Detect unsent DISCONNECT messages

This change uses a ChannelInterceptor (inserted at index 0) to detect
when a DISCONNECT message is precluded from being sent on the
clientInboundChannel. This can happen if another interceptor allows
a runtime exception out from preSend or returns false.

It is crucial for such messages to be processed, so when detected
they're processed still.

Issue: SPR-12218

spring-messaging/src/main/java/org/springframework/messaging/simp/broker/AbstractBrokerMessageHandler.java

@@ -30,6 +30,11 @@ import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageHandler;
 import org.springframework.messaging.SubscribableChannel;
+import org.springframework.messaging.simp.SimpMessageHeaderAccessor;
+import org.springframework.messaging.simp.SimpMessageType;
+import org.springframework.messaging.support.AbstractMessageChannel;
+import org.springframework.messaging.support.ChannelInterceptor;
+import org.springframework.messaging.support.ChannelInterceptorAdapter;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 
@@ -69,6 +74,8 @@ public abstract class AbstractBrokerMessageHandler
 
 	private final Object lifecycleMonitor = new Object();
 
+	private ChannelInterceptor unsentDisconnectInterceptor = new UnsentDisconnectChannelInterceptor();
+
 
 	/**
 	 * Constructor with no destination prefixes (matches all destinations).
@@ -165,6 +172,9 @@ public abstract class AbstractBrokerMessageHandler
 			}
 			this.clientInboundChannel.subscribe(this);
 			this.brokerChannel.subscribe(this);
+			if (this.clientInboundChannel instanceof AbstractMessageChannel) {
+				((AbstractMessageChannel) this.clientInboundChannel).addInterceptor(0, this.unsentDisconnectInterceptor);
+			}
 			startInternal();
 			this.running = true;
 			if (logger.isInfoEnabled()) {
@@ -185,6 +195,9 @@ public abstract class AbstractBrokerMessageHandler
 			stopInternal();
 			this.clientInboundChannel.unsubscribe(this);
 			this.brokerChannel.unsubscribe(this);
+			if (this.clientInboundChannel instanceof AbstractMessageChannel) {
+				((AbstractMessageChannel) this.clientInboundChannel).removeInterceptor(this.unsentDisconnectInterceptor);
+			}
 			this.running = false;
 			if (logger.isDebugEnabled()) {
 				logger.info("Stopped.");
@@ -264,4 +277,21 @@ public abstract class AbstractBrokerMessageHandler
 		}
 	}
 
+
+	/**
+	 * Detect unsent DISCONNECT messages and process them anyway.
+	 */
+	private class UnsentDisconnectChannelInterceptor extends ChannelInterceptorAdapter {
+
+		@Override
+		public void afterSendCompletion(Message<?> message, MessageChannel channel, boolean sent, Exception ex) {
+			if (!sent) {
+				SimpMessageType messageType = SimpMessageHeaderAccessor.getMessageType(message.getHeaders());
+				if (SimpMessageType.DISCONNECT.equals(messageType)) {
+					logger.debug("Detected unsent DISCONNECT message. Processing anyway.");
+					handleMessage(message);
+				}
+			}
+		}
+	}
 }

spring-messaging/src/main/java/org/springframework/messaging/support/AbstractMessageChannel.java

@@ -81,6 +81,13 @@ public abstract class AbstractMessageChannel implements MessageChannel, BeanName
 		this.interceptors.add(interceptor);
 	}
 
+	/**
+	 * Add a channel interceptor at the specified index.
+	 */
+	public void addInterceptor(int index, ChannelInterceptor interceptor) {
+		this.interceptors.add(index, interceptor);
+	}
+
 	/**
 	 * Return a read-only list of the configured interceptors.
 	 */
@@ -88,6 +95,13 @@ public abstract class AbstractMessageChannel implements MessageChannel, BeanName
 		return Collections.unmodifiableList(this.interceptors);
 	}
 
+	/**
+	 * Remove the given interceptor.
+	 */
+	public boolean removeInterceptor(ChannelInterceptor interceptor) {
+		return this.interceptors.remove(interceptor);
+	}
+
 
 	@Override
 	public final boolean send(Message<?> message) {