Fix race condition in transition from UNSUBSCRIBED->COMPLETED

- Ensure completion signal (normal/exception) will be delivered to
the subscriber when transition from UNSUBSCRIBED->COMPLETED

- According to the specification "Publisher.subscribe MUST call onSubscribe
on the provided Subscriber prior to any other signals to that Subscriber" so
ensure onComplete/onError signals will be called AFTER onSubscribe signal.

Issue: SPR-16207

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

@@ -52,6 +52,11 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 
 	private volatile long demand;
 
+	private volatile boolean publisherCompleted;
+
+	@Nullable
+	private volatile Throwable publisherError;
+
 	@SuppressWarnings("rawtypes")
 	private static final AtomicLongFieldUpdater<AbstractListenerReadPublisher> DEMAND_FIELD_UPDATER =
 			AtomicLongFieldUpdater.newUpdater(AbstractListenerReadPublisher.class, "demand");
@@ -208,15 +213,54 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 			<T> void subscribe(AbstractListenerReadPublisher<T> publisher, Subscriber<? super T> subscriber) {
 				Assert.notNull(publisher, "Publisher must not be null");
 				Assert.notNull(subscriber, "Subscriber must not be null");
-				if (publisher.changeState(this, NO_DEMAND)) {
+				if (publisher.changeState(this, SUBSCRIBING)) {
 					Subscription subscription = new ReadSubscription(publisher);
 					publisher.subscriber = subscriber;
 					subscriber.onSubscribe(subscription);
+					publisher.changeState(SUBSCRIBING, NO_DEMAND);
+					if (publisher.publisherCompleted) {
+						publisher.onAllDataRead();
+					}
+					Throwable publisherError = publisher.publisherError;
+					if (publisherError != null) {
+						publisher.onError(publisherError);
+					}
 				}
 				else {
 					throw new IllegalStateException(toString());
 				}
 			}
+
+			@Override
+			<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
+				publisher.publisherCompleted = true;
+			}
+
+			@Override
+			<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable t) {
+				publisher.publisherError = t;
+			}
+		},
+
+		SUBSCRIBING {
+			<T> void request(AbstractListenerReadPublisher<T> publisher, long n) {
+				if (Operators.validate(n)) {
+					Operators.addCap(DEMAND_FIELD_UPDATER, publisher, n);
+					if (publisher.changeState(this, DEMAND)) {
+						publisher.checkOnDataAvailable();
+					}
+				}
+			}
+
+			@Override
+			<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
+				publisher.publisherCompleted = true;
+			}
+
+			@Override
+			<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable t) {
+				publisher.publisherError = t;
+			}
 		},
 
 		/**

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

@@ -119,18 +119,17 @@ class WriteResultPublisher implements Publisher<Void> {
 			@Override
 			void subscribe(WriteResultPublisher publisher, Subscriber<? super Void> subscriber) {
 				Assert.notNull(subscriber, "Subscriber must not be null");
-				publisher.subscriber = subscriber;
-				if (publisher.changeState(this, SUBSCRIBED)) {
+				if (publisher.changeState(this, SUBSCRIBING)) {
 					Subscription subscription = new ResponseBodyWriteResultSubscription(publisher);
+					publisher.subscriber = subscriber;
 					subscriber.onSubscribe(subscription);
+					publisher.changeState(SUBSCRIBING, SUBSCRIBED);
 					if (publisher.publisherCompleted) {
 						publisher.publishComplete();
 					}
-					else {
-						Throwable publisherError = publisher.publisherError;
-						if (publisherError != null) {
-							publisher.publishError(publisherError);
-						}
+					Throwable publisherError = publisher.publisherError;
+					if (publisherError != null) {
+						publisher.publishError(publisherError);
 					}
 				}
 				else {
@@ -147,6 +146,21 @@ class WriteResultPublisher implements Publisher<Void> {
 			}
 		},
 
+		SUBSCRIBING {
+			@Override
+			void request(WriteResultPublisher publisher, long n) {
+				Operators.validate(n);
+			}
+			@Override
+			void publishComplete(WriteResultPublisher publisher) {
+				publisher.publisherCompleted = true;
+			}
+			@Override
+			void publishError(WriteResultPublisher publisher, Throwable t) {
+				publisher.publisherError = t;
+			}
+		},
+
 		SUBSCRIBED {
 			@Override
 			void request(WriteResultPublisher publisher, long n) {
@@ -183,14 +197,6 @@ class WriteResultPublisher implements Publisher<Void> {
 			void cancel(WriteResultPublisher publisher) {
 				// ignore
 			}
-			@Override
-			void publishComplete(WriteResultPublisher publisher) {
-				// ignore
-			}
-			@Override
-			void publishError(WriteResultPublisher publisher, Throwable t) {
-				// ignore
-			}
 		};
 
 		void subscribe(WriteResultPublisher publisher, Subscriber<? super Void> subscriber) {
@@ -208,11 +214,11 @@ class WriteResultPublisher implements Publisher<Void> {
 		}
 
 		void publishComplete(WriteResultPublisher publisher) {
-			throw new IllegalStateException(toString());
+			// ignore
 		}
 
 		void publishError(WriteResultPublisher publisher, Throwable t) {
-			throw new IllegalStateException(toString());
+			// ignore
 		}
 	}