Fix an inconsistency issue in io.BytesIO.write() where the buffer was exported
twice, which could lead to unexpected data overwrites and position drift when
the buffer changes between exports.
PyObject_GetBuffer() can execute user code (e.g. via __buffer__), which may
close or otherwise mutate a BytesIO object while write() or writelines()
is in progress. This could invalidate the internal buffer and lead to a
use-after-free.
Ensure that PyObject_GetBuffer() is called before validation checks.
Fix error in assertion which causes failure if pos is equal to PY_SSIZE_T_MAX.
Fix undefined behavior in read() and readinto() if pos is larger that the size
of the underlying buffer.
Centralize `io` tests into the `test_io` module so they are easier to
find and work on. This will make it easier to split `test_general` which
takes 30+ seconds in a debug build on my machine.
This renames `test_bufio` to be `test_bufferedio` so that it matches
the implementation file name (`bufferedio.c`).
Validation performed:
Tests are run in parallel after change:
```bash
./python.exe -m test test_io -uall,largefile,extralargefile -M12G -j8
```
Docstring reformat in `test_io/__init__.py` looks reasonable:
```python
>>> import test.test_io
>>> help(test.test_io)
```
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
