[GitHub] [arrow] bkietz commented on pull request #7181: ARROW-8799: [C++][Parquet] NestedListReader needs to handle empty item batches

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bkietz commented on pull request #7181:
URL: https://github.com/apache/arrow/pull/7181#issuecomment-636937045


   @emkornfield @wesm In adding a unit test I've become uncertain of the `ColumnReader` contract and whether my solution upholds it
   
   - [ColumnReader::NextBatch's doccomment](https://github.com/apache/arrow/blob/6716bbd/cpp/src/parquet/arrow/reader.h#L254-L255) states that when no data remains null should be yielded (which I read as: a null ChunkedArray).
   - Instead the tests [assert that the chunked array contain a single null chunk](https://github.com/apache/arrow/blob/6716bbd25ead03ad4774c8d1caa612a8f66e853c/cpp/src/parquet/arrow/arrow_reader_writer_test.cc#L502-L504)
   - When reading into a dictionary `LeafReader` does neither of these and instead yields an empty `ChunkedArray` (for which `NestedListReader` on master is unprepared, causing ARROW-8799).
   
   If modifying `NestedListReader` as I have here is unsatisfactory, I could change `TransferDictionary` to ensure `LeafReader` yields `ChunkedArray{nullptr}` when its out of data. What do you think?


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