Release cached item in ChannelSendOperator

1. If the write Subscriber cancels with the item cached, release it.

2. If the write Publisher emits an error while the item is cached, when
the write Subscriber subscribes, release the cached item and emit the
error signal.

Closes gh-22720

spring-web/src/main/java/org/springframework/http/server/reactive/ChannelSendOperator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,6 +28,8 @@ import reactor.core.publisher.Mono;
 import reactor.core.publisher.Operators;
 import reactor.util.context.Context;
 
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
@@ -279,13 +281,20 @@ public class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 		}
 
 		private boolean emitCachedSignals() {
-			if (this.item != null) {
-				requiredWriteSubscriber().onNext(this.item);
-			}
 			if (this.error != null) {
-				requiredWriteSubscriber().onError(this.error);
+				try {
+					requiredWriteSubscriber().onError(this.error);
+				}
+				finally {
+					releaseCachedItem();
+				}
 				return true;
 			}
+			T item = this.item;
+			this.item = null;
+			if (item != null) {
+				requiredWriteSubscriber().onNext(item);
+			}
 			if (this.completed) {
 				requiredWriteSubscriber().onComplete();
 				return true;
@@ -298,7 +307,22 @@ public class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 			Subscription s = this.subscription;
 			if (s != null) {
 				this.subscription = null;
-				s.cancel();
+				try {
+					s.cancel();
+				}
+				finally {
+					releaseCachedItem();
+				}
+			}
+		}
+
+		private void releaseCachedItem() {
+			synchronized (this) {
+				Object item = this.item;
+				if (item instanceof DataBuffer) {
+					DataBufferUtils.release((DataBuffer) item);
+				}
+				this.item = null;
 			}
 		}