WebSession creation does not block

Closes gh-24027

spring-web/src/main/java/org/springframework/web/server/session/InMemoryWebSessionStore.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -29,6 +29,7 @@ import java.util.concurrent.atomic.AtomicReference;
 import java.util.concurrent.locks.ReentrantLock;
 
 import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 import org.springframework.util.Assert;
 import org.springframework.util.IdGenerator;
@@ -111,9 +112,14 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 
 	@Override
 	public Mono<WebSession> createWebSession() {
+
+		// Opportunity to clean expired sessions
 		Instant now = this.clock.instant();
 		this.expiredSessionChecker.checkIfNecessary(now);
-		return Mono.fromSupplier(() -> new InMemoryWebSession(now));
+
+		return Mono.fromSupplier(() -> new InMemoryWebSession(now))
+				.subscribeOn(Schedulers.boundedElastic())
+				.cast(WebSession.class);
 	}
 
 	@Override

spring-web/src/test/java/org/springframework/web/server/session/InMemoryWebSessionStoreTests.java

@@ -22,7 +22,10 @@ import java.time.Instant;
 import java.util.Map;
 import java.util.stream.IntStream;
 
+import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.web.server.WebSession;
@@ -56,6 +59,14 @@ public class InMemoryWebSessionStoreTests {
 		assertThat(session.isStarted()).isTrue();
 	}
 
+	@Disabled // TODO: remove if/when Blockhound is enabled
+	@Test // gh-24027
+	public void createSessionDoesNotBlock() {
+		Mono.defer(() -> this.store.createWebSession())
+				.subscribeOn(Schedulers.parallel())
+				.block();
+	}
+
 	@Test
 	public void retrieveExpiredSession() {
 		WebSession session = this.store.createWebSession().block();