Add a test.

Author: ZeroIntensity

Lib/test/test_atexit.py

@@ -4,6 +4,7 @@
 import unittest
 from test import support
 from test.support import script_helper
+from test.support import threading_helper
 
 
 class GeneralTest(unittest.TestCase):
@@ -46,6 +47,34 @@ def test_atexit_instances(self):
         self.assertEqual(res.out.decode().splitlines(), ["atexit2", "atexit1"])
         self.assertFalse(res.err)
 
+    @threading_helper.requires_working_threading()
+    @support.requires_resource("cpu")
+    @unittest.skipUnless(support.Py_GIL_DISABLED, "only meaningful without the GIL")
+    def test_atexit_thread_safety(self):
+        # GH-126907: atexit was not thread safe on the free-threaded build
+
+        # I'm not certain this needs to be in a script runner, but
+        # let's do it anyway.
+        code = textwrap.dedent("""
+        from threading import Thread
+
+        def dummy():
+            pass
+
+
+        def thready():
+            for _ in range(100):
+                atexit.register(dummy)
+                atexit._clear()
+                atexit.register(dummy)
+                atexit.unregister(dummy)
+
+
+        for x in range(100):
+            Thread(target=thready, args=()).start()
+        """)
+        script_helper.assert_python_ok("-c", code)
+
 
 @support.cpython_only
 class SubinterpreterTest(unittest.TestCase):