MM-23503: Fix race in websocket_client writer (#14160)

* MM-23503: Fix race in websocket_client writer

We create a separate writer goroutine where all the writes happen.

* Fixing the case of double close

* Incorporate review comments

* Use CAS

* Fix incorrect comment

* Check if client is closed in pingHandler too

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

model/websocket_client_test.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gorilla/websocket"
+	"github.com/stretchr/testify/require"
+)
+
+func dummyWebsocketHandler(t *testing.T) http.HandlerFunc {
+	return func(w http.ResponseWriter, req *http.Request) {
+		upgrader := &websocket.Upgrader{
+			ReadBufferSize:  1024,
+			WriteBufferSize: 1024,
+		}
+		conn, err := upgrader.Upgrade(w, req, nil)
+		var buf []byte
+		for {
+			_, buf, err = conn.ReadMessage()
+			if err != nil {
+				break
+			}
+			t.Logf("%s\n", buf)
+			err = conn.WriteMessage(websocket.PingMessage, []byte("ping"))
+			if err != nil {
+				break
+			}
+		}
+		if _, ok := err.(*websocket.CloseError); !ok {
+			require.NoError(t, err)
+		}
+	}
+}
+
+// TestWebSocketRace needs to be run with -race to verify that
+// there is no race.
+func TestWebSocketRace(t *testing.T) {
+	s := httptest.NewServer(dummyWebsocketHandler(t))
+	defer s.Close()
+
+	url := strings.Replace(s.URL, "http://", "ws://", 1)
+	cli, err := NewWebSocketClient4(url, "authToken")
+	require.Nil(t, err)
+
+	cli.Listen()
+
+	for i := 0; i < 10; i++ {
+		time.Sleep(500 * time.Millisecond)
+		cli.UserTyping("channel", "parentId")
+	}
+}