Use SESSION_NOT_RELIABLE when no messages received

When a WebSocket session is closed after not having received any
messages, we'll use SESSION_NOT_RELIABLE to indicate to other parts
of the session closing code not to send anything further (e.g.
SockJS "Go Away!" frame).

Issue: SPR-11884

spring-websocket/src/main/java/org/springframework/web/socket/CloseStatus.java

@@ -135,11 +135,10 @@ public final class CloseStatus {
 
 
 	/**
-	 * Indicates that a session has become unreliable (e.g. timed out while sending
-	 * a message) and extra care should be exercised while closing the session in
-	 * order to avoid locking additional threads.
-	 *
-	 * <p><strong>NOTE:</strong> Spring Framework specific status code.
+	 * A status code for use within the framework the indicate a session has
+	 * become unreliable (e.g. timed out while sending a message) and extra
+	 * care should be exercised, e.g. avoid sending any further data to the
+	 * client that may be done during normal shutdown.
 	 */
 	public static final CloseStatus SESSION_NOT_RELIABLE = new CloseStatus(4500);
 

spring-websocket/src/main/java/org/springframework/web/socket/messaging/SubProtocolWebSocketHandler.java

@@ -356,8 +356,13 @@ private String resolveSessionId(Message<?> message) {
 	}
 
 	/**
-	 * Periodically check sessions to ensure they have received at least one
-	 * message or otherwise close them.
+	 * When a session is connected through a higher-level protocol it has a chance
+	 * to use heartbeat management to shut down sessions that are too slow to send
+	 * or receive messages. However, after a WebSocketSession is established and
+	 * before the higher level protocol is fully connected there is a possibility
+	 * for sessions to hang. This method checks and closes any sessions that have
+	 * been connected for more than 60 seconds without having received a single
+	 * message.
 	 */
 	private void checkSessions() throws IOException {
 		long currentTime = System.currentTimeMillis();
@@ -380,7 +385,7 @@ private void checkSessions() throws IOException {
 								"Closing " + holder.getSession() + ".");
 					}
 					try {
-						session.close(CloseStatus.PROTOCOL_ERROR);
+						session.close(CloseStatus.SESSION_NOT_RELIABLE);
 					}
 					catch (Throwable t) {
 						logger.error("Failure while closing " + session, t);