gh-137894: Fix segmentation fault in deeply nested filter() iterator chains

Lib/test/test_builtin.py

@@ -1132,6 +1132,22 @@ def test_filter_dealloc(self):
         del i
         gc.collect()
 
+    @support.skip_wasi_stack_overflow()
+    @support.skip_emscripten_stack_overflow()
+    @support.requires_resource('cpu')
+    def test_filter_deep_nesting_recursion_error(self):
+        # gh-137894: Test that deeply nested filter() iterator chains
+        # raise RecursionError instead of causing segmentation fault.
+        # This verifies that the tp_clear method prevents stack overflow
+        # during garbage collection of cyclic references.
+        i = filter(bool, range(1000000))
+        for _ in range(100000):
+            i = filter(bool, i)
+
+        # Should raise RecursionError, not segmentation fault
+        with self.assertRaises(RecursionError):
+            list(i)
+
     def test_getattr(self):
         self.assertTrue(getattr(sys, 'stdout') is sys.stdout)
         self.assertRaises(TypeError, getattr)

Misc/NEWS.d/next/Core_and_Builtins/2025-08-18-08-14-52.gh-issue-137894.SrkIA_.rst

@@ -0,0 +1,6 @@
+Fix segmentation fault in deeply nested :func:`filter` iterator chains by
+adding a ``tp_clear`` method to the filter object. Previously, creating
+thousands of nested filter iterators and then consuming them would cause
+a stack overflow during garbage collection, leading to interpreter crashes.
+Now such cases properly raise :exc:`RecursionError` instead of crashing
+the interpreter.

Python/bltinmodule.c

@@ -579,6 +579,14 @@ filter_traverse(PyObject *self, visitproc visit, void *arg)
     return 0;
 }
 
+static int
+filter_clear(filterobject *lz)
+{
+    Py_CLEAR(lz->it);
+    Py_CLEAR(lz->func);
+    return 0;
+}
+
 static PyObject *
 filter_next(PyObject *self)
 {
@@ -660,8 +668,8 @@ PyTypeObject PyFilter_Type = {
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
         Py_TPFLAGS_BASETYPE,            /* tp_flags */
     filter_doc,                         /* tp_doc */
-    filter_traverse,                    /* tp_traverse */
-    0,                                  /* tp_clear */
+    (traverseproc)filter_traverse,      /* tp_traverse */
+    (inquiry)filter_clear,              /* tp_clear */
     0,                                  /* tp_richcompare */
     0,                                  /* tp_weaklistoffset */
     PyObject_SelfIter,                  /* tp_iter */