@Override
    public boolean next(NullWritable key,
                        CompactorInputSplit compactorInputSplit) throws IOException {
      if (split != null) {
        compactorInputSplit.set(split);
        split = null;
        return true;
      }
      return false;
    }
This input format returns its own input split as a value.  This is because our splits
contain information needed to properly construct the writer.  Crazy, huh?
ql/src/java/org/apache/hadoop/hive/ql/txn/compactor/MRCompactor.java