gh-137422: Fix race condition in PyImport_AddModuleRef (gh-141822)

Lib/test/test_free_threading/test_capi.py

@@ -0,0 +1,47 @@
+import ctypes
+import sys
+import unittest
+
+from test.support import threading_helper
+from test.support.threading_helper import run_concurrently
+
+
+_PyImport_AddModuleRef = ctypes.pythonapi.PyImport_AddModuleRef
+_PyImport_AddModuleRef.argtypes = (ctypes.c_char_p,)
+_PyImport_AddModuleRef.restype = ctypes.py_object
+
+
+@threading_helper.requires_working_threading()
+class TestImportCAPI(unittest.TestCase):
+    def test_pyimport_addmoduleref_thread_safe(self):
+        # gh-137422: Concurrent calls to PyImport_AddModuleRef with the same
+        # module name must return the same module object.
+
+        NUM_ITERS = 10
+        NTHREADS = 4
+
+        module_name = f"test_free_threading_addmoduleref_{id(self)}"
+        module_name_bytes = module_name.encode()
+        sys.modules.pop(module_name, None)
+        results = []
+
+        def worker():
+            module = _PyImport_AddModuleRef(module_name_bytes)
+            results.append(module)
+
+        for _ in range(NUM_ITERS):
+            try:
+                run_concurrently(worker_func=worker, nthreads=NTHREADS)
+                self.assertEqual(len(results), NTHREADS)
+                reference = results[0]
+                for module in results[1:]:
+                    self.assertIs(module, reference)
+                self.assertIn(module_name, sys.modules)
+                self.assertIs(sys.modules[module_name], reference)
+            finally:
+                results.clear()
+                sys.modules.pop(module_name, None)
+
+
+if __name__ == "__main__":
+    unittest.main()