Support TCP Fast Open about socket2

[zonyitoo]

Well, setting the TCP_FASTOPEN flag could be in socket2, but the whole TFO process (client making connection, server setting up acceptor socket) may have to be put in tokio-tfo.

I am ok to make a PR for setting TCP_FASTOPEN for sockets.

[alexcrichton]

I don't know much about this myself, but PRs are most welcome to explore the design space here!

[Thomasdezeeuw]

@zonyitoo still interested in contributing this?

[zonyitoo]

I made a few steps but couldn't find a good design for it.

[Thomasdezeeuw]

We could just expose the system calls/flags if we can't get a nice cross-platform API.

[zonyitoo]

https://github.com/zonyitoo/tokio-tfo

Made a simple repo for this.

[Thomasdezeeuw]

@zonyitoo do you still think this should go into Socket2? Or is your library a better place to keep it?

[Thomasdezeeuw]

Well, setting the TCP_FASTOPEN flag could be in socket2, but the whole TFO process (client making connection, server setting up acceptor socket) may have to be put in tokio-tfo.

I am ok to make a PR for setting TCP_FASTOPEN for sockets.

Sounds good.

[zonyitoo]

https://github.com/zonyitoo/tokio-tfo/blob/3e23d00c4258884db73cba49753fa37f5f9fa7ca/src/sys/unix/linux.rs#L388-L419

There is a problem about how to design an unified API. Linux's TCP_FASTOPEN accepts an integer value reprecenting the length of TFO accept queue. On the other platforms, including Windows, macOS, FreeBSD, it is only a switch with 0 and 1.

[Thomasdezeeuw]

We could document the behaviour, i.e. 0 disables, >= 1 enables, on Linux it changes the queue length.