presto-main/src/main/java/io/prestosql/operator/WorkProcessor.java/WorkProcessor/ProcessState
class WorkProcessor:        private ProcessState(Type type, @Nullable T result, @Nullable ListenableFuture<?> blocked)
        {
            this.type = requireNonNull(type, "type is null");
            this.result = result;
            this.blocked = blocked;
        }