net: add support for FileConn, FilePacketConn, FileListener on windows

[mattn]

Issue for discussion.

FileListener is useful to implement graceful-restart server. ex: https://github.com/lestrrat/go-server-starter

I know windows doesn't have a way to pass FDs to external process like UNIX OSs.
But it's not impossible. WSADuplicateSocket can export intormation of file descriptor as WSAPROTOCOL_INFO to specified process. This structure is possible to be written to a file as byte array.

http://msdn.microsoft.com/ja-jp/library/windows/desktop/ms741565(v=vs.85).aspx

How about implementation of os.File.dup, os.FileListener on windows?

https://codereview.appspot.com/177590043/

[gopherbot]

CL https://golang.org/cl/8683 mentions this issue.

[mikioh]

This issue might be unfixable because of the architectural differences between Unix-variants and Windows. One good news is that Go 1.12 supports #24331; you may use os.File.SyscallConn instead of net.File{Conn,PacketConn,Listener} functions.

[evanmoses-okta]

@mikioh I've been trying to wrap a net.Listener around the syscall.Handle I get from windows sockets APIs (e.g. WSASocket). SyscallConn doesn't really help me with that...does it? I'd love to be able to do something like this:

	pi := syscall.WSAProtocolInfo{}
        //fill pi by some means
	fd, err := xwin.WSASocket(pi.AddressFamily, pi.SocketType, pi.Protocol, &pi, 0, 0)
	if err != nil {
		return nil, err
	}
        listener, err := net.FileListener(os.NewFile(uintptr(fd), "listener"))

But of course this doesn't work because of this issue.