docs: private DirectoryLoaderAdaptor getDirectory(final String indexName) {
        DirectoryLoaderAdaptor adapter = openDirectories.get(indexName);
        if (adapter == null) {
           synchronized (openDirectories) {
              adapter = openDirectories.get(indexName);
              if (adapter == null) {
                 final File path = new File(this.rootDirectory, indexName);
                 final FSDirectory directory = openLuceneDirectory(path);
                 adapter = new DirectoryLoaderAdaptor(directory, indexName, autoChunkSize, affinitySegmentId);
                 openDirectories.put(indexName, adapter);
              }
           }
        }
        return adapter;
     }