Use Reactor's new Schedulers.boundedElastic()

Prior to this commit, Spring Boot would use `Schedulers.elastic()` when
required to process blocking tasks in a reactive environment.
reactor/reactor-core#1804 introduced a new scheduler,
`Schedulers.boundedElastic()` that behaves quite similarly but:

* will limit the number of workers thread
* will queue tasks if no worker thread is available and reject them is
the queue is exceeds a limit

This allows Spring Boot to schedule blocking tasks as before and allows
greater flexibility.

Fixes gh-18269
See gh-18276

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/web/reactive/AbstractWebFluxEndpointHandlerMapping.java

@@ -223,7 +223,8 @@ public abstract class AbstractWebFluxEndpointHandlerMapping extends RequestMappi
 
 	/**
 	 * An {@link OperationInvoker} that performs the invocation of a blocking operation on
-	 * a separate thread using Reactor's {@link Schedulers#elastic() elastic scheduler}.
+	 * a separate thread using Reactor's {@link Schedulers#boundedElastic() bounded
+	 * elastic scheduler}.
 	 */
 	protected static final class ElasticSchedulerInvoker implements OperationInvoker {
 
@@ -235,7 +236,7 @@ public abstract class AbstractWebFluxEndpointHandlerMapping extends RequestMappi
 
 		@Override
 		public Object invoke(InvocationContext context) {
-			return Mono.fromCallable(() -> this.invoker.invoke(context)).subscribeOn(Schedulers.elastic());
+			return Mono.fromCallable(() -> this.invoker.invoke(context)).subscribeOn(Schedulers.boundedElastic());
 		}
 
 	}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthIndicatorReactiveAdapter.java

@@ -43,7 +43,7 @@ public class HealthIndicatorReactiveAdapter implements ReactiveHealthIndicator {
 
 	@Override
 	public Mono<Health> health() {
-		return Mono.fromCallable(this.delegate::health).subscribeOn(Schedulers.elastic());
+		return Mono.fromCallable(this.delegate::health).subscribeOn(Schedulers.boundedElastic());
 	}
 
 }

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/redis/RedisReactiveHealthIndicator.java

@@ -56,7 +56,8 @@ public class RedisReactiveHealthIndicator extends AbstractReactiveHealthIndicato
 	}
 
 	private Mono<ReactiveRedisConnection> getConnection() {
-		return Mono.fromSupplier(this.connectionFactory::getReactiveConnection).subscribeOn(Schedulers.parallel());
+		return Mono.fromSupplier(this.connectionFactory::getReactiveConnection)
+				.subscribeOn(Schedulers.boundedElastic());
 	}
 
 	private Health up(Health.Builder builder, Properties info) {

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -171,7 +171,7 @@
 		<quartz.version>2.3.1</quartz.version>
 		<querydsl.version>4.2.1</querydsl.version>
 		<rabbit-amqp-client.version>5.7.3</rabbit-amqp-client.version>
-		<reactor-bom.version>Dysprosium-RC1</reactor-bom.version>
+		<reactor-bom.version>Dysprosium-BUILD-SNAPSHOT</reactor-bom.version>
 		<rest-assured.version>3.3.0</rest-assured.version>
 		<reactive-streams.version>1.0.3</reactive-streams.version>
 		<rsocket.version>1.0.0-RC3</rsocket.version>