> 2. os.ftruncate doesn't exist on Windows.

Steve Dower wrote a patch and the latest version looks good to me:
https://bugs.python.org/issue23668#msg240385

+            if 0 in os.fsencode(file):
+                raise ValueError('embedded null character')

IMO the check must be implemented in os.open() (in path_converter). It doesn't look right to have a differen behaviour on UNIX and Windows. path_converter() calls PyUnicode_FSConverter() on UNIX and this function checks for embedded null bytes.

So I would prefer to see the issue #23668 fixed and path_converter() fixed, instead of pushing pyio_fileio_fix.patch.