> We could fill the buffer while moving its start point to 0. I guess this
> behavior would require a new function (or a new parameter to
> Modules/_io/bufferedio.c:_bufferedreader_fill_buffer() ).
> If you are ok with that I could write a patch.

The buffer is used for both reading and writing and you have to be
careful when shifting it. Besides, the same change (or similar) should
also be done in the Python implementation (in _pyio.py).

If you come up with a patch, please add some tests and check the whole
regression suite passes.