udp: Windows: abstract the GetSocketOption away too.

Windows is weird, put this stuff in the impl. specific sub file too.

udp.go

@@ -27,13 +27,12 @@ func setUDPSocketOptions(conn *net.UDPConn) error {
 	sa, err := syscall.Getsockname(int(file.Fd()))
 	switch sa.(type) {
 	case *syscall.SockaddrInet6:
-		// dual stack. See http://stackoverflow.com/questions/1618240/how-to-support-both-ipv4-and-ipv6-connections
-		v6only, err := syscall.GetsockoptInt(int(file.Fd()), syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY)
+		v6only, err := getUDPSocketOption6Only(conn)
 		if err != nil {
 			return err
 		}
 		setUDPSocketOptions6(conn)
-		if v6only == 0 {
+		if !v6only {
 			setUDPSocketOptions4(conn)
 		}
 	case *syscall.SockaddrInet4:

udp_linux.go

@@ -38,3 +38,16 @@ func setUDPSocketOptions6(conn *net.UDPConn) error {
 	}
 	return nil
 }
+
+// getUDPSocketOption6Only return true if the socket is v6 only and false when it is v4/v6 combined
+// (dualstack).
+func getUDPSocketOption6Only(conn *net.UDPConn) (bool, error) {
+	// We got the .File() in NewUDPConn, this this will work.
+	file, _ := conn.File()
+	// dual stack. See http://stackoverflow.com/questions/1618240/how-to-support-both-ipv4-and-ipv6-connections
+	v6only, err := syscall.GetsockoptInt(int(file.Fd()), syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY)
+	if err != nil {
+		return false, err
+	}
+	return v6only == 1, nil
+}

udp_other.go

@@ -14,3 +14,5 @@ import (
 
 func setUDPSocketOptions4(conn *net.UDPConn) error { return nil }
 func setUDPSocketOptions6(conn *net.UDPConn) error { return nil }
+
+func getUDPSocketOption6Only(conn *net.UDPConn) (bool, error) { return false, nil }