No way to cleanly terminate a dedicated UDP receive thread

[LorenVS]

Given the outcome of #23194, there is currently no way within rust to cleanly terminate a UDP receive loop thread. If shutdown() is not possible, the only other option I see is to expose a recv_from_timeout of some sort on UdpSocket, so that the thread can periodically check whether a message has been sent on a channel, and terminate when such a message is received.

After thinking about this briefly, I suppose it is possible to craft a special UDP packet and send it to the locally bound UDP port, and have the receive thread terminate when it receives that special packet. I don't think this is necessarily a good way to force people to cleanly terminate such a loop, but it is an option right now.

[alexcrichton]

In general this starts breaching into the async I/O territory as what you probably "really want to do" is wait on some form of pipe plus a UDP socket and then send something on the pipe to have the call be unblocked. It looks like the OS-provided shutdown isn't always what we want, so there's no direct analog for supporting this, and as such it would require quite a bit of extra infrastructure.

As a result, I'm going to close this and add its use case to rust-lang/rfcs#957 instead. Thanks for the issue though!