Failed BytesIO reinitialization mutates the stream's position and contents

[ngoldbaum]

Bug report

Bug description:

Python 3.16.0a0 (heads/main:09117bc, Sep  6 2026, 15:04:05) [Clang 21.0.0 (clang-2100.1.1.101)] on darwin
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>>> import io
...
... stream = io.BytesIO(b"abcd")
... stream.seek(2)
... view = stream.getbuffer()
... try:
...     stream.__init__(b"replacement")
... except BufferError:
...     pass
... print(stream.tell(), stream.getvalue(), bytes(view))
... # 0 b'' b'abcd'
... view.release()
...
0 b'' b'abcd'

This is happening because the implementation of BytesIO.__init__ resets state before checking for the BufferError:

cpython/Modules/_io/bytesio.c

Lines 1129 to 1137 in 09117bc

	/* 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;
	}

CPython versions tested on:

3.16, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-157048: check for buffer errors before mutating in BytesIO.__init__ #157049
• [3.14] gh-157048: check for buffer errors before mutating in BytesIO.__init__ (GH-157049) #157068
• [3.15] gh-157048: check for buffer errors before mutating in BytesIO.__init__ (GH-157049) #157069
• [3.13] gh-157048: check for buffer errors before mutating in BytesIO.__init__ (GH-157049) #157070