fix(arrow/array): ignore zero-count union appends

arrow/array/union.go

@@ -1201,6 +1201,10 @@ func (b *DenseUnionBuilder) AppendNull() {
 // for a DenseUnion this is more efficient than calling AppendNull multiple
 // times in a loop
 func (b *DenseUnionBuilder) AppendNulls(n int) {
+	if n <= 0 {
+		return
+	}
+
 	// only append 1 null to the child builder, use the same offset twice
 	firstChildCode := b.codes[0]
 	childBuilder := b.typeIDtoBuilder[firstChildCode]
@@ -1228,6 +1232,10 @@ func (b *DenseUnionBuilder) AppendEmptyValue() {
 // at that value using the offsets n times. That makes this more efficient
 // than calling AppendEmptyValue multiple times.
 func (b *DenseUnionBuilder) AppendEmptyValues(n int) {
+	if n <= 0 {
+		return
+	}
+
 	// only append 1 null to the child builder, use the same offset twice
 	firstChildCode := b.codes[0]
 	childBuilder := b.typeIDtoBuilder[firstChildCode]

arrow/array/union_test.go

@@ -1148,6 +1148,30 @@ func TestUnions(t *testing.T) {
 	suite.Run(t, new(UnionBuilderSuite))
 }
 
+func TestDenseUnionBuilderZeroBulkAppendDoesNotMutateChildren(t *testing.T) {
+	mem := memory.NewCheckedAllocator(memory.NewGoAllocator())
+	defer mem.AssertSize(t, 0)
+
+	typ := arrow.DenseUnionOf(
+		[]arrow.Field{{Name: "value", Type: arrow.PrimitiveTypes.Int32}},
+		[]arrow.UnionTypeCode{0},
+	)
+	builder := array.NewDenseUnionBuilder(mem, typ)
+	defer builder.Release()
+
+	builder.AppendNulls(0)
+	builder.AppendEmptyValues(0)
+	assert.Zero(t, builder.Len())
+	assert.Zero(t, builder.Child(0).Len())
+
+	builder.Append(0)
+	builder.Child(0).(*array.Int32Builder).Append(42)
+	result := builder.NewDenseUnionArray()
+	defer result.Release()
+	assert.EqualValues(t, 0, result.ValueOffset(0))
+	assert.EqualValues(t, 42, result.Field(0).(*array.Int32).Value(0))
+}
+
 func TestNestedUnionStructDict(t *testing.T) {
 	// ARROW-18274
 	dt1 := arrow.SparseUnionOf([]arrow.Field{