lib/model, lib/protocol: Send ClusterConfig on config change (fixes #7020) (#7018)

lib/protocol/protocol.go

@@ -322,11 +322,9 @@ func (c *rawConnection) Request(ctx context.Context, folder string, name string,
 }
 
 // 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) {
 	select {
 	case c.clusterConfigBox <- &config:
-		close(c.clusterConfigBox)
 	case <-c.closed:
 	}
 }
@@ -386,13 +384,12 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 		switch msg := msg.(type) {
 		case *ClusterConfig:
 			l.Debugln("read ClusterConfig message")
-			if state != stateInitial {
-				return fmt.Errorf("protocol error: cluster config message in state %d", state)
+			if state == stateInitial {
+				state = stateReady
 			}
 			if err := c.receiver.ClusterConfig(c.id, *msg); err != nil {
 				return errors.Wrap(err, "receiver error")
 			}
-			state = stateReady
 
 		case *Index:
 			l.Debugln("read Index message")
@@ -683,6 +680,12 @@ func (c *rawConnection) writerLoop() {
 	}
 	for {
 		select {
+		case cc := <-c.clusterConfigBox:
+			err := c.writeMessage(cc)
+			if err != nil {
+				c.internalClose(err)
+				return
+			}
 		case hm := <-c.outbox:
 			err := c.writeMessage(hm.msg)
 			if hm.done != nil {