Verify session existence before update in ReactiveRedisOperationsSessionRepository

Currently, ReactiveRedisOperationsSessionRepository#save does not ensure session's existence before executing update. This can result in an invalid session record in Redis, since write use only delta, and in turn to error while retrieving the invalid session record.

This commit adds check for session existence if session is being updated.

Closes gh-1111

spring-session-data-redis/src/integration-test/java/org/springframework/session/data/redis/ReactiveRedisOperationsSessionRepositoryITests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.session.data.redis;
 
+import java.time.Instant;
+
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
@@ -191,6 +193,28 @@ public class ReactiveRedisOperationsSessionRepositoryITests extends AbstractRedi
 		assertThat(this.repository.findById(originalId).block()).isNull();
 	}
 
+	// gh-1111
+	@Test
+	public void changeSessionSaveOldSessionInstance() {
+		ReactiveRedisOperationsSessionRepository.RedisSession toSave = this.repository
+				.createSession().block();
+		String sessionId = toSave.getId();
+
+		this.repository.save(toSave).block();
+
+		ReactiveRedisOperationsSessionRepository.RedisSession session = this.repository
+				.findById(sessionId).block();
+		session.changeSessionId();
+		session.setLastAccessedTime(Instant.now());
+		this.repository.save(session).block();
+
+		toSave.setLastAccessedTime(Instant.now());
+		this.repository.save(toSave).block();
+
+		assertThat(this.repository.findById(sessionId).block()).isNull();
+		assertThat(this.repository.findById(session.getId()).block()).isNotNull();
+	}
+
 	@Configuration
 	@EnableRedisWebSession
 	static class Config extends BaseConfig {