public Path getOldPath() {
    long currentFilenum = this.filenum.get();
    Path oldPath = null;
    if (currentFilenum > 0) {
      // ComputeFilename will take care of meta wal filename
      oldPath = computeFilename(currentFilenum);
    } // I presume if currentFilenum is <= 0, this is first file and null for oldPath if fine?
    return oldPath;
  }
retrieve the next path to use for writing. Increments the internal filenum.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AbstractFSWAL.java