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

Lib/test/test_free_threading/test_interpreters.py

@@ -0,0 +1,28 @@
+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: On a free-threaded debug build, destroying a
+        # subinterpreter used to trip an overly-restrictive assertion in
+        # _PyMem_mi_page_reclaimed and abort 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/Core_and_Builtins/2026-07-07-00-00-00.gh-issue-153176.Kx7pQ2.rst

@@ -0,0 +1,4 @@
+Fix a crash on free-threaded debug builds when destroying a subinterpreter.
+An overly-restrictive assertion in ``_PyMem_mi_page_reclaimed`` wrongly
+assumed the reclaimed page's owning thread state always matched the current
+thread state, which is not true during stop-the-world events.

Objects/obmalloc.c

@@ -211,8 +211,10 @@ _PyMem_mi_page_reclaimed(mi_page_t *page)
     if (page->qsbr_goal != 0) {
         if (mi_page_all_free(page)) {
             assert(page->qsbr_node.next == NULL);
+            // We may be reclaiming a page belonging to a different thread
+            // during a stop-the-world event. Find the _PyThreadStateImpl for
+            // the page.
             _PyThreadStateImpl *tstate = tstate_from_heap(mi_page_heap(page));
-            assert(tstate == (_PyThreadStateImpl *)_PyThreadState_GET());
             page->retire_expire = 0;
             llist_insert_tail(&tstate->mimalloc.page_list, &page->qsbr_node);
         }