Fix SwitchOnFirst hanging subscription

reactor-core/src/main/java/reactor/core/publisher/FluxSwitchOnFirst.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2023 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2018-2024 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -716,7 +716,7 @@ public final void subscribe(CoreSubscriber<? super T> inboundSubscriber) {
 				return;
 			}
 
-			if (hasInboundClosedPrematurely(previousState)) {
+			if (hasInboundCancelled(previousState) ||hasInboundClosedPrematurely(previousState)) {
 				Operators.error(inboundSubscriber, new CancellationException("FluxSwitchOnFirst has already been cancelled"));
 				return;
 			}

reactor-core/src/test/java/reactor/core/publisher/FluxSwitchOnFirstTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2022 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2018-2024 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -45,6 +45,7 @@
 import reactor.core.scheduler.Schedulers;
 import reactor.test.MockUtils;
 import reactor.test.StepVerifier;
+import reactor.test.StepVerifierOptions;
 import reactor.test.publisher.TestPublisher;
 import reactor.test.scheduler.VirtualTimeScheduler;
 import reactor.test.util.RaceTestUtils;
@@ -1386,6 +1387,35 @@ public void shouldCancelSourceOnOnDownstreamTerminal() {
         assertThat(testPublisher.wasCancelled()).isTrue();
     }
 
+	@Test
+	void shouldErrorWhenResubscribingAfterCancel() {
+		TestPublisher<Long> testPublisher = TestPublisher.create();
+
+		AtomicReference<Flux<Long>> intercepted = new AtomicReference<>();
+
+		Flux<Flux<Long>> flux =
+				testPublisher.flux().switchOnFirst((s, f) -> {
+					intercepted.set(f);
+					// Due to wrapping we can avoid subscribing to f and subscribe later
+					// (re-subscribing is not allowed)
+					return Mono.just(f);
+				});
+
+		StepVerifier.create(flux)
+		            .expectSubscription()
+		            .then(() -> testPublisher.next(1L))
+					.thenCancel()
+		            .verify(Duration.ofSeconds(2));
+
+		Flux<Long> switchOnFirstMain = intercepted.get();
+
+		StepVerifier.create(switchOnFirstMain,
+				            StepVerifierOptions.create()
+				                               .scenarioName("Expect immediate onError from SwitchOnFirstMain as the outer operator has completed"))
+		            .expectError(IllegalStateException.class)
+		            .verify(Duration.ofSeconds(1));
+	}
+
     @Test
     public void scanOperator(){
     	Flux<Integer> parent = Flux.just(1);