Improve logging in STOMP broker relay

Ignore DISCONNECT messages if already disconnected. This can occur if
the client explicitly sends a DISCONNECT frame and then closes the
socket quickly. The closing of the WebSocket sessions also sends a
DISCONNECT upstream to ensure the broker is aware.

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompBrokerRelayMessageHandler.java

@@ -18,6 +18,7 @@ package org.springframework.messaging.simp.stomp;
 
 import java.util.Collection;
 import java.util.Map;
+import java.util.concurrent.Callable;
 import java.util.concurrent.ConcurrentHashMap;
 
 import org.springframework.messaging.Message;
@@ -36,6 +37,7 @@ import org.springframework.messaging.tcp.reactor.ReactorTcpClient;
 import org.springframework.util.Assert;
 import org.springframework.util.concurrent.ListenableFuture;
 import org.springframework.util.concurrent.ListenableFutureCallback;
+import org.springframework.util.concurrent.ListenableFutureTask;
 
 /**
  * A {@link org.springframework.messaging.MessageHandler} that handles messages by forwarding them to a STOMP broker.
@@ -68,6 +70,8 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 
 	private static final byte[] EMPTY_PAYLOAD = new byte[0];
 
+	private static final ListenableFutureTask<Void> EMPTY_TASK = new ListenableFutureTask<Void>(new VoidCallable());
+
 	// STOMP recommends error of margin for receiving heartbeats
 	private static final long HEARTBEAT_MULTIPLIER = 3;
 
@@ -627,6 +631,9 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 
 			if (!this.isStompConnected) {
 				if (this.isRemoteClientSession) {
+					if (StompCommand.DISCONNECT.equals(StompHeaderAccessor.wrap(message).getCommand())) {
+						return EMPTY_TASK;
+					}
 					// Should never happen
 					throw new IllegalStateException("Unexpected client message " + message +
 							(this.tcpConnection != null ?
@@ -681,16 +688,6 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 		 */
 		public void clearConnection() {
 
-			this.isStompConnected = false;
-
-			try {
-				TcpConnection<byte[]> conn = this.tcpConnection;
-				this.tcpConnection = null;
-				if (conn != null) {
-					conn.close();
-				}
-			}
-			finally {
 			if (this.isRemoteClientSession) {
 				if (logger.isDebugEnabled()) {
 					logger.debug("Removing session '" + sessionId + "' (total remaining=" +
@@ -698,6 +695,13 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 				}
 				StompBrokerRelayMessageHandler.this.connectionHandlers.remove(this.sessionId);
 			}
+
+			this.isStompConnected = false;
+
+			TcpConnection<byte[]> conn = this.tcpConnection;
+			this.tcpConnection = null;
+			if (conn != null) {
+				conn.close();
 			}
 		}
 
@@ -754,4 +758,12 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 		}
 	}
 
+	private static class VoidCallable implements Callable<Void> {
+
+		@Override
+		public Void call() throws Exception {
+			return null;
+		}
+	}
+
 }