First dirty fix. There are still a few methods I want to look into

conn.go

@@ -28,6 +28,7 @@ type Conn struct {
 	readTimeout  time.Duration
 	writeTimeout time.Duration
 	closed       bool
+	closeMutex   *sync.Mutex
 	options      *connOptions
 }
 
@@ -68,7 +69,8 @@ func Connect(conn io.ReadWriteCloser, opts ...func(*Conn) error) (*Conn, error)
 	writer := frame.NewWriter(conn)
 
 	c := &Conn{
-		conn: conn,
+		conn:       conn,
+		closeMutex: &sync.Mutex{},
 	}
 
 	options, err := newConnOptions(c, opts)
@@ -296,6 +298,8 @@ func processLoop(c *Conn, writer *frame.Writer) {
 					close(ch)
 				}
 
+				c.closeMutex.Lock()
+				defer c.closeMutex.Unlock()
 				c.closed = true
 				c.conn.Close()
 
@@ -366,6 +370,8 @@ func sendError(m map[string]chan *frame.Frame, err error) {
 // with the STOMP server is closed and any further attempt to write
 // to the server will fail.
 func (c *Conn) Disconnect() error {
+	c.closeMutex.Lock()
+	defer c.closeMutex.Unlock()
 	if c.closed {
 		return nil
 	}
@@ -389,6 +395,8 @@ func (c *Conn) Disconnect() error {
 // This method should be used only as last resort when there are fatal
 // network errors that prevent to do a proper disconnect from the server.
 func (c *Conn) MustDisconnect() error {
+	c.closeMutex.Lock()
+	defer c.closeMutex.Unlock()
 	if c.closed {
 		return nil
 	}
@@ -410,6 +418,8 @@ func (c *Conn) MustDisconnect() error {
 // Any number of options can be specified in opts. See the examples for usage. Options include whether
 // to receive a RECEIPT, should the content-length be suppressed, and sending custom header entries.
 func (c *Conn) Send(destination, contentType string, body []byte, opts ...func(*frame.Frame) error) error {
+	c.closeMutex.Lock()
+	defer c.closeMutex.Unlock()
 	if c.closed {
 		return ErrAlreadyClosed
 	}
@@ -551,10 +561,13 @@ func (c *Conn) Subscribe(destination string, ack AckMode, opts ...func(*frame.Fr
 	}
 	go sub.readLoop(ch)
 
+	// TODO is this safe? There is no check if writeCh is actually open.
 	c.writeCh <- request
 	return sub, nil
 }
 
+// TODO check further for race conditions
+
 // Ack acknowledges a message received from the STOMP server.
 // If the message was received on a subscription with AckMode == AckAuto,
 // then no operation is performed.