Properly handle completion while in READING state

Closes gh-26834

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

@@ -68,10 +68,10 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 	@Nullable
 	private volatile Subscriber<? super T> subscriber;
 
-	private volatile boolean completionBeforeDemand;
+	private volatile boolean completionPending;
 
 	@Nullable
-	private volatile Throwable errorBeforeDemand;
+	private volatile Throwable errorPending;
 
 	private final String logPrefix;
 
@@ -228,21 +228,24 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 		}
 	}
 
-	private void handleCompletionOrErrorBeforeDemand() {
+	private boolean handlePendingCompletionOrError() {
 		State state = this.state.get();
-		if (!state.equals(State.UNSUBSCRIBED) && !state.equals(State.SUBSCRIBING)) {
-			if (this.completionBeforeDemand) {
-				rsReadLogger.trace(getLogPrefix() + "Completed before demand");
+		if (state.equals(State.DEMAND) || state.equals(State.NO_DEMAND)) {
+			if (this.completionPending) {
+				rsReadLogger.trace(getLogPrefix() + "Processing pending completion");
 				this.state.get().onAllDataRead(this);
+				return true;
 			}
-			Throwable ex = this.errorBeforeDemand;
+			Throwable ex = this.errorPending;
 			if (ex != null) {
 				if (rsReadLogger.isTraceEnabled()) {
-					rsReadLogger.trace(getLogPrefix() + "Completed with error before demand: " + ex);
+					rsReadLogger.trace(getLogPrefix() + "Processing pending completion with error: " + ex);
 				}
 				this.state.get().onError(this, ex);
+				return true;
 			}
 		}
+		return false;
 	}
 
 	private Subscription createSubscription() {
@@ -305,7 +308,7 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 					publisher.subscriber = subscriber;
 					subscriber.onSubscribe(subscription);
 					publisher.changeState(SUBSCRIBING, NO_DEMAND);
-					publisher.handleCompletionOrErrorBeforeDemand();
+					publisher.handlePendingCompletionOrError();
 				}
 				else {
 					throw new IllegalStateException("Failed to transition to SUBSCRIBING, " +
@@ -315,14 +318,14 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 
 			@Override
 			<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
-				publisher.completionBeforeDemand = true;
-				publisher.handleCompletionOrErrorBeforeDemand();
+				publisher.completionPending = true;
+				publisher.handlePendingCompletionOrError();
 			}
 
 			@Override
 			<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable ex) {
-				publisher.errorBeforeDemand = ex;
-				publisher.handleCompletionOrErrorBeforeDemand();
+				publisher.errorPending = ex;
+				publisher.handlePendingCompletionOrError();
 			}
 		},
 
@@ -341,14 +344,14 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 
 			@Override
 			<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
-				publisher.completionBeforeDemand = true;
-				publisher.handleCompletionOrErrorBeforeDemand();
+				publisher.completionPending = true;
+				publisher.handlePendingCompletionOrError();
 			}
 
 			@Override
 			<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable ex) {
-				publisher.errorBeforeDemand = ex;
-				publisher.handleCompletionOrErrorBeforeDemand();
+				publisher.errorPending = ex;
+				publisher.handlePendingCompletionOrError();
 			}
 		},
 
@@ -379,14 +382,17 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 						boolean demandAvailable = publisher.readAndPublish();
 						if (demandAvailable) {
 							publisher.changeToDemandState(READING);
+							publisher.handlePendingCompletionOrError();
 						}
 						else {
 							publisher.readingPaused();
 							if (publisher.changeState(READING, NO_DEMAND)) {
-								// Demand may have arrived since readAndPublish returned
-								long r = publisher.demand;
-								if (r > 0) {
-									publisher.changeToDemandState(NO_DEMAND);
+								if (!publisher.handlePendingCompletionOrError()) {
+									// Demand may have arrived since readAndPublish returned
+									long r = publisher.demand;
+									if (r > 0) {
+										publisher.changeToDemandState(NO_DEMAND);
+									}
 								}
 							}
 						}
@@ -408,6 +414,18 @@ public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {
 					publisher.changeToDemandState(NO_DEMAND);
 				}
 			}
+
+			@Override
+			<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
+				publisher.completionPending = true;
+				publisher.handlePendingCompletionOrError();
+			}
+
+			@Override
+			<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable ex) {
+				publisher.errorPending = ex;
+				publisher.handlePendingCompletionOrError();
+			}
 		},
 
 		COMPLETED {

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

@@ -151,10 +151,11 @@ public abstract class AbstractListenerWriteProcessor<T> implements Processor<T,
 	 * container.
 	 */
 	public final void onWritePossible() {
+		State state = this.state.get();
 		if (rsWriteLogger.isTraceEnabled()) {
-			rsWriteLogger.trace(getLogPrefix() + "onWritePossible");
+			rsWriteLogger.trace(getLogPrefix() + "onWritePossible [" + state + "]");
 		}
-		this.state.get().onWritePossible(this);
+		state.onWritePossible(this);
 	}
 
 	/**