Bug report

The following code

from io import BytesIO
memio = BytesIO(b"1234567890")
buf = memio.getbuffer()
del memio
a = [buf]
del buf
a.append(a)
del a
import gc
gc.collect()

raises an unraisable exception:

Exception ignored in: <_io.BytesIO object at 0x7fe2404d0d70>
Traceback (most recent call last):
  File "<python-input-9>", line 1, in <module>
BufferError: Existing exports of data: object cannot be re-sized

Unraisable exception during garbage collection is a sign of a bug.

It is only for the C implementation. The Python implementation raises an unraisable exception in different situation. See #111330.