version_dict)],
-            "@platforms//os:windows": ["python3.dll", "libs/python{major}{minor}.lib".format(**version_dict)],
+            "@platforms//os:windows": [
+                "python3.dll",
+                "python{major}{minor}.dll".format(**version_dict),
+                "libs/python{major}{minor}.lib".format(**version_dict),
+                "libs/python3.lib",
+            ],
         }),
     )
 

tests/cc/current_py_cc_libs/BUILD.bazel

@@ -33,3 +33,21 @@ cc_test(
         "@rules_python//python/cc:current_py_cc_libs",
     ],
 )
+
+# This is technically a headers test, but since the pyconfig.h header
+# designates the appropriate lib to link on Win+MSVC, this test verifies that
+# the expected Windows libraries are all present in the expected location.
+# Since we define the Py_LIMITED_API macro, we expect the linker to go search
+# for libs/python3.lib.
+# buildifier: disable=native-cc
+cc_test(
+    name = "python_libs_linking_windows_test",
+    srcs = ["python_libs_linking_test.cc"],
+    defines = ["Py_LIMITED_API=0x030A0000"],
+    env = {"HELLO": "world"},
+    target_compatible_with = ["@platforms//os:windows"],
+    deps = [
+        "@rules_python//python/cc:current_py_cc_headers",
+        "@rules_python//python/cc:current_py_cc_libs",
+    ],
+)

tests/cc/current_py_cc_libs/python_libs_linking_test.cc

@@ -12,7 +12,7 @@ int main(int argc, char** argv) {
   // To make it actually run, more custom initialization is necessary.
   // See https://docs.python.org/3/c-api/intro.html#embedding-python
   Py_Initialize();
-  PyRun_SimpleString("print('Hello, world')\n");
+  Py_BytesMain(argc, argv);
   Py_Finalize();
   return 0;
 }