presto-main/src/main/java/io/prestosql/operator/PartitionedLookupSource.java/OuterPositionTracker/getOuterPositionIterator
class OuterPositionTracker:            public OuterPositionIterator getOuterPositionIterator()
            {
                // touching atomic values ensures memory visibility between commit and getVisitedPositions
                verify(referenceCount.get() == 0);
                finished.set(true);
                return new PartitionedLookupOuterPositionIterator(lookupSources, visitedPositions);
            }