Handle error in apply of writeFunction

Closes gh-23175

Author: rstoyanchev

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

@@ -181,7 +181,15 @@ public final void onNext(T item) {
 				else if (this.state == State.NEW) {
 					this.item = item;
 					this.state = State.FIRST_SIGNAL_RECEIVED;
-					writeFunction.apply(this).subscribe(this.writeCompletionBarrier);
+					Publisher<Void> result;
+					try {
+						result = writeFunction.apply(this);
+					}
+					catch (Throwable ex) {
+						this.writeCompletionBarrier.onError(ex);
+						return;
+					}
+					result.subscribe(this.writeCompletionBarrier);
 				}
 				else {
 					if (this.subscription != null) {
@@ -230,7 +238,15 @@ public final void onComplete() {
 				else if (this.state == State.NEW) {
 					this.completed = true;
 					this.state = State.FIRST_SIGNAL_RECEIVED;
-					writeFunction.apply(this).subscribe(this.writeCompletionBarrier);
+					Publisher<Void> result;
+					try {
+						result = writeFunction.apply(this);
+					}
+					catch (Throwable ex) {
+						this.writeCompletionBarrier.onError(ex);
+						return;
+					}
+					result.subscribe(this.writeCompletionBarrier);
 				}
 				else {
 					this.completed = true;

spring-web/src/test/java/org/springframework/http/server/reactive/ChannelSendOperatorTests.java

@@ -92,7 +92,7 @@ public void writeOneItem() throws Exception {
 
 
 	@Test
-	public void writeMultipleItems() throws Exception {
+	public void writeMultipleItems() {
 		List<String> items = Arrays.asList("one", "two", "three");
 		Mono<Void> completion = Flux.fromIterable(items).as(this::sendOperator);
 		Signal<Void> signal = completion.materialize().block();
@@ -108,7 +108,7 @@ public void writeMultipleItems() throws Exception {
 	}
 
 	@Test
-	public void errorAfterMultipleItems() throws Exception {
+	public void errorAfterMultipleItems() {
 		IllegalStateException error = new IllegalStateException("boo");
 		Flux<String> publisher = Flux.generate(() -> 0, (idx , subscriber) -> {
 			int i = ++idx;
@@ -213,6 +213,25 @@ public void errorFromWriteFunctionWhileItemCached() {
 		bufferFactory.checkForLeaks();
 	}
 
+	@Test // gh-23175
+	public void errorInWriteFunction() {
+
+		StepVerifier
+				.create(new ChannelSendOperator<>(Mono.just("one"), p -> {
+					throw new IllegalStateException("boo");
+				}))
+				.expectErrorMessage("boo")
+				.verify(Duration.ofMillis(5000));
+
+		StepVerifier
+				.create(new ChannelSendOperator<>(Mono.empty(), p -> {
+					throw new IllegalStateException("boo");
+				}))
+				.expectErrorMessage("boo")
+				.verify(Duration.ofMillis(5000));
+	}
+
+
 	private <T> Mono<Void> sendOperator(Publisher<String> source){
 		return new ChannelSendOperator<>(source, writer::send);
 	}