Native asynchronous file io

[buhman]

I think this would be fun to explore, once we have nothing better to do:

• https://lwn.net/Articles/724198/
• async file I/O #20 (comment)

Once a mildly sane (platform-specific) implementation exists, it shouldn't be too hard to extend wrap_file or open_file appropriately.

[njsmith]

Huh! Apparently Linux 4.14 actually added something useful here: https://kernelnewbies.org/Linux_4.14#Asynchronous_buffered_I.2FO_support

It's actually pretty sensible: it lets you do a "non-blocking read", so if the data is in the buffer cache you can just return it, before spending the overhead on dispatching to a thread.

Adding support to CPython's wrappers is bpo-31368. What would be even nicer is if they add a nonblocking option to the io readers.

[njsmith]

I also just filed bpo-32561, which would make this MUCH easier for us to use.
Edit: Now python/cpython#76742