presto-main/src/main/java/io/prestosql/operator/index/IndexLookupSourceFactory.java/IndexLookupSourceFactory/whenBuildFinishes
class IndexLookupSourceFactory:    @Override
    public ListenableFuture<?> whenBuildFinishes()
    {
        return Futures.transformAsync(
                whenTaskContextSet,
                ignored -> 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()),
                directExecutor());
    }