lib/protocol: Don't send any messages before cluster config (#5646)

* lib/model: Send cluster config before releasing pmut

* reshuffle

* add model.connReady to track cluster-config status

* Corrected comments/strings

* do it in protocol

lib/protocol/protocol.go

@@ -182,11 +182,12 @@ type rawConnection struct {
 	nextID    int32
 	nextIDMut sync.Mutex
 
-	outbox        chan asyncMessage
-	closed        chan struct{}
-	closeOnce     sync.Once
-	sendCloseOnce sync.Once
-	compression   Compression
+	sentClusterConfig chan struct{}
+	outbox            chan asyncMessage
+	closed            chan struct{}
+	closeOnce         sync.Once
+	sendCloseOnce     sync.Once
+	compression       Compression
 }
 
 type asyncResult struct {
@@ -220,15 +221,16 @@ func NewConnection(deviceID DeviceID, reader io.Reader, writer io.Writer, receiv
 	cw := &countingWriter{Writer: writer}
 
 	c := rawConnection{
-		id:          deviceID,
-		name:        name,
-		receiver:    nativeModel{receiver},
-		cr:          cr,
-		cw:          cw,
-		awaiting:    make(map[int32]chan asyncResult),
-		outbox:      make(chan asyncMessage),
-		closed:      make(chan struct{}),
-		compression: compress,
+		id:                deviceID,
+		name:              name,
+		receiver:          nativeModel{receiver},
+		cr:                cr,
+		cw:                cw,
+		awaiting:          make(map[int32]chan asyncResult),
+		sentClusterConfig: make(chan struct{}),
+		outbox:            make(chan asyncMessage),
+		closed:            make(chan struct{}),
+		compression:       compress,
 	}
 
 	return wireFormatConnection{&c}
@@ -322,9 +324,20 @@ 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).
 func (c *rawConnection) ClusterConfig(config ClusterConfig) {
-	c.send(&config, nil)
+	select {
+	case <-c.sentClusterConfig:
+		return
+	case <-c.closed:
+		return
+	default:
+	}
+	if err := c.writeMessage(asyncMessage{&config, nil}); err != nil {
+		c.internalClose(err)
+	}
+	close(c.sentClusterConfig)
 }
 
 func (c *rawConnection) Closed() bool {
@@ -628,13 +641,20 @@ func (c *rawConnection) handleResponse(resp Response) {
 }
 
 func (c *rawConnection) send(msg message, done chan struct{}) bool {
+	defer func() {
+		if done != nil {
+			close(done)
+		}
+	}()
+	select {
+	case <-c.sentClusterConfig:
+	case <-c.closed:
+		return false
+	}
 	select {
 	case c.outbox <- asyncMessage{msg, done}:
 		return true
 	case <-c.closed:
-		if done != nil {
-			close(done)
-		}
 		return false
 	}
 }