code: File findInProgressEditsFile(long startTxId)
    throws IOException {
      File ret = findFile(NameNodeDirType.EDITS,
          getInProgressEditsFileName(startTxId));
      if (ret == null) {
        throw new IOException(
            "No edits file for txid " + startTxId + "-in progress");
      }
      return ret;
    }
docs: Return the first readable inprogress edits file for the given txid.