lib/protocol: Don't send anything else before cluster config (#5741)

lib/protocol/protocol.go

@@ -184,6 +184,7 @@ type rawConnection struct {
 
 	inbox                 chan message
 	outbox                chan asyncMessage
+	clusterConfigBox      chan *ClusterConfig
 	dispatcherLoopStopped chan struct{}
 	closed                chan struct{}
 	closeOnce             sync.Once
@@ -230,6 +231,7 @@ func NewConnection(deviceID DeviceID, reader io.Reader, writer io.Writer, receiv
 		awaiting:              make(map[int32]chan asyncResult),
 		inbox:                 make(chan message),
 		outbox:                make(chan asyncMessage),
+		clusterConfigBox:      make(chan *ClusterConfig),
 		dispatcherLoopStopped: make(chan struct{}),
 		closed:                make(chan struct{}),
 		compression:           compress,
@@ -327,9 +329,11 @@ func (c *rawConnection) Request(folder string, name string, offset int64, size i
 	return res.val, res.err
 }
 
-// ClusterConfig send the cluster configuration message to the peer and returns any error
+// ClusterConfig sends the cluster configuration message to the peer.
+// It must be called just once (as per BEP), otherwise it will panic.
 func (c *rawConnection) ClusterConfig(config ClusterConfig) {
-	c.send(&config, nil)
+	c.clusterConfigBox <- &config
+	close(c.clusterConfigBox)
 }
 
 func (c *rawConnection) Closed() bool {
@@ -657,6 +661,16 @@ func (c *rawConnection) send(msg message, done chan struct{}) bool {
 }
 
 func (c *rawConnection) writerLoop() {
+	select {
+	case cc := <-c.clusterConfigBox:
+		err := c.writeMessage(cc)
+		if err != nil {
+			c.internalClose(err)
+			return
+		}
+	case <-c.closed:
+		return
+	}
 	for {
 		select {
 		case hm := <-c.outbox: