Create UdpSocket method that exposes async_io

[diondokter]

Motivation

We're building a PTP library and need to be able to read timestamp information from received UDP packet. This is not possible with Tokio's APIs directly of course, so we have to make the specific syscall ourselves.

We're making our PTP stack async, so receiving a packet has to be async as well.

All async udp functions that need to send or receive use the private internal async_io function from the Registration. This isn't available to use from the outside while the polling version of this API is public (named try_io).

Solution

I made a new method just like the existing try_io that allows access to the inner API.
With this we can plug in our own syscall that can read the socket together with the options so a timestamp is returned.

[Darksonn]

Can you open a feature request issue for this?

If we decide to add a convenience wrapper around try_io, then it would need to be added to all of our socket types - not just UdpSocket.

[diondokter]

Can you open a feature request issue for this?

I will! I looked at TCP to see if it had that function and saw it didn't. However, looking more carefully I now see that e.g. stream and unix sockets also have that function.

[Darksonn]

I'm ok with adding this, but it would need to be added to all of our IO resources.

[satakuma]

Closing in favor of #5512.