gh-153176: Fix crash in _PyMem_mi_page_reclaimed on free-threaded debug builds

Lib/test/test_free_threading/test_interpreters.py

@@ -0,0 +1,29 @@
+import textwrap
+import unittest
+
+from test.support import import_helper, script_helper
+
+
+# Make sure _testinternalcapi is available before running the test.
+import_helper.import_module('_testinternalcapi')
+
+
+class InterpreterTeardownTests(unittest.TestCase):
+    def test_destroy_subinterpreter_does_not_abort(self):
+        # gh-153176: destroy_interpreter(basic=True) used to call
+        # PyThreadState_Clear() on a non-current thread state, which on a
+        # free-threaded debug build reclaimed mimalloc pages into a heap not
+        # owned by the current thread and aborted the process.  Run the
+        # reproduction in a subprocess so that a regression surfaces as a
+        # non-zero exit / SIGABRT instead of killing the test runner.
+        script = textwrap.dedent("""
+            import _testinternalcapi
+
+            interpid = _testinternalcapi.create_interpreter()
+            _testinternalcapi.destroy_interpreter(interpid, basic=True)
+        """)
+        script_helper.assert_python_ok('-c', script)
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Tests/2026-07-22-00-00-00.gh-issue-153176.mZ8nQ4.rst

@@ -0,0 +1,3 @@
+Fix ``_testinternalcapi.destroy_interpreter()`` calling
+``PyThreadState_Clear()`` on a non-current thread state, which aborted
+free-threaded debug builds when destroying a subinterpreter.

Modules/_testinternalcapi.c

@@ -2301,8 +2301,7 @@ destroy_interpreter(PyObject *self, PyObject *args, PyObject *kwargs)
         }
         t2 = PyThreadState_New(interp);
         prev = PyThreadState_Swap(t2);
-        PyThreadState_Clear(t1);
-        PyThreadState_Delete(t1);
+        // t1 is deliberately left alive; Py_EndInterpreter() must clean it up.
         Py_EndInterpreter(t2);
         PyThreadState_Swap(prev);
     }