presto-main/src/main/java/io/prestosql/execution/scheduler/FixedSourcePartitionedScheduler.java/AsGroupedSourceScheduler/AsGroupedSourceScheduler
class AsGroupedSourceScheduler:        public AsGroupedSourceScheduler(SourceScheduler sourceScheduler)
        {
            this.sourceScheduler = requireNonNull(sourceScheduler, "sourceScheduler is null");
            pendingCompleted = new ArrayList<>();
        }