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gh-103503: prevent map iterator stack overflow #155848

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b6a8921
gh-103503: prevent map iterator stack overflow
BHUVANSH855 Aug 15, 2026
e31f673
gh-103503: add NEWS entry
BHUVANSH855 Aug 15, 2026
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8 changes: 8 additions & 0 deletions Lib/test/test_builtin.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1432,6 +1432,14 @@ def badfunc(x):
raise RuntimeError
self.assertRaises(RuntimeError, list, map(badfunc, range(5)))

it = []
for _ in range(100_000):
it = map(int, it)
with self.assertRaisesRegex(
RecursionError, r"Stack overflow .* while iterating"
):
list(it)

def test_map_pickle(self):
for proto in range(pickle.HIGHEST_PROTOCOL + 1):
m1 = map(map_char, "Is this the real life?")
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-08-15-12-14-08.gh-issue-103503.SZni1r.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Deeply nested ``map()`` iterators now raise ``RecursionError`` instead of
causing a stack overflow.
14 changes: 9 additions & 5 deletions Python/bltinmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1572,11 +1572,16 @@ map_next(PyObject *self)
{
mapobject *lz = _mapobject_CAST(self);
Py_ssize_t i;
Py_ssize_t nargs = 0;
PyObject *small_stack[_PY_FASTCALL_SMALL_STACK];
PyObject **stack;
PyObject **stack = NULL;
PyObject *result = NULL;
PyThreadState *tstate = _PyThreadState_GET();

if (_Py_EnterRecursiveCallTstate(tstate, " while calling map()")) {
return NULL;
}

const Py_ssize_t niters = PyTuple_GET_SIZE(lz->iters);
if (niters <= (Py_ssize_t)Py_ARRAY_LENGTH(small_stack)) {
stack = small_stack;
Expand All @@ -1585,11 +1590,9 @@ map_next(PyObject *self)
stack = PyMem_Malloc(niters * sizeof(stack[0]));
if (stack == NULL) {
_PyErr_NoMemory(tstate);
return NULL;
goto exit;
}
}

Py_ssize_t nargs = 0;
for (i = 0; i < niters; i++) {
PyObject *it = PyTuple_GET_ITEM(lz->iters, i);
PyObject *val = Py_TYPE(it)->tp_iternext(it);
Expand Down Expand Up @@ -1652,9 +1655,10 @@ map_next(PyObject *self)
for (i = 0; i < nargs; i++) {
Py_DECREF(stack[i]);
}
if (stack != small_stack) {
if (stack != NULL && stack != small_stack) {
PyMem_Free(stack);
}
_Py_LeaveRecursiveCallTstate(tstate);
return result;
}

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