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[3.14] gh-157048: check for buffer errors before mutating in BytesIO.__init__ (GH-157049) #157068

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Merged
ngoldbaum merged 1 commit into from
Sep 7, 2026
+19 −4
Merged

[3.14] gh-157048: check for buffer errors before mutating in BytesIO.__init__ (GH-157049) #157068

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12 changes: 12 additions & 0 deletions Lib/test/test_memoryio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -923,6 +923,18 @@ def test_cow_mutable(self):
memio = self.ioclass(ba)
self.assertEqual(sys.getrefcount(ba), old_rc)

def test_write_with_export(self):
memio = self.ioclass(b"abcd")
memio.seek(2)
with memio.getbuffer() as view:
self.assertRaises(BufferError, memio.__init__, b"replacement")
self.assertEqual(memio.tell(), 2)
self.assertEqual(memio.getvalue(), b"abcd")
self.assertEqual(bytes(view), b"abcd")
memio.write(b"X")
self.assertEqual(memio.getvalue(), b"abXd")


class CStringIOTest(PyStringIOTest):
ioclass = io.StringIO
UnsupportedOperation = io.UnsupportedOperation
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-09-06-15-23-24.gh-issue-157048.UtFyiJ.rst
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@@ -0,0 +1,2 @@
Preserve the position and contents of :class:`io.BytesIO` when
:meth:`!BytesIO.__init__` fails because a buffer is exported.
9 changes: 5 additions & 4 deletions Modules/_io/bytesio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1078,15 +1078,16 @@ static int
_io_BytesIO___init___impl(bytesio *self, PyObject *initvalue)
/*[clinic end generated code: output=65c0c51e24c5b621 input=3da5a74ee4c4f1ac]*/
{
/* In case, __init__ is called multiple times. */
self->string_size = 0;
self->pos = 0;

if (FT_ATOMIC_LOAD_SSIZE_RELAXED(self->exports) > 0) {
PyErr_SetString(PyExc_BufferError,
"Existing exports of data: object cannot be re-sized");
return -1;
}

/* In case, __init__ is called multiple times. */
self->string_size = 0;
self->pos = 0;

if (initvalue && initvalue != Py_None) {
if (PyBytes_CheckExact(initvalue)) {
Py_XSETREF(self->buf, Py_NewRef(initvalue));
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