PartitionSenderRootExec creates a PartitionerDecorator and saves it in the partitioner field. The creation of the partitioner happens in the createPartitioner method. This method get's called by the main fragment thread. The partitioner field is accessed by the fragment thread during normal execution but it can also be accessed by the receivingFragmentFinished method which is a callback executed by the event processor thread. Because multiple threads can access the partitioner field synchronization is done on creation and on when receivingFragmentFinished. However, the close method can also be called by the event processor thread, and the close method does not synchronize before accessing the partitioner field. Since synchronization is not done the event processor thread may have an old reference to the partitioner when a query cancellation is done. Since it has an old reference the current partitioner can may not be cleared and a memory leak may occur.