docs: MappableJournalSegmentReader<E> createReader() {
      checkOpen();
      MappableJournalSegmentReader<E> reader = new MappableJournalSegmentReader<>(
          openChannel(file.file()), this, maxEntrySize, index, namespace);
      MappedByteBuffer buffer = writer.buffer();
      if (buffer != null) {
        reader.map(buffer);
      }
      readers.add(reader);
      return reader;
    }