Cleanup udp.go somewhat (#662)

* Avoid hard-coding required UDP OOB buffer size

* Simplify parseDstFromOOB if statements

* Use self-invoking function for udpOOBSize variable

udp.go

@@ -9,6 +9,22 @@ import (
 	"golang.org/x/net/ipv6"
 )
 
+// This is the required size of the OOB buffer to pass to ReadMsgUDP.
+var udpOOBSize = func() int {
+	// We can't know whether we'll get an IPv4 control message or an
+	// IPv6 control message ahead of time. To get around this, we size
+	// the buffer equal to the largest of the two.
+
+	oob4 := ipv4.NewControlMessage(ipv4.FlagDst | ipv4.FlagInterface)
+	oob6 := ipv6.NewControlMessage(ipv6.FlagDst | ipv6.FlagInterface)
+
+	if len(oob4) > len(oob6) {
+		return len(oob4)
+	}
+
+	return len(oob6)
+}()
+
 // SessionUDP holds the remote address and the associated
 // out-of-band data.
 type SessionUDP struct {
@@ -22,7 +38,7 @@ func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
 // ReadFromSessionUDP acts just like net.UDPConn.ReadFrom(), but returns a session object instead of a
 // net.UDPAddr.
 func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
-	oob := make([]byte, 40)
+	oob := make([]byte, udpOOBSize)
 	n, oobn, _, raddr, err := conn.ReadMsgUDP(b, oob)
 	if err != nil {
 		return n, nil, err
@@ -53,18 +69,15 @@ func parseDstFromOOB(oob []byte) net.IP {
 	// Start with IPv6 and then fallback to IPv4
 	// TODO(fastest963): Figure out a way to prefer one or the other. Looking at
 	// the lvl of the header for a 0 or 41 isn't cross-platform.
-	var dst net.IP
 	cm6 := new(ipv6.ControlMessage)
-	if cm6.Parse(oob) == nil {
-		dst = cm6.Dst
+	if cm6.Parse(oob) == nil && cm6.Dst != nil {
+		return cm6.Dst
 	}
-	if dst == nil {
-		cm4 := new(ipv4.ControlMessage)
-		if cm4.Parse(oob) == nil {
-			dst = cm4.Dst
-		}
+	cm4 := new(ipv4.ControlMessage)
+	if cm4.Parse(oob) == nil && cm4.Dst != nil {
+		return cm4.Dst
 	}
-	return dst
+	return nil
 }
 
 // correctSource takes oob data and returns new oob data with the Src equal to the Dst