presto-main/src/main/java/io/prestosql/operator/PartitionedLookupSourceFactory.java/PartitionedLookupSourceFactory/whenBuildFinishes
class PartitionedLookupSourceFactory:    @Override
    public ListenableFuture<?> whenBuildFinishes()
    {
        return transform(
                this.createLookupSourceProvider(),
                lookupSourceProvider -> {
                    // Close the lookupSourceProvider we just created.
                    // The only reason we created it is to wait until lookup source is ready.
                    lookupSourceProvider.close();
                    return null;
                },
                directExecutor());
    }