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bpo-15108: Prevent accessing the result tuple from Python in PySequence_Tuple #24510

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bpo-15108: Prevent accessing the result tuple from Python in PySequence_Tuple #24510

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20 changes: 20 additions & 0 deletions Lib/test/test_iter.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@

import sys
import unittest
import gc
from test.support import run_unittest, cpython_only
from test.support.os_helper import TESTFN, unlink
from test.support import check_free_after_iterating, ALWAYS_EQ, NEVER_EQ
Expand Down Expand Up @@ -1036,6 +1037,25 @@ def test_error_iter(self):
self.assertRaises(TypeError, iter, typ())
self.assertRaises(ZeroDivisionError, iter, BadIterableClass())

def test_access_result_tuple_while_iterating(self):
TAG = object()

def monitor():
lst = [x for x in gc.get_referrers(TAG) if isinstance(x, tuple)]
# This would be the result tuple if is accessible mid-iteration
t = lst[0]
print(t)
return t

def my_iter():
yield TAG
t = monitor()
breakpoint()
for x in range(10):
yield x

self.assertRaises(IndexError, tuple, my_iter())


def test_main():
run_unittest(TestCase)
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2021-02-12-00-56-06.bpo-15108.KKSAxV.rst
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@@ -0,0 +1,3 @@
Fixed an issue that could cause a crash when accessing an incomplete tuple
when collecting an iterable into a :class:`tuple` using :c:func:`PySequence_Tuple`.
Patch by Pablo Galindo.
10 changes: 10 additions & 0 deletions Objects/abstract.c
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Expand Up @@ -1993,6 +1993,9 @@ PySequence_Tuple(PyObject *v)
if (result == NULL)
goto Fail;

// bpo-15108: Code can access the result tuple while being
// incomplete when calling PyIter_Next().
PyObject_GC_UnTrack(result);
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I would prefer to add a new API to create an untracked tuple: https://bugs.python.org/issue15108#msg387029

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Let's discuss int the issue, then I can modify the PR :)

/* Fill the tuple. */
for (j = 0; ; ++j) {
PyObject *item = PyIter_Next(it);
Expand Down Expand Up @@ -2022,10 +2025,17 @@ PySequence_Tuple(PyObject *v)
Py_DECREF(item);
goto Fail;
}
// Resizing could track the tuple again
PyObject_GC_UnTrack(result);
}
PyTuple_SET_ITEM(result, j, item);
}

// No more calls can go back into Python, so is safe
// to re-track the tuple.

PyObject_GC_Track(result);

/* Cut tuple back if guess was too large. */
if (j < n &&
_PyTuple_Resize(&result, j) != 0)
Expand Down