presto-main/src/main/java/io/prestosql/operator/LookupJoinOperator.java/JoinProcessor/isBlocked
class JoinProcessor:        private ListenableFuture<?> isBlocked()
        {
            if (!spillInProgress.isDone()) {
                // Input spilling can happen only after lookupSourceProviderFuture was done.
                return spillInProgress;
            }
            if (unspilledLookupSource.isPresent()) {
                // Unspilling can happen only after lookupSourceProviderFuture was done.
                return unspilledLookupSource.get();
            }

            if (finishing) {
                return NOT_BLOCKED;
            }

            return lookupSourceProviderFuture;
        }