Fix cache cleanup issue in DefaultSubscriptionRegistry

Issue: SPR-11931

Author: sdeleuze

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

@@ -96,7 +96,7 @@ protected void removeSubscriptionInternal(String sessionId, String subsId, Messa
 		SessionSubscriptionInfo info = this.subscriptionRegistry.getSubscriptions(sessionId);
 		if (info != null) {
 			String destination = info.removeSubscription(subsId);
-			if (destination != null && info.getSubscriptions(destination) == null) {
+			if (destination != null) {
 				this.destinationCache.updateAfterRemovedSubscription(destination, sessionId, subsId);
 			}
 		}

spring-messaging/src/test/java/org/springframework/messaging/simp/broker/DefaultSubscriptionRegistryTests.java

@@ -270,6 +270,22 @@ public void unregisterSubscription() {
 		assertEquals(subscriptionIds, sort(actual.get(sessIds.get(2))));
 	}
 
+	// SPR-11931
+
+	@Test
+	public void registerTwiceAndUnregisterSubscriptions() {
+
+		this.registry.registerSubscription(subscribeMessage("sess01", "subs01", "/foo"));
+		this.registry.registerSubscription(subscribeMessage("sess01", "subs02", "/foo"));
+		MultiValueMap<String, String> actual = this.registry.findSubscriptions(message("/foo"));
+		assertEquals("Expected 1 element", 1, actual.size());
+
+		this.registry.unregisterSubscription(unsubscribeMessage("sess01", "subs01"));
+		this.registry.unregisterSubscription(unsubscribeMessage("sess01", "subs02"));
+		actual = this.registry.findSubscriptions(message("/foo"));
+		assertEquals("Expected no element", 0, actual.size());
+	}
+
 	@Test
 	public void unregisterAllSubscriptions() {