code: private void setEndFileMarkerFile(String fileName) {
      int fileIndex = parseFileIndex(fileName);
      if (endFileNumber == -1) {
        // First time finding the end-marker file.
        endFileNumber = fileIndex;
        logger.atInfo().log("Found end-marker file '%s' with index %s", fileName, endFileNumber);
  
        // Sanity-check known filenames against the endFileNumber.
        for (String knownFile : knownFileSet) {
          int knownFileIndex = parseFileIndex(knownFile);
          Preconditions.checkState(
              knownFileIndex <= endFileNumber,
              "Found known file '%s' with index %s, which isn't less than or "
                  + "equal to than endFileNumber %s!",
              knownFile, knownFileIndex, endFileNumber);
        }
      } else {
        // If we found it before, make sure the file we're looking at has the same index.
        Preconditions.checkState(fileIndex == endFileNumber,
            "Found new end-marker file '%s' with index %s but already have endFileNumber %s!",
            fileName, fileIndex, endFileNumber);
      }
    }
docs: Record a specific file as being the 0-record end of stream marker.