presto-main/src/main/java/io/prestosql/operator/PartitionedLookupSourceFactory.java/PartitionedLookupSourceFactory/getOuterPositionIterator
class PartitionedLookupSourceFactory:    @Override
    public OuterPositionIterator getOuterPositionIterator()
    {
        TrackingLookupSourceSupplier lookupSourceSupplier;

        lock.writeLock().lock();
        try {
            checkState(this.lookupSourceSupplier != null, "lookup source not ready yet");
            lookupSourceSupplier = this.lookupSourceSupplier;
        }
        finally {
            lock.writeLock().unlock();
        }

        return lookupSourceSupplier.getOuterPositionIterator();
    }