Fix guard against multiple subscriptions

This commit changes the guard against multiple subscriptions, as the
previously used doOnSubscribe hook could not function as guard in
certain scenarios.

Closes gh-32727

Author: poutsma

spring-web/src/main/java/org/springframework/http/client/reactive/AbstractClientHttpResponse.java

@@ -16,8 +16,12 @@
 
 package org.springframework.http.client.reactive;
 
+import java.util.Objects;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+import org.reactivestreams.Publisher;
+import org.reactivestreams.Subscriber;
+import org.reactivestreams.Subscription;
 import reactor.core.publisher.Flux;
 
 import org.springframework.core.io.buffer.DataBuffer;
@@ -55,16 +59,7 @@ protected AbstractClientHttpResponse(HttpStatusCode statusCode, HttpHeaders head
 		this.statusCode = statusCode;
 		this.headers = headers;
 		this.cookies = cookies;
-		this.body = singleSubscription(body);
-	}
-
-	private static Flux<DataBuffer> singleSubscription(Flux<DataBuffer> body) {
-		AtomicBoolean subscribed = new AtomicBoolean();
-		return body.doOnSubscribe(s -> {
-			if (!subscribed.compareAndSet(false, true)) {
-				throw new IllegalStateException("The client response body can only be consumed once");
-			}
-		});
+		this.body = Flux.from(new SingleSubscriberPublisher<>(body));
 	}
 
 
@@ -87,4 +82,39 @@ public MultiValueMap<String, ResponseCookie> getCookies() {
 	public Flux<DataBuffer> getBody() {
 		return this.body;
 	}
+
+
+	private static final class SingleSubscriberPublisher<T> implements Publisher<T> {
+
+		private static final Subscription NO_OP_SUBSCRIPTION = new Subscription() {
+			@Override
+			public void request(long l) {
+			}
+
+			@Override
+			public void cancel() {
+			}
+		};
+
+		private final Publisher<T> delegate;
+
+		private final AtomicBoolean subscribed = new AtomicBoolean();
+
+
+		public SingleSubscriberPublisher(Publisher<T> delegate) {
+			this.delegate = delegate;
+		}
+
+		@Override
+		public void subscribe(Subscriber<? super T> subscriber) {
+			Objects.requireNonNull(subscriber, "Subscriber must not be null");
+			if (this.subscribed.compareAndSet(false, true)) {
+				this.delegate.subscribe(subscriber);
+			}
+			else {
+				subscriber.onSubscribe(NO_OP_SUBSCRIPTION);
+				subscriber.onError(new IllegalStateException("The client response body can only be consumed once"));
+			}
+		}
+	}
 }