presto-main/src/main/java/io/prestosql/operator/PagesSpatialIndexFactory.java/PagesSpatialIndexFactory/createPagesSpatialIndex
class PagesSpatialIndexFactory:    public synchronized ListenableFuture<PagesSpatialIndex> createPagesSpatialIndex()
    {
        checkState(!destroyed.isDone(), "already destroyed");
        if (pagesSpatialIndex != null) {
            return immediateFuture(pagesSpatialIndex.get());
        }

        SettableFuture<PagesSpatialIndex> future = SettableFuture.create();
        pagesSpatialIndexFutures.add(future);
        return future;
    }