Support for closing Channel and yielding socket file descriptor (without closing the fd)

[weissi]

Sometimes I need to write integration tests where I basically just need a connected socket (without doing anything to it). But somewhat later I want to hand this file descriptor into a library which then does networking with the already-open socket.

The only way I can achieve this today is:

1. Use socket+connect to get a raw socket
2. ... do test setup ...
3. Use library (or ClientBootstrap.withConnectedSocket(...)) to then kick off the actual processing

It's annoying that I can't currently use SwiftNIO to perform step 1. I mean I can easily create a socket and not do anything with it, but I can't get the file descriptor of the underlying socket. What I'd love to do is something like

let channel = try ClientBootstrap(...)
   .connect(host: ..., port: ...)
   .wait()
let fileDescriptorPromise = channel.eventLoop.makePromise(of: CInt.self)
try channel.triggerOutboundUserEvent(SpecialCloseChannelAndGiveMeTheSocketFD(fileDescriptorPromise)).wait()
let fd = try fileDescriptorPromise.futureResult.wait()

[...]

let channelAgain = try ClientBootstrap(...)
    .channelInitializer { channel in
        // actual setup
    }
    .withConnectedSocket(fd)
    .wait()

The semantic I would want is: Do everything exactly like a close(mode: .all), except when NIO would normally call close(fd), don't do that and provide me with the (now unmanaged by NIO) fd.