Update eviction queue before size

See gh-25298

Author: rstoyanchev

spring-messaging/src/main/java/org/springframework/messaging/simp/broker/DefaultSubscriptionRegistry.java

@@ -263,8 +263,9 @@ public LinkedMultiValueMap<String, String> getSubscriptions(String destination)
 			if (sessionIdToSubscriptionIds == null) {
 				sessionIdToSubscriptionIds = this.destinationCache.computeIfAbsent(destination, _destination -> {
 					LinkedMultiValueMap<String, String> matches = computeMatchingSubscriptions(destination);
-					this.cacheSize.incrementAndGet();
+					// Update queue first, so that cacheSize <= queue.size(
 					this.cacheEvictionPolicy.add(destination);
+					this.cacheSize.incrementAndGet();
 					return matches;
 				});
 				ensureCacheLimit();
@@ -309,7 +310,9 @@ private void ensureCacheLimit() {
 			if (size > cacheLimit) {
 				do {
 					if (this.cacheSize.compareAndSet(size, size - 1)) {
-						this.destinationCache.remove(this.cacheEvictionPolicy.poll());
+						// Remove (vs poll): we expect an element
+						String head = this.cacheEvictionPolicy.remove();
+						this.destinationCache.remove(head);
 					}
 				} while ((size = this.cacheSize.get()) > cacheLimit);
 			}