bpo-17852: Maintain a list of BufferedWriter objects. Flush them on exit. by nascheme · Pull Request #3372 · python/cpython

In Python 3, the buffer and the underlying file object are separate
and so the order in which objects are finalized matters.  This is
unlike Python 2 where the file and buffer were a single object and
finalization was done for both at the same time.  In Python 3, if
the file is finalized and closed before the buffer then the data in
the buffer is lost.

This change adds a doubly linked list of open file buffers.  An atexit
hook ensures they are flushed before proceeding with interpreter
shutdown.  This is addition does not remove the need to properly close
files as there are other reasons why buffered data could get lost during
finalization.

Initial patch by Armin Rigo.

The previous implementation was not careful enough to avoid
causing issues in multi-threaded cases.  Check for buf->ok
and buf->finalizing before actually doing the flush.  Also,
increase the refcnt to ensure the object does not disappear.

We can't use _Py_PyAtExit() as it only supports registering a single
callback.  It is used by the atexit module and so we can't use it.  We
can't use Py_AtExit() either because it calls functions too late in the
interpreter shutdown process.  Instead, create io._flush_all_buffers.
In io.py, register it with the atexit module.