[ARROW-308] UnionListWriter.setPosition() should not call startList()

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.1.0
• Component/s: Java
• External issue URL: https://github.com/apache/arrow/issues/15730
• Assignee: Abdel Hakim Deneche
• Reporter: Abdel Hakim Deneche
• Created: 28/Sep/16 02:03
• Updated: 11/Jan/23 07:08
• Resolved: 29/Sep/16 21:36

Description

UnionListWriter.setPosition() is implemented as follows:

  @Override
  public void setPosition(int index) {
    super.setPosition(index);
    startList();
  }

It works fine, but if you run the following code:

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

    rootWriter.start();
    rootWriter.bigInt("int").writeBigInt(0);
    rootWriter.list("list").startList();
    rootWriter.list("list").bigInt().writeBigInt(0);
    rootWriter.list("list").endList();
    rootWriter.end();

    rootWriter.setPosition(1);
    rootWriter.start();
    rootWriter.bigInt("int").writeBigInt(1);
    rootWriter.end();

    rootWriter.setPosition(2);
    rootWriter.bigInt("int").writeBigInt(2);
    rootWriter.start();
    rootWriter.list("list").startList();
    rootWriter.list("list").bigInt().writeBigInt(2);
    rootWriter.list("list").endList();
    rootWriter.end();
    writer.setValueCount(3);

    for (int i = 0; i < 3; i++) {
      parent.getReader().setPosition(i);
      System.out.printf("%d: %s%n", i, parent.getReader().readObject());
    }

You get:

0: {"root":{"int":0,"list":[0]}}
1: {"root":{"int":1,"list":[]}}
2: {"root":{"int":2,"list":[2]}}

Even though we didn't write anything in the 2nd row "list", it shows up as empty instead of null. I tracked the problem to UnionListWriter.setPosition() calling startList() which marks the row as not null even if we don't write anything to it.