I observed this while running org.apache.activemq.artemis.tests.integration.client.ConcurrentCreateDeleteProduceTest.testConcurrentProduceCreateAndDelete in loop:

java.lang.AssertionError: null is not a legitimate internal value. Concurrent Modification?
at io.netty.util.collection.IntObjectHashMap.toExternal(IntObjectHashMap.java:103) ~[netty-common-4.1.109.Final.jar:4.1.109.Final]
at io.netty.util.collection.IntObjectHashMap.access$900(IntObjectHashMap.java:37) ~[netty-common-4.1.109.Final.jar:4.1.109.Final]
at io.netty.util.collection.IntObjectHashMap$PrimitiveIterator.value(IntObjectHashMap.java:650) ~[netty-common-4.1.109.Final.jar:4.1.109.Final]
at io.netty.util.collection.IntObjectHashMap$2$1.next(IntObjectHashMap.java:234) ~[netty-common-4.1.109.Final.jar:4.1.109.Final]
at org.apache.activemq.artemis.core.paging.cursor.impl.PageSubscriptionImpl.destroy(PageSubscriptionImpl.java:642) ~[classes/:?]
at org.apache.activemq.artemis.core.server.impl.QueueImpl.destroyPaging(QueueImpl.java:2382) ~[classes/:?]
at org.apache.activemq.artemis.core.server.impl.QueueImpl.deleteQueue(QueueImpl.java:2441) ~[classes/:?]
at org.apache.activemq.artemis.core.server.impl.ActiveMQServerImpl.destroyQueue(ActiveMQServerImpl.java:2512) ~[classes/:?]
at org.apache.activemq.artemis.core.server.impl.ActiveMQServerImpl.destroyQueue(ActiveMQServerImpl.java:2461) ~[classes/:?]
at org.apache.activemq.artemis.core.server.impl.ServerSessionImpl.deleteQueue(ServerSessionImpl.java:1212) ~[classes/:?]
at org.apache.activemq.artemis.core.server.impl.ServerSessionImpl.deleteQueue(ServerSessionImpl.java:1196) ~[classes/:?]
at org.apache.activemq.artemis.core.protocol.core.ServerSessionPacketHandler.slowPacketHandler(ServerSessionPacketHandler.java:436) ~[classes/:?]

On the occasion I had a fix for ARTEMIS-4165 in place, but the issue seems orthogonal to me.

The cleanup methods are clearing the Netty collection out of abundance of caution (to help out GC). The fix here is just to stop doing that.

As part of this fix I'm also adding a check-leak test to make sure such objects are not leaking.