python
diff --git a/Lib/test/test_iter.py b/Lib/test/test_iter.py
@@ -7,6 +7,9 @@
 from test.support import check_free_after_iterating, ALWAYS_EQ, NEVER_EQ
 import pickle
 import collections.abc
+import functools
+import contextlib
+import builtins
 
 # Test result of triple loop (too big to inline)
 TRIPLETS = [(0, 0, 0), (0, 0, 1), (0, 0, 2),
@@ -91,6 +94,12 @@ def __call__(self):
             raise IndexError # Emergency stop
         return i
 
+class EmptyIterClass:
+    def __len__(self):
+        return 0
+    def __getitem__(self, i):
+        raise StopIteration
+
 # Main test suite
 
 class TestCase(unittest.TestCase):
@@ -238,6 +247,78 @@ def test_mutating_seq_class_exhausted_iter(self):
         self.assertEqual(list(empit), [5, 6])
         self.assertEqual(list(a), [0, 1, 2, 3, 4, 5, 6])
 
+    def test_reduce_mutating_builtins_iter(self):
+        # This is a reproducer of issue #101765
+        # where iter `__reduce__` calls could lead to a segfault or SystemError
+        # depending on the order of C argument evaluation, which is undefined
+
+        # Backup builtins
+        builtins_dict = builtins.__dict__
+        orig = {"iter": iter, "reversed": reversed}
+
+        def run(builtin_name, item, sentinel=None):
+            it = iter(item) if sentinel is None else iter(item, sentinel)
+
+            class CustomStr:
+                def __init__(self, name, iterator):
+                    self.name = name
+                    self.iterator = iterator
+                def __hash__(self):
+                    return hash(self.name)
+                def __eq__(self, other):
+                    # Here we exhaust our iterator, possibly changing
+                    # its `it_seq` pointer to NULL
+                    # The `__reduce__` call should correctly get
+                    # the pointers after this call
+                    list(self.iterator)
+                    return other == self.name
+
+            # del is required here
+            # to not prematurely call __eq__ from
+            # the hash collision with the old key
+            del builtins_dict[builtin_name]
+            builtins_dict[CustomStr(builtin_name, it)] = orig[builtin_name]
+
+            return it.__reduce__()
+
+        types = [
+            (EmptyIterClass(),),
+            (bytes(8),),
+            (bytearray(8),),
+            ((1, 2, 3),),
+            (lambda: 0, 0),
+            (tuple[int],)  # GenericAlias
+        ]
+
+        try:
+            run_iter = functools.partial(run, "iter")
+            # The returned value of `__reduce__` should not only be valid
+            # but also *empty*, as `it` was exhausted during `__eq__`
+            # i.e "xyz" returns (iter, ("",))
+            self.assertEqual(run_iter("xyz"), (orig["iter"], ("",)))
+            self.assertEqual(run_iter([1, 2, 3]), (orig["iter"], ([],)))
+
+            # _PyEval_GetBuiltin is also called for `reversed` in a branch of
+            # listiter_reduce_general
+            self.assertEqual(
+                run("reversed", orig["reversed"](list(range(8)))),
+                (iter, ([],))
+            )
+
+            for case in types:
+                self.assertEqual(run_iter(*case), (orig["iter"], ((),)))
+        finally:
+            # Restore original builtins
+            for key, func in orig.items():
+                # need to suppress KeyErrors in case
+                # a failed test deletes the key without setting anything
+                with contextlib.suppress(KeyError):
+                    # del is required here
+                    # to not invoke our custom __eq__ from
+                    # the hash collision with the old key
+                    del builtins_dict[key]
+                builtins_dict[key] = func
+
     # Test a new_style class with __iter__ but no next() method
     def test_new_style_iter_class(self):
         class IterClass(object):

diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-02-10-07-21-47.gh-issue-101765.MO5LlC.rst b/Misc/NEWS.d/next/Core and Builtins/2023-02-10-07-21-47.gh-issue-101765.MO5LlC.rst
@@ -0,0 +1 @@
+Fix SystemError / segmentation fault in iter ``__reduce__`` when internal access of ``builtins.__dict__`` keys mutates the iter object.
diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
@@ -2391,11 +2391,16 @@ PyDoc_STRVAR(length_hint_doc,
 static PyObject *
 bytearrayiter_reduce(bytesiterobject *it, PyObject *Py_UNUSED(ignored))
 {
+    PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
+
+    /* _PyEval_GetBuiltin can invoke arbitrary code,
+     * call must be before access of iterator pointers.
+     * see issue #101765 */
+
     if (it->it_seq != NULL) {
-        return Py_BuildValue("N(O)n", _PyEval_GetBuiltin(&_Py_ID(iter)),
-                             it->it_seq, it->it_index);
+        return Py_BuildValue("N(O)n", iter, it->it_seq, it->it_index);
     } else {
-        return Py_BuildValue("N(())", _PyEval_GetBuiltin(&_Py_ID(iter)));
+        return Py_BuildValue("N(())", iter);
     }
 }
 

diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
@@ -3169,11 +3169,16 @@ PyDoc_STRVAR(length_hint_doc,
 static PyObject *
 striter_reduce(striterobject *it, PyObject *Py_UNUSED(ignored))
 {
+    PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
+
+    /* _PyEval_GetBuiltin can invoke arbitrary code,
+     * call must be before access of iterator pointers.
+     * see issue #101765 */
+
     if (it->it_seq != NULL) {
-        return Py_BuildValue("N(O)n", _PyEval_GetBuiltin(&_Py_ID(iter)),
-                             it->it_seq, it->it_index);
+        return Py_BuildValue("N(O)n", iter, it->it_seq, it->it_index);
     } else {
-        return Py_BuildValue("N(())", _PyEval_GetBuiltin(&_Py_ID(iter)));
+        return Py_BuildValue("N(())", iter);
     }
 }
 

diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
@@ -877,8 +877,17 @@ ga_iter_clear(PyObject *self) {
 static PyObject *
 ga_iter_reduce(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
+    PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
     gaiterobject *gi = (gaiterobject *)self;
-    return Py_BuildValue("N(O)", _PyEval_GetBuiltin(&_Py_ID(iter)), gi->obj);
+
+    /* _PyEval_GetBuiltin can invoke arbitrary code,
+     * call must be before access of iterator pointers.
+     * see issue #101765 */
+
+    if (gi->obj)
+        return Py_BuildValue("N(O)", iter, gi->obj);
+    else
+        return Py_BuildValue("N(())", iter);
 }
 
 static PyMethodDef ga_iter_methods[] = {

diff --git a/Objects/iterobject.c b/Objects/iterobject.c
@@ -102,11 +102,16 @@ PyDoc_STRVAR(length_hint_doc, "Private method returning an estimate of len(list(
 static PyObject *
 iter_reduce(seqiterobject *it, PyObject *Py_UNUSED(ignored))
 {
+    PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
+
+    /* _PyEval_GetBuiltin can invoke arbitrary code,
+     * call must be before access of iterator pointers.
+     * see issue #101765 */
+
     if (it->it_seq != NULL)
-        return Py_BuildValue("N(O)n", _PyEval_GetBuiltin(&_Py_ID(iter)),
-                             it->it_seq, it->it_index);
+        return Py_BuildValue("N(O)n", iter, it->it_seq, it->it_index);
     else
-        return Py_BuildValue("N(())", _PyEval_GetBuiltin(&_Py_ID(iter)));
+        return Py_BuildValue("N(())", iter);
 }
 
