MM-24397: Reusing the read buffer while reading messages from websockets (#14368)

* MM-24397: Reusing the read buffer while reading messages from websockets

The core problem was that conn.ReadMessage allocated a buffer every time it was read.
This created heavy amount of allocations every single time we read a message from the websocket.

To avoid this, we bypass the ReadMessage which was more of a helper method,
and actually call the NextReader which returns a reader object.
We can then reuse a single byte.Buffer instance to read the object unmarshal
into a WebSocketEvent object. This gets rid of the allocation in the read path completly
and allows GC more time to do other tasks.

* Incorporate review comments

* Move reset buffer to top of loop

* Cleanup further

* Fix test

* Final fix

model/websocket_client.go

@@ -30,6 +30,8 @@ type writeMessage struct {
 	data    interface{}
 }
 
+const avgReadMsgSizeBytes = 1024
+
 // WebSocketClient stores the necessary information required to
 // communicate with a WebSocket endpoint.
 type WebSocketClient struct {
@@ -167,19 +169,30 @@ func (wsc *WebSocketClient) Listen() {
 			close(wsc.ResponseChannel)
 			close(wsc.quitPingWatchdog)
 		}()
+		var buf bytes.Buffer
+		buf.Grow(avgReadMsgSizeBytes)
 
 		for {
-			var rawMsg json.RawMessage
-			var err error
-			if _, rawMsg, err = wsc.Conn.ReadMessage(); err != nil {
+			// Reset buffer.
+			buf.Reset()
+			_, r, err := wsc.Conn.NextReader()
+			if err != nil {
 				if !websocket.IsCloseError(err, websocket.CloseNormalClosure, websocket.CloseNoStatusReceived) {
 					wsc.ListenError = NewAppError("NewWebSocketClient", "model.websocket_client.connect_fail.app_error", nil, err.Error(), http.StatusInternalServerError)
 				}
-
+				return
+			}
+			// Use pre-allocated buffer.
+			_, err = buf.ReadFrom(r)
+			if err != nil {
+				// This should use a different error ID, but en.json is not imported anyways.
+				// It's a different bug altogether but we let it be for now.
+				// See MM-24520.
+				wsc.ListenError = NewAppError("NewWebSocketClient", "model.websocket_client.connect_fail.app_error", nil, err.Error(), http.StatusInternalServerError)
 				return
 			}
 
-			event := WebSocketEventFromJson(bytes.NewReader(rawMsg))
+			event := WebSocketEventFromJson(bytes.NewReader(buf.Bytes()))
 			if event == nil {
 				continue
 			}
@@ -189,7 +202,7 @@ func (wsc *WebSocketClient) Listen() {
 			}
 
 			var response WebSocketResponse
-			if err := json.Unmarshal(rawMsg, &response); err == nil && response.IsValid() {
+			if err := json.Unmarshal(buf.Bytes(), &response); err == nil && response.IsValid() {
 				wsc.ResponseChannel <- &response
 				continue
 			}