Flatten goroutine inside goroutine in Transfer.In (#890)

* Flatten goroutine inside goroutine in Transfer.In

* Return an error for unknown question types

Previously this would just be silently ignored leaving nothing to close
the returned channel or return an error.

xfr.go

@@ -35,30 +35,36 @@ type Transfer struct {
 //	channel, err := transfer.In(message, master)
 //
 func (t *Transfer) In(q *Msg, a string) (env chan *Envelope, err error) {
+	switch q.Question[0].Qtype {
+	case TypeAXFR, TypeIXFR:
+	default:
+		return nil, &Error{"unsupported question type"}
+	}
+
 	timeout := dnsTimeout
 	if t.DialTimeout != 0 {
 		timeout = t.DialTimeout
 	}
+
 	if t.Conn == nil {
 		t.Conn, err = DialTimeout("tcp", a, timeout)
 		if err != nil {
 			return nil, err
 		}
 	}
+
 	if err := t.WriteMsg(q); err != nil {
 		return nil, err
 	}
+
 	env = make(chan *Envelope)
-	go func() {
-		if q.Question[0].Qtype == TypeAXFR {
-			go t.inAxfr(q, env)
-			return
-		}
-		if q.Question[0].Qtype == TypeIXFR {
-			go t.inIxfr(q, env)
-			return
-		}
-	}()
+	switch q.Question[0].Qtype {
+	case TypeAXFR:
+		go t.inAxfr(q, env)
+	case TypeIXFR:
+		go t.inIxfr(q, env)
+	}
+
 	return env, nil
 }