handler function is spawn after receiver is attached

.gitignore

@@ -7,6 +7,7 @@
 # Folders
 _obj
 _test
+.idea
 
 # Architecture specific extensions/prefixes
 *.[568vq]

.travis.yml

@@ -2,8 +2,8 @@ sudo: false
 language: go
 
 go:
-  - "1.10.x"
-  - "1.9.x"
+  - "1.13.x"
+  - "1.12.x"
   - tip
 
 notifications:

sockjs/rawwebsocket.go

@@ -29,12 +29,12 @@ func (h *handler) rawWebsocket(rw http.ResponseWriter, req *http.Request) {
 	sessID := ""
 	sess := newSession(req, sessID, h.options.DisconnectDelay, h.options.HeartbeatDelay)
 	sess.raw = true
-	if h.handlerFunc != nil {
-		go h.handlerFunc(sess)
-	}
 
 	receiver := newRawWsReceiver(conn, h.options.WebsocketWriteTimeout)
 	sess.attachReceiver(receiver)
+	if h.handlerFunc != nil {
+		go h.handlerFunc(sess)
+	}
 	readCloseCh := make(chan struct{})
 	go func() {
 		for {

sockjs/websocket.go

@@ -35,12 +35,11 @@ func (h *handler) sockjsWebsocket(rw http.ResponseWriter, req *http.Request) {
 	}
 	sessID, _ := h.parseSessionID(req.URL)
 	sess := newSession(req, sessID, h.options.DisconnectDelay, h.options.HeartbeatDelay)
+	receiver := newWsReceiver(conn, h.options.WebsocketWriteTimeout)
+	sess.attachReceiver(receiver)
 	if h.handlerFunc != nil {
 		go h.handlerFunc(sess)
 	}
-
-	receiver := newWsReceiver(conn, h.options.WebsocketWriteTimeout)
-	sess.attachReceiver(receiver)
 	readCloseCh := make(chan struct{})
 	go func() {
 		var d []string