Avro adapter: setPosition doesn't work with BaseVariableWidthVectors

[sap1ens]

Describe the bug, including details regarding any error messages, version, and platform.

BaseVariableWidthVector-based vectors (VarBinaryVector, VarCharVector) expect the data to be written sequentially. This means Consumer.setPosition is not safe to use. Imagine the following scenario:

• write index 0
• write index 2
• write index 1
• write index 3

On the last step, BaseVariableWidthVector.fillHoles erases the data written for the index 2:

  protected final void fillHoles(int index) {
    for (int i = lastSet + 1; i < index; i++) {
      setBytes(i, emptyByteArray, 0, emptyByteArray.length);
    }
    lastSet = index - 1;
  }

Here, index would be 3, lastSet + 1 would be 2 (1 + 1), so setBytes(2, emptyByteArray, ...) is called.

Not sure what the right solution is here, but at a minimum, the library shouldn't allow calling setPosition for Consumers using BaseVariableWidthVector vectors.

[lidavidm]

CC @martin-traverse (or are you two working together?)

[sap1ens]

or are you two working together?

nope :)