allow ping text messages

this is required as browsers don't allow to directly send ping and receive pong messages. Therefore, the client is forced to send pings as text messages and pongs must be received as text messages. Otherwise, the client has no information if the connection is actually stale or still alive.

rest-api/src/main/java/tools/simrail/backend/api/event/EventWebsocketHandler.java

@@ -32,6 +32,7 @@
 import org.springframework.web.socket.CloseStatus;
 import org.springframework.web.socket.PingMessage;
 import org.springframework.web.socket.PongMessage;
+import org.springframework.web.socket.TextMessage;
 import org.springframework.web.socket.WebSocketHandler;
 import org.springframework.web.socket.WebSocketMessage;
 import org.springframework.web.socket.WebSocketSession;
@@ -54,8 +55,8 @@ public EventWebsocketHandler(@Nonnull EventSessionManager sessionManager) {
   @Override
   @SuppressWarnings("unchecked")
   public void afterConnectionEstablished(@Nonnull WebSocketSession session) {
-    // don't allow any data to be sent from the client
-    session.setTextMessageSizeLimit(0);
+    // don't allow any data to be sent from the client, except for ping messages
+    session.setTextMessageSizeLimit(4);
     session.setBinaryMessageSizeLimit(0);
 
     // get the frame types added by the handshake interceptor and register the session
@@ -79,6 +80,8 @@ public void handleMessage(@Nonnull WebSocketSession session, @Nonnull WebSocketM
       case PingMessage _ -> eventSession.sendPong();
       // received pong from the client, mark the client as alive
       case PongMessage _ -> eventSession.handlePongReceive();
+      // client sends a ping in form of a text message, respond with a pong text message
+      case TextMessage tm when tm.getPayload().equals("ping") -> eventSession.sendPongText();
       // reject all other type of websocket messages being sent by the client
       default -> session.close(CloseStatus.NOT_ACCEPTABLE);
     }

rest-api/src/main/java/tools/simrail/backend/api/event/session/EventWebsocketSession.java

@@ -85,6 +85,13 @@ public void sendPong() throws IOException {
     this.webSocketSession.sendMessage(new PongMessage());
   }
 
+  /**
+   * Sends out a text message with the data "pong".
+   */
+  public void sendPongText() throws IOException {
+    this.sendText("pong");
+  }
+
   /**
    * Sends the given text as a text frame to the websocket connection.
    *