private/server: Windows TCP fast-open support detection

Implements tryEnableFastOpen by creating a localhost socket and enabling
the TCP_FASTOPEN socket option. On builds where TCP_FASTOPEN isn't
available, setting the socket option fails and tryEnableFastOpen returns
false.

This was verified on a supporting build (latest Windows 10) and an
unsupporting build (Windows 8.1, couldn't find an ISO for an older
Windows 10 build).

Change-Id: I497117dc2f04acdd2b0cc836e20d12d69076b939

private/server/fastopen_windows.go

@@ -4,22 +4,44 @@
 package server
 
 import (
+	"context"
+	"net"
+	"sync"
 	"syscall"
 
 	"go.uber.org/zap"
 )
 
-const tcpFastOpenServer = 15
+const tcpFastOpen = 15 // Corresponds to TCP_FASTOPEN from MS SDK
 
 func setTCPFastOpen(fd uintptr, queue int) error {
-	return syscall.SetsockoptInt(syscall.Handle(fd), syscall.IPPROTO_TCP, tcpFastOpenServer, 1)
+	return syscall.SetsockoptInt(syscall.Handle(fd), syscall.IPPROTO_TCP, tcpFastOpen, 1)
 }
 
+var tryInitFastOpenOnce sync.Once
+var initFastOpenPossiblyEnabled bool
+
 // tryInitFastOpen returns true if fastopen support is possibly enabled.
 func tryInitFastOpen(*zap.Logger) bool {
-	// should we log or check something along the lines of
-	// netsh int tcp set global fastopen=enabled
-	// netsh int tcp set global fastopenfallback=disabled
-	// ?
-	return false
+	tryInitFastOpenOnce.Do(func() {
+		// TCP-FASTOPEN is supported as of Windows 10 build 1607, but is
+		// enabled per socket. If the socket option isn't supported then the
+		// call to opt-in will fail. So as long as we can set up a listening
+		// socket with the right socket option set, we should be good.
+		if listener, err := (&net.ListenConfig{
+			Control: func(network, addr string, c syscall.RawConn) error {
+				var sockOptErr error
+				if controlErr := c.Control(func(fd uintptr) {
+					sockOptErr = setTCPFastOpen(fd, 0) // queue is unused
+				}); controlErr != nil {
+					return controlErr
+				}
+				return sockOptErr
+			},
+		}).Listen(context.Background(), "tcp", "127.0.0.1:0"); err == nil {
+			listener.Close()
+			initFastOpenPossiblyEnabled = true
+		}
+	})
+	return initFastOpenPossiblyEnabled
 }