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gh-141311: Avoid assertion in BytesIO readinto #141333

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gh-141311: Avoid assertion in BytesIO readinto #141333

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db81df6
gh-141311: Avoid assertion in BytesIO readinto
cmaloney Nov 10, 2025
34ed126
Tweak comment for correctness.
cmaloney Nov 10, 2025
2c84efe
Simplify by using sys.maxsize
cmaloney Nov 10, 2025
d119904
Reference io.BytesIO.readinto directly
cmaloney Nov 10, 2025
d78cc84
Resolve read_bytes_lock_held and audit other cases
cmaloney Nov 11, 2025
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8 changes: 8 additions & 0 deletions Lib/test/test_io/test_memoryio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -552,6 +552,14 @@ def test_relative_seek(self):
memio.seek(1, 1)
self.assertEqual(memio.read(), buf[1:])

def test_issue141311(self):
memio = self.ioclass()
# Seek allows PY_SSIZE_T_MAX, read should handle that.
# Past end of buffer read should always return 0 (EOF).
self.assertEqual(sys.maxsize, memio.seek(sys.maxsize))
buf = bytearray(2)
self.assertEqual(0, memio.readinto(buf))

def test_unicode(self):
memio = self.ioclass()

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2025-11-09-18-55-13.gh-issue-141311.qZ3swc.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix assertion failure in :class:`io.BytesIO` implementation of
:func:`~io.BufferedIOBase.readinto` when the current offset is at the max
offset and readinto is called.
5 changes: 3 additions & 2 deletions Modules/_io/bytesio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -609,8 +609,9 @@ _io_BytesIO_readinto_impl(bytesio *self, Py_buffer *buffer)
n = self->string_size - self->pos;
if (len > n) {
len = n;
if (len < 0)
len = 0;
if (len < 0) {
return PyLong_FromSsize_t(0);
}
}

assert(self->pos + len < PY_SSIZE_T_MAX);
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@serhiy-storchaka serhiy-storchaka Nov 10, 2025

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Isn't it enough to fix this assertion?

Suggested change
assert(self->pos + len < PY_SSIZE_T_MAX);
assert(self->pos + len <= PY_SSIZE_T_MAX);

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@cmaloney cmaloney Nov 10, 2025

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It will but I worry about the later PyBytes_AS_STRING(self->buf) + self->pos as self->pos is PY_SSIZE_T_MAX and adding a large integer to a pointer then doing a memcpy from that computed address (even 0 bytes) feels like a path to undefined behavior to me. Happy to implement whichever you'd prefer.

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@serhiy-storchaka serhiy-storchaka Nov 10, 2025

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Good point. If adding PY_SSIZE_T_MAX causes troubles, adding slightly smaller value will also cause troubles. Actually, if pos larger than the size of the array, ot is an undefined behaviour. So, a specoal case for empty read is needed too.

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