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gh-154672: Fix use-after-free in itertools.zip_longest via re-entrant iterator #154673

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gh-154672: Fix use-after-free in itertools.zip_longest via re-entrant iterator #154673

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a6a20ba
gh-000000: Fix use-after-free in itertools.count via re-entrant step
tonghuaroot Jul 25, 2026
cd78520
gh-154672: Fix use-after-free in itertools.zip_longest via re-entrant…
tonghuaroot Jul 25, 2026
1f51f66
Remove stale NEWS file from count fix
tonghuaroot Jul 25, 2026
f21835b
Fix test: guard against infinite recursion in re-entrant iterator
tonghuaroot Jul 25, 2026
54a489a
Rewrite test to exhaust sibling iterator without self-recursion
tonghuaroot Jul 25, 2026
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20 changes: 20 additions & 0 deletions Lib/test/test_itertools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1505,6 +1505,26 @@ def test_zip_longest_result_gc(self):
gc.collect()
self.assertTrue(gc.is_tracked(next(it)))

def test_zip_longest_reentrant_iterator(self):
# A re-entrant iterator exhaust must not cause use-after-free.
class ExhaustingIter:
def __init__(self, it, sibling):
self.it = it
self.sibling = sibling
self.first = True
def __iter__(self):
return self
def __next__(self):
val = next(self.it)
if self.first:
self.first = False
for _ in self.sibling:
pass
return val
sib = iter([2, 3])
zl = zip_longest(ExhaustingIter(iter([1, 4]), sib), sib)
list(zl)

@support.cpython_only
def test_pairwise_result_gc(self):
# Ditto for pairwise.
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-07-25-12-30-00.gh-issue-154672.ZlUAF2.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix use-after-free in :func:`itertools.zip_longest` when a re-entrant
iterator callback exhausts a sibling iterator. Patch by tonghuaroot.
18 changes: 14 additions & 4 deletions Modules/itertoolsmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3977,19 +3977,24 @@ zip_longest_next_lock_held(PyObject *op)
if (it == NULL) {
item = Py_NewRef(lz->fillvalue);
} else {
Py_INCREF(it);
item = PyIter_Next(it);
if (item == NULL) {
lz->numactive -= 1;
if (lz->numactive == 0 || PyErr_Occurred()) {
lz->numactive = 0;
Py_DECREF(it);
Py_DECREF(result);
return NULL;
} else {
item = Py_NewRef(lz->fillvalue);
PyTuple_SET_ITEM(lz->ittuple, i, NULL);
Py_DECREF(it);
if (PyTuple_GET_ITEM(lz->ittuple, i) != NULL) {
PyTuple_SET_ITEM(lz->ittuple, i, NULL);
Py_DECREF(it);
}
}
}
Py_DECREF(it);
}
olditem = PyTuple_GET_ITEM(result, i);
PyTuple_SET_ITEM(result, i, item);
Expand All @@ -4007,19 +4012,24 @@ zip_longest_next_lock_held(PyObject *op)
if (it == NULL) {
item = Py_NewRef(lz->fillvalue);
} else {
Py_INCREF(it);
item = PyIter_Next(it);
if (item == NULL) {
lz->numactive -= 1;
if (lz->numactive == 0 || PyErr_Occurred()) {
lz->numactive = 0;
Py_DECREF(it);
Py_DECREF(result);
return NULL;
} else {
item = Py_NewRef(lz->fillvalue);
PyTuple_SET_ITEM(lz->ittuple, i, NULL);
Py_DECREF(it);
if (PyTuple_GET_ITEM(lz->ittuple, i) != NULL) {
PyTuple_SET_ITEM(lz->ittuple, i, NULL);
Py_DECREF(it);
}
}
}
Py_DECREF(it);
}
PyTuple_SET_ITEM(result, i, item);
}
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