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[3.14] gh-156995: Fix _PyBytes_Resize() in Free Threading (#156996) by vstinner · Pull Request #157373 · python/cpython · GitHub

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[3.14] gh-156995: Fix _PyBytes_Resize() in Free Threading (#156996) #157373

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Merged
vstinner merged 2 commits into from
Sep 13, 2026
Conversation 6 Commits 2 Checks 46 Files changed
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[3.14] gh-156995: Fix _PyBytes_Resize() in Free Threading (#156996) #157373

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gh-156995: Fix _PyBytes_Resize() in Free Threading (#156996)
StanFromIreland Sep 10, 2026
75afa46
Add Changelog entry
vstinner Sep 12, 2026
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14 changes: 13 additions & 1 deletion Lib/test/test_bytes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1507,7 +1507,6 @@ def test_resize(self):
self.assertRaises(MemoryError, bytearray().resize, sys.maxsize)
self.assertRaises(MemoryError, bytearray(1000).resize, sys.maxsize)


def test_setitem(self):
def setitem_as_mapping(b, i, val):
b[i] = val
Expand Down Expand Up @@ -2841,5 +2840,18 @@ def resize_stress(ba):
with threading_helper.start_threads(threads):
pass

@threading_helper.reap_threads
@threading_helper.requires_working_threading()
def test_free_threading_bytearray_resize_other_thread(self):
# Shrinking a bytearray whose buffer another thread owns must not
# adopt the immortal single-byte bytes object a the buffer.
ba = bytearray(b'abc')
thread = threading.Thread(target=ba.resize, args=(1,))
with threading_helper.start_threads([thread]):
pass
ba[0] = ord('X')
self.assertEqual(ba, bytearray(b'X'))
self.assertEqual(ord(b'a'), ord('a'))

if __name__ == "__main__":
unittest.main()
52 changes: 34 additions & 18 deletions Lib/test/test_capi/test_bytes.py
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
import sys
import unittest
from test.support import import_helper

Expand Down Expand Up @@ -231,26 +232,41 @@ def test_decodeescape(self):

def test_resize(self):
"""Test _PyBytes_Resize()"""
resize = _testcapi.bytes_resize
_resize = _testcapi.bytes_resize

def resize(obj, size, new):
result = _resize(obj, size, new)
if 1 <= len(result):
if new or size != len(obj):
# gh-156995: Make sure that the result is a fresh object.
# Previously, _PyBytes_Resize(&obj, 1) returned a singleton
# if _PyObject_IsUniquelyReferenced() is false.
self.assertEqual(sys.getrefcount(result), 1)
self.assertFalse(sys._is_immortal(result))
else:
# check that the result is the empty bytes string singleton
self.assertTrue(sys._is_immortal(result))
return result

for new in True, False:
self.assertEqual(resize(b'abc', 0, new), b'')
self.assertEqual(resize(b'abc', 1, new), b'a')
self.assertEqual(resize(b'abc', 2, new), b'ab')
self.assertEqual(resize(b'abc', 3, new), b'abc')
b = resize(b'abc', 4, new)
self.assertEqual(len(b), 4)
self.assertEqual(b[:3], b'abc')

self.assertEqual(resize(b'a', 0, new), b'')
self.assertEqual(resize(b'a', 1, new), b'a')
b = resize(b'a', 2, new)
self.assertEqual(len(b), 2)
self.assertEqual(b[:1], b'a')

self.assertEqual(resize(b'', 0, new), b'')
self.assertEqual(len(resize(b'', 1, new)), 1)
self.assertEqual(len(resize(b'', 2, new)), 2)
with self.subTest(new=new):
self.assertEqual(resize(b'abc', 0, new), b'')
self.assertEqual(resize(b'abc', 1, new), b'a')
self.assertEqual(resize(b'abc', 2, new), b'ab')
self.assertEqual(resize(b'abc', 3, new), b'abc')
b = resize(b'abc', 4, new)
self.assertEqual(len(b), 4)
self.assertEqual(b[:3], b'abc')

self.assertEqual(resize(b'a', 0, new), b'')
self.assertEqual(resize(b'a', 1, new), b'a')
b = resize(b'a', 2, new)
self.assertEqual(len(b), 2)
self.assertEqual(b[:1], b'a')

self.assertEqual(resize(b'', 0, new), b'')
self.assertEqual(len(resize(b'', 1, new)), 1)
self.assertEqual(len(resize(b'', 2, new)), 2)

self.assertRaises(SystemError, resize, b'abc', -1, False)
self.assertRaises(SystemError, resize, bytearray(b'abc'), 3, False)
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/C_API/2026-09-12-21-00-42.gh-issue-156995.UVjqQe.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
On Free Threading, ``_PyBytes_Resize(&obj, 1)`` no longer returns a single byte
singleton is the current thread is different than the thread which created the

cmaloney Sep 14, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality
Suggested change
singleton is the current thread is different than the thread which created the
singleton if the current thread is different than the thread which created the

nit, is -> if

vstinner Sep 14, 2026 •
edited
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Ooops, I wrote #157477 to fix the typo. Thanks for the post-merge review :-)
object. Patch by Stan Ulbrych.
14 changes: 6 additions & 8 deletions Objects/bytesobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3243,14 +3243,12 @@ _PyBytes_Resize(PyObject **pv, Py_ssize_t newsize)
return 0;
}
if (!_PyObject_IsUniquelyReferenced(v)) {
if (oldsize < newsize) {
*pv = _PyBytes_FromSize(newsize, 0);
if (*pv) {
memcpy(PyBytes_AS_STRING(*pv), PyBytes_AS_STRING(v), oldsize);
}
}
else {
*pv = PyBytes_FromStringAndSize(PyBytes_AS_STRING(v), newsize);
// Allocate and then copy so we don't get a shared immortal
// one-character singleton!
*pv = _PyBytes_FromSize(newsize, 0);
if (*pv) {
memcpy(PyBytes_AS_STRING(*pv), PyBytes_AS_STRING(v),
Py_MIN(oldsize, newsize));
}
Py_DECREF(v);
return (*pv == NULL) ? -1 : 0;
Expand Down
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