gh-140080: Fix atexit with low memory

Lib/test/test_exceptions.py

@@ -342,6 +342,32 @@ def testMemoryErrorBigSource(self, size):
         with self.assertRaisesRegex(OverflowError, "Parser column offset overflow"):
             compile(src, '<fragment>', 'exec')
 
+    @cpython_only
+    # Python built with Py_TRACE_REFS fail with a fatal error in
+    # _PyRefchain_Trace() on memory allocation error.
+    @unittest.skipIf(support.Py_TRACE_REFS, 'cannot test Py_TRACE_REFS build')
+    def test_atexit_with_low_memory(self):
+        # gh-140080: Test that setting low memory after registering an atexit
+        # callback doesn't cause an infinite loop during finalization.
+        user_input = dedent("""
+        import atexit
+        import _testcapi
+
+        def callback():
+            pass
+
+        atexit.register(callback)
+        # Simulate low memory condition
+        _testcapi.set_nomemory(0)
+        """)
+        with SuppressCrashReport():
+            with script_helper.spawn_python('-c', user_input) as p:
+                p.wait()
+                output = p.stdout.read()
+
+        # The key point is that the process should exit (not hang)
+        self.assertIn(p.returncode, (0, 1))
+
     @cpython_only
     def testSettingException(self):
         # test that setting an exception at the C level works even if the

Misc/NEWS.d/next/Core_and_Builtins/2025-10-14-20-18-31.gh-issue-140080.8ROjxW.rst

@@ -0,0 +1,2 @@
+Fix: ``atexit_callfuncs`` need to atexit_cleanup the state when copy is NULL
+to avoid the low memory error then recursive error.

Modules/atexitmodule.c

@@ -112,6 +112,8 @@ atexit_callfuncs(struct atexit_state *state)
     {
         PyErr_FormatUnraisable("Exception ignored while "
                                "copying atexit callbacks");
+        // gh-140080: need to cleanup to prevent recursive when low memory
+        atexit_cleanup(state);
         return;
     }