>> I've a few ideas how to add 0-copy support to protocols.

> I'd be interesting to hear about them.  The main challenge IMHO is to find a way to allow a readinto()-like functionality.

Exposing sendfile() should be straightforward. I started implementing it years ago but I gave up pretty soon because asyncio had no solid test framework for testing an actual data transfer between two sockets and that basically represented a blocker. Basically back then all recv() / send() related tests were mocks. Not sure about the current situation but if that has changed I would be happy to contribute a PR (I was the one who contributed socket.sendfile()).

As for a readinto()-like functionality: the only thing I'm aware of is splice() syscall but it's Linux only. It must be noted that also sendfile() is UNIX only. Windows achieve the same via TransmitFile.