[Java] Incorrect getBufferSize for a dense union vector

[wotbrew]

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

DenseUnionVector.getBufferSizeFor takes a count parameter. My expectation is the count represents the number of elements in the union you wish to account for.

However, that count is passed directly to internalStruct.getBufferSizeFor, which I suspect is a bug.

This is because struct fields normally have the same valueCount, but this is not likely true when used in a union. If your children have different lengths:

• for fixed vectors, you will calculate the size of the leg contents incorrectly
• dynamic vectors like VarBinary may require reading buffer contents to find the data size, potentially causing an out-of-bounds dereference

Observed on 13.0.0 and 12.0.1.

Repro:

package org.apache.arrow.vector.complex;

import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
import static org.junit.jupiter.api.Assertions.assertTrue;

import java.util.Arrays;
import java.util.List;

import org.apache.arrow.memory.BoundsChecking;
import org.apache.arrow.memory.BufferAllocator;
import org.apache.arrow.memory.RootAllocator;
import org.apache.arrow.vector.BaseValueVector;
import org.apache.arrow.vector.complex.DenseUnionVector;
import org.apache.arrow.vector.holders.NullableIntHolder;
import org.apache.arrow.vector.holders.NullableVarBinaryHolder;
import org.apache.arrow.vector.types.UnionMode;
import org.apache.arrow.vector.types.pojo.ArrowType;
import org.apache.arrow.vector.types.pojo.Field;
import org.apache.arrow.vector.types.pojo.FieldType;
import org.junit.jupiter.api.Test;

public class TestDuvBufferCount {
  @Test
  public void testBufferSize() {
    try (BufferAllocator allocator = new RootAllocator();
         DenseUnionVector duv = new DenseUnionVector("duv", allocator,
                 FieldType.nullable(new ArrowType.Union(UnionMode.Dense, null)), null)) {

      List<Field> fields = Arrays.asList(
              new Field("a", FieldType.notNullable(new ArrowType.Int(32, true)), null),
              new Field("b", FieldType.notNullable(new ArrowType.Binary()), null)
      );

      duv.initializeChildrenFromFields(fields);

      NullableIntHolder nih = new NullableIntHolder();
      NullableVarBinaryHolder nvbh = new NullableVarBinaryHolder();

      int ac = BaseValueVector.INITIAL_VALUE_ALLOCATION + 1;
      for (int i = 0; i < ac; i++) {
        duv.setTypeId(i, (byte) 0);
        duv.setSafe(i, nih);
      }

      int bc = 1;
      for (int i = 0; i < bc; i++) {
        duv.setTypeId(i + ac, (byte) 1);
        duv.setSafe(i + ac, nvbh);
      }

      duv.setValueCount(ac + bc);

      // will not necessarily see an error unless bounds checking is on.
      assertTrue(BoundsChecking.BOUNDS_CHECKING_ENABLED);
      assertDoesNotThrow(duv::getBufferSize);
    }
  }
}

Component(s)

Java