Avoid duplicate upstream subscription during reactive cache put

This commit fixes an issue where a Cacheable method which returns a
Flux (or multi-value publisher) will be invoked once, but the returned
publisher is actually subscribed twice.

By using the Reactor `tap` operator, we ensure that we can emit values
downstream AND accumulate emitted values into the List with a single
subscription.

The SignalListener additionally handles scenarios involving cancel,
for instance in case of a `take(1)` in the chain. In that case values
emitted up until that point will have been stored into the List buffer,
so we can still put it in the cache. In case of error, no caching occurs
and the internal buffer is cleared. This implementation also protects
against competing onComplete/onError signals and cancel signals.

Closes gh-32370

spring-context/src/test/java/org/springframework/cache/annotation/ReactiveCachingTests.java

@@ -61,7 +61,7 @@ class ReactiveCachingTests {
 		Long r3 = service.cacheFuture(key).join();
 
 		assertThat(r1).isNotNull();
-		assertThat(r1).isSameAs(r2).isSameAs(r3);
+		assertThat(r1).as("cacheFuture").isSameAs(r2).isSameAs(r3);
 
 		key = new Object();
 
@@ -70,7 +70,7 @@ class ReactiveCachingTests {
 		r3 = service.cacheMono(key).block();
 
 		assertThat(r1).isNotNull();
-		assertThat(r1).isSameAs(r2).isSameAs(r3);
+		assertThat(r1).as("cacheMono").isSameAs(r2).isSameAs(r3);
 
 		key = new Object();
 
@@ -79,7 +79,7 @@ class ReactiveCachingTests {
 		r3 = service.cacheFlux(key).blockFirst();
 
 		assertThat(r1).isNotNull();
-		assertThat(r1).isSameAs(r2).isSameAs(r3);
+		assertThat(r1).as("cacheFlux blockFirst").isSameAs(r2).isSameAs(r3);
 
 		key = new Object();
 
@@ -88,7 +88,7 @@ class ReactiveCachingTests {
 		List<Long> l3 = service.cacheFlux(key).collectList().block();
 
 		assertThat(l1).isNotNull();
-		assertThat(l1).isEqualTo(l2).isEqualTo(l3);
+		assertThat(l1).as("cacheFlux collectList").isEqualTo(l2).isEqualTo(l3);
 
 		key = new Object();
 
@@ -97,7 +97,7 @@ class ReactiveCachingTests {
 		r3 = service.cacheMono(key).block();
 
 		assertThat(r1).isNotNull();
-		assertThat(r1).isSameAs(r2).isSameAs(r3);
+		assertThat(r1).as("cacheMono common key").isSameAs(r2).isSameAs(r3);
 
 		// Same key as for Mono, reusing its cached value
 
@@ -106,12 +106,11 @@ class ReactiveCachingTests {
 		r3 = service.cacheFlux(key).blockFirst();
 
 		assertThat(r1).isNotNull();
-		assertThat(r1).isSameAs(r2).isSameAs(r3);
+		assertThat(r1).as("cacheFlux blockFirst common key").isSameAs(r2).isSameAs(r3);
 
 		ctx.close();
 	}
 
-
 	@CacheConfig(cacheNames = "first")
 	static class ReactiveCacheableService {
 
@@ -124,12 +123,16 @@ class ReactiveCachingTests {
 
 		@Cacheable
 		Mono<Long> cacheMono(Object arg) {
-			return Mono.just(this.counter.getAndIncrement());
+			// here counter not only reflects invocations of cacheMono but subscriptions to
+			// the returned Mono as well. See https://github.com/spring-projects/spring-framework/issues/32370
+			return Mono.defer(() -> Mono.just(this.counter.getAndIncrement()));
 		}
 
 		@Cacheable
 		Flux<Long> cacheFlux(Object arg) {
-			return Flux.just(this.counter.getAndIncrement(), 0L);
+			// here counter not only reflects invocations of cacheFlux but subscriptions to
+			// the returned Flux as well. See https://github.com/spring-projects/spring-framework/issues/32370
+			return Flux.defer(() -> Flux.just(this.counter.getAndIncrement(), 0L));
 		}
 	}