Don't send expirations del events when expiration time does not change.

Expiration time is rounded up, so in case of many request per second from a
single session we can skip sending of delete event to reduce the traffic.
This is good because actually at the moment the redis repository is subscribed
for this events, even though it ignores them.

Fix spring-projectsgh-315

spring-session/src/main/java/org/springframework/session/data/redis/RedisSessionExpirationPolicy.java

@@ -68,14 +68,16 @@ public void onDelete(ExpiringSession session) {
 
 	public void onExpirationUpdated(Long originalExpirationTimeInMilli, ExpiringSession session) {
 		String keyToExpire = "expires:" + session.getId();
+		long toExpire = roundUpToNextMinute(expiresInMillis(session));
+
 		if(originalExpirationTimeInMilli != null) {
 			long originalRoundedUp = roundUpToNextMinute(originalExpirationTimeInMilli);
-			String expireKey = getExpirationKey(originalRoundedUp);
-			redis.boundSetOps(expireKey).remove(keyToExpire);
+			if(toExpire != originalRoundedUp) {
+				String expireKey = getExpirationKey(originalRoundedUp);
+				redis.boundSetOps(expireKey).remove(keyToExpire);
+			}
 		}
 
-		long toExpire = roundUpToNextMinute(expiresInMillis(session));
-
 		String expireKey = getExpirationKey(toExpire);
 		BoundSetOperations<Object, Object> expireOperations = redis.boundSetOps(expireKey);
 		expireOperations.add(keyToExpire);

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

@@ -59,7 +59,7 @@ public void setup() {
 		RedisOperationsSessionRepository repository = new RedisOperationsSessionRepository(sessionRedisOperations);
 		policy = new RedisSessionExpirationPolicy(sessionRedisOperations, repository);
 		session = new MapSession();
-		session.setLastAccessedTime(1429116694665L);
+		session.setLastAccessedTime(1429116694675L);
 		session.setId("12345");
 
 		when(sessionRedisOperations.boundSetOps(anyString())).thenReturn(setOperations);
@@ -81,6 +81,19 @@ public void onExpirationUpdatedRemovesOriginalExpirationTimeRoundedUp() throws E
 		verify(setOperations).remove("expires:"+ session.getId());
 	}
 
+	@Test
+	public void onExpirationUpdatedDoNotSendDeleteWhenExpirationTimeDoesNotChange() throws Exception {
+		long originalExpirationTimeInMs = RedisSessionExpirationPolicy.expiresInMillis(session) - 10;
+		long originalRoundedToNextMinInMs = RedisSessionExpirationPolicy.roundUpToNextMinute(originalExpirationTimeInMs);
+		String originalExpireKey = policy.getExpirationKey(originalRoundedToNextMinInMs);
+
+		policy.onExpirationUpdated(originalExpirationTimeInMs, session);
+
+		// verify the original is not removed
+		verify(sessionRedisOperations).boundSetOps(originalExpireKey);
+		verify(setOperations, never()).remove("expires:"+ session.getId());
+	}
+
 	@Test
 	public void onExpirationUpdatedAddsExpirationTimeRoundedUp() throws Exception {
 		long expirationTimeInMs = RedisSessionExpirationPolicy.expiresInMillis(session);