docs: public ListenableFuture<?> lendPagesSpatialIndex(Supplier<PagesSpatialIndex> pagesSpatialIndex)
      {
          requireNonNull(pagesSpatialIndex, "pagesSpatialIndex is null");
  
          if (activeProbeOperators.getFreeFuture().isDone()) {
              return NOT_BLOCKED;
          }
  
          List<SettableFuture<PagesSpatialIndex>> settableFutures;
          synchronized (this) {
              verify(this.pagesSpatialIndex == null);
              this.pagesSpatialIndex = pagesSpatialIndex;
              settableFutures = ImmutableList.copyOf(pagesSpatialIndexFutures);
              pagesSpatialIndexFutures.clear();
          }
  
          for (SettableFuture<PagesSpatialIndex> settableFuture : settableFutures) {
              settableFuture.set(pagesSpatialIndex.get());
          }
  
          return activeProbeOperators.getFreeFuture();
      }