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

Lib/test/test_builtin.py

@@ -1132,6 +1132,18 @@ def test_filter_dealloc(self):
         del i
         gc.collect()
 
+
+    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)
+
+        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 +1,3 @@
+Fix segmentation fault in deeply nested :func:`filter` iterator chains.
+Deeply nested filter iterators now properly raise :exc:`RecursionError`
+instead of crashing the interpreter.

Python/bltinmodule.c

@@ -579,6 +579,15 @@ filter_traverse(PyObject *self, visitproc visit, void *arg)
     return 0;
 }
 
+static int
+filter_clear(PyObject *self)
+{
+    filterobject *lz = _filterobject_CAST(self);
+    Py_CLEAR(lz->it);
+    Py_CLEAR(lz->func);
+    return 0;
+}
+
 static PyObject *
 filter_next(PyObject *self)
 {
@@ -661,7 +670,7 @@ PyTypeObject PyFilter_Type = {
         Py_TPFLAGS_BASETYPE,            /* tp_flags */
     filter_doc,                         /* tp_doc */
     filter_traverse,                    /* tp_traverse */
-    0,                                  /* tp_clear */
+    filter_clear,              /* tp_clear */
     0,                                  /* tp_richcompare */
     0,                                  /* tp_weaklistoffset */
     PyObject_SelfIter,                  /* tp_iter */