> > This new patch also fixes importlib.
> 
> """
>                 path_tmp = path + '.tmp'
>                 with _io.FileIO(path_tmp, 'wb') as file:
>                     file.write(data)
>                 _os.rename(path_tmp, path)
> """
> 
> I don't know exactly the context in which this code runs, but you can
> have a corruption if multiple processes try to write the bytecode file
> at the same time, since they'll all open the .tmp file: it should be
> opened with O_EXCL.

Or perhaps append the PID to the name of the temp file ?
(easier done in Python than in C :-))

> Also, as a side note, I'm wondering whether this type of check:
> """
>             if not sys.platform.startswith('win'):
>                 # On POSIX-like platforms, renaming is atomic
> """
> 
> couldn't be rewritten as
> """
>             if os.name == 'posix':
>                 # On POSIX-like platforms, renaming is atomic
> """

No, because os.py is not available to importlib (which must be
bootstrappable early). See the _bootstrap.py header for information
about what is available; this is also why we use FileIO instead of
open().

> Fox example, does OS-X report as POSIX?

I think so.