 PyDoc_STRVAR(reduce_doc, "Return state information for pickling.");
@@ -239,11 +244,16 @@ calliter_iternext(calliterobject *it)
 static PyObject *
 calliter_reduce(calliterobject *it, PyObject *Py_UNUSED(ignored))
 {
+    PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
+
+    /* _PyEval_GetBuiltin can invoke arbitrary code,
+     * call must be before access of iterator pointers.
+     * see issue #101765 */
+
     if (it->it_callable != NULL && it->it_sentinel != NULL)
-        return Py_BuildValue("N(OO)", _PyEval_GetBuiltin(&_Py_ID(iter)),
-                             it->it_callable, it->it_sentinel);
+        return Py_BuildValue("N(OO)", iter, it->it_callable, it->it_sentinel);
     else
-        return Py_BuildValue("N(())", _PyEval_GetBuiltin(&_Py_ID(iter)));
+        return Py_BuildValue("N(())", iter);
 }
 
 static PyMethodDef calliter_methods[] = {

diff --git a/Objects/listobject.c b/Objects/listobject.c
@@ -3444,18 +3444,22 @@ listiter_reduce_general(void *_it, int forward)
 {
     PyObject *list;
 
+    /* _PyEval_GetBuiltin can invoke arbitrary code,
+     * call must be before access of iterator pointers.
+     * see issue #101765 */
+
     /* the objects are not the same, index is of different types! */
     if (forward) {
+        PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
         _PyListIterObject *it = (_PyListIterObject *)_it;
         if (it->it_seq) {
-            return Py_BuildValue("N(O)n", _PyEval_GetBuiltin(&_Py_ID(iter)),
-                                 it->it_seq, it->it_index);
+            return Py_BuildValue("N(O)n", iter, it->it_seq, it->it_index);
         }
     } else {
+        PyObject *reversed = _PyEval_GetBuiltin(&_Py_ID(reversed));
         listreviterobject *it = (listreviterobject *)_it;
         if (it->it_seq) {
-            return Py_BuildValue("N(O)n", _PyEval_GetBuiltin(&_Py_ID(reversed)),
-                                 it->it_seq, it->it_index);
+            return Py_BuildValue("N(O)n", reversed, it->it_seq, it->it_index);
         }
     }
     /* empty iterator, create an empty list */

diff --git a/Objects/tupleobject.c b/Objects/tupleobject.c
@@ -1048,11 +1048,16 @@ PyDoc_STRVAR(length_hint_doc, "Private method returning an estimate of len(list(
 static PyObject *
 tupleiter_reduce(_PyTupleIterObject *it, PyObject *Py_UNUSED(ignored))
 {
+    PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
+
+    /* _PyEval_GetBuiltin can invoke arbitrary code,
+     * call must be before access of iterator pointers.
+     * see issue #101765 */
+
     if (it->it_seq)
-        return Py_BuildValue("N(O)n", _PyEval_GetBuiltin(&_Py_ID(iter)),
-                             it->it_seq, it->it_index);
+        return Py_BuildValue("N(O)n", iter, it->it_seq, it->it_index);
     else
-        return Py_BuildValue("N(())", _PyEval_GetBuiltin(&_Py_ID(iter)));
+        return Py_BuildValue("N(())", iter);
 }
 
 static PyObject *

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
@@ -14784,14 +14784,19 @@ PyDoc_STRVAR(length_hint_doc, "Private method returning an estimate of len(list(
 static PyObject *
 unicodeiter_reduce(unicodeiterobject *it, PyObject *Py_UNUSED(ignored))
 {
+    PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
+
+    /* _PyEval_GetBuiltin can invoke arbitrary code,
+     * call must be before access of iterator pointers.
+     * see issue #101765 */
+
     if (it->it_seq != NULL) {
-        return Py_BuildValue("N(O)n", _PyEval_GetBuiltin(&_Py_ID(iter)),
-                             it->it_seq, it->it_index);
+        return Py_BuildValue("N(O)n", iter, it->it_seq, it->it_index);
     } else {
         PyObject *u = unicode_new_empty();
         if (u == NULL)
             return NULL;
-        return Py_BuildValue("N(N)", _PyEval_GetBuiltin(&_Py_ID(iter)), u);
+        return Py_BuildValue("N(N)", iter, u);
     }
 }
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix SystemError / segmentation fault in iter ``__reduce__`` when internal access of ``builtins.__dict__`` keys mutates the iter object.