Fix the build on macos.

bazel/workspace_defs.bzl

@@ -34,17 +34,23 @@ cc_library(
 )
 """
 
-def _find_python_dir(repository_ctx):
-  py_program = "import sysconfig; print(sysconfig.get_config_var('INCLUDEPY'), end='')"
-  result = repository_ctx.execute(["python3", "-c", py_program])
+_build_defs_file = """
+EXT_SUFFIX = "%s"
+"""
+
+def _get_config_var(repository_ctx, name):
+  py_program = "import sysconfig; print(sysconfig.get_config_var('%s'), end='')"
+  result = repository_ctx.execute(["python3", "-c", py_program % (name)])
   if result.return_code != 0:
     fail("No python3 executable available on the system")
   return result.stdout
 
 def _python_headers_impl(repository_ctx):
-  path = _find_python_dir(repository_ctx)
+  path = _get_config_var(repository_ctx, "INCLUDEPY")
+  ext_suffix = _get_config_var(repository_ctx, "EXT_SUFFIX")
   repository_ctx.symlink(path, "python")
   repository_ctx.file("BUILD.bazel", _build_file)
+  repository_ctx.file("build_defs.bzl", _build_defs_file % ext_suffix)
 
 # The python_headers() repository rule exposes Python headers from the system.
 #

python/BUILD

@@ -27,6 +27,10 @@ load(
     "//bazel:build_defs.bzl",
     "UPB_DEFAULT_COPTS",
 )
+load(
+    "@python_headers//:build_defs.bzl",
+    "EXT_SUFFIX",
+)
 
 cc_binary(
     name = "message",
@@ -45,7 +49,10 @@ cc_binary(
     ],
     # We use a linker script to hide all symbols except the entry point for
     # the module.
-    linkopts = ["-Wl,--version-script,$(location :version_script.lds)"],
+    linkopts = select({
+        "@platforms//os:linux": ["-Wl,--version-script,$(location :version_script.lds)"],
+        "@platforms//os:macos": ["-Wl,-exported_symbol", "-Wl,_PyInit__message"],
+    }),
     linkshared = True,
     linkstatic = True,
     deps = [
@@ -60,7 +67,7 @@ cc_binary(
 genrule(
     name = "message_ext",
     srcs = [":message"],
-    outs = ["google/protobuf/pyext/_message.cpython-39-x86_64-linux-gnu.so"],
+    outs = ["google/protobuf/pyext/_message" + EXT_SUFFIX],
     cmd = "cp $< $@",
 )