[ARROW-337] UnionListWriter.list() is doing more than it should, this can cause data corruption

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.2.0
• Component/s: Java
• External issue URL: https://github.com/apache/arrow/issues/15815
• Assignee: Abdel Hakim Deneche
• Reporter: Abdel Hakim Deneche
• Created: 14/Oct/16 17:44
• Updated: 11/Jan/23 07:08
• Resolved: 26/Oct/16 19:09

Description

If you run the following code:

    MapVector parent = new MapVector("parent", allocator, null);
    ComplexWriter writer = new ComplexWriterImpl("root", parent);
    MapWriter rootWriter = writer.rootAsMap();

    ListWriter listWriter = rootWriter.list("list");
    ListWriter list = listWriter.list();

    rootWriter.start();
    {
      listWriter.startList();
      {
        list.startList();
        list.bigInt().writeBigInt(0);
        list.endList();
      }
      {
        list.startList();
        list.bigInt().writeBigInt(1);
        list.endList();
      }
      listWriter.endList();
    }
    rootWriter.end();

    writer.setValueCount(1);

    MapReader rootReader = new SingleMapReaderImpl(parent).reader("root");

    System.out.println(rootReader.reader("list").readObject());

You should expect it to print

[[0],[1]]

but it actually prints

[[0,1]]

If you change the code so that UnionListWriter.list() is called along with startList() then the code works fine:

    MapVector parent = new MapVector("parent", allocator, null);
    ComplexWriter writer = new ComplexWriterImpl("root", parent);
    MapWriter rootWriter = writer.rootAsMap();

    rootWriter.start();
    {
      ListWriter listWriter = rootWriter.list("mylist");
      listWriter.startList();
      {
        ListWriter list = listWriter.list();
        list.startList();
        list.bigInt().writeBigInt(0);
        list.endList();
      }
      {
        ListWriter list = listWriter.list();
        list.startList();
        list.bigInt().writeBigInt(1);
        list.endList();
      }
      listWriter.endList();
    }
    rootWriter.end();

    writer.setValueCount(1);

    MapReader rootReader = new SingleMapReaderImpl(parent).reader("root");

    System.out.println(rootReader.reader("mylist").readObject());