presto-main/src/main/java/io/prestosql/operator/WorkProcessor.java/TransformationState/blocked
class TransformationState:        public static <T> TransformationState<T> blocked(ListenableFuture<?> blocked)
        {
            return new TransformationState<>(Type.BLOCKED, false, null, requireNonNull(blocked, "blocked is null"));
        }