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[3.13] gh-132713: Fix repr(list) race condition (#132801) #132809

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Merged
merged 2 commits into from
Apr 23, 2025
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[3.13] gh-132713: Fix repr(list) race condition (#132801) #132809

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2edc865
gh-132713: Fix repr(list) race condition (#132801)
vstinner Apr 22, 2025
00f556b
Simplify list_repr_impl()
vstinner Apr 23, 2025
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13 changes: 13 additions & 0 deletions Lib/test/test_list.py
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Original file line number Diff line number Diff line change
Expand Up @@ -116,6 +116,19 @@ def test_list_resize_overflow(self):
with self.assertRaises((MemoryError, OverflowError)):
lst *= size

def test_repr_mutate(self):
class Obj:
@staticmethod
def __repr__():
try:
mylist.pop()
except IndexError:
pass
return 'obj'

mylist = [Obj() for _ in range(5)]
self.assertEqual(repr(mylist), '[obj, obj, obj]')

def test_repr_large(self):
# Check the repr of large list objects
def check(n):
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2025-04-22-16-38-43.gh-issue-132713.mBWTSZ.rst
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@@ -0,0 +1,2 @@
Fix ``repr(list)`` race condition: hold a strong reference to the item while
calling ``repr(item)``. Patch by Victor Stinner.
8 changes: 7 additions & 1 deletion Objects/listobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -585,6 +585,7 @@ list_repr_impl(PyListObject *v)
{
PyObject *s;
_PyUnicodeWriter writer;
PyObject *item = NULL;
Py_ssize_t i = Py_ReprEnter((PyObject*)v);
if (i != 0) {
return i > 0 ? PyUnicode_FromString("[...]") : NULL;
Expand All @@ -601,12 +602,15 @@ list_repr_impl(PyListObject *v)
/* Do repr() on each element. Note that this may mutate the list,
so must refetch the list size on each iteration. */
for (i = 0; i < Py_SIZE(v); ++i) {
/* Hold a strong reference since repr(item) can mutate the list */
item = Py_NewRef(v->ob_item[i]);

if (i > 0) {
if (_PyUnicodeWriter_WriteASCIIString(&writer, ", ", 2) < 0)
goto error;
}

s = PyObject_Repr(v->ob_item[i]);
s = PyObject_Repr(item);
if (s == NULL)
goto error;

Expand All @@ -615,6 +619,7 @@ list_repr_impl(PyListObject *v)
goto error;
}
Py_DECREF(s);
Py_CLEAR(item);
}

writer.overallocate = 0;
Expand All @@ -625,6 +630,7 @@ list_repr_impl(PyListObject *v)
return _PyUnicodeWriter_Finish(&writer);

error:
Py_XDECREF(item);
_PyUnicodeWriter_Dealloc(&writer);
Py_ReprLeave((PyObject *)v);
return NULL;
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