From d1488ea3cba18951c0c1a9d33d4c44157a99eaad Mon Sep 17 00:00:00 2001
From: Minh Vu <vuhoangminh97@gmail.com>
Date: Sat, 25 Jul 2026 22:42:49 +0200
Subject: [PATCH] fix(arrow/array): ignore zero-count union appends

---
 arrow/array/union.go      |  8 ++++++++
 arrow/array/union_test.go | 24 ++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/arrow/array/union.go b/arrow/array/union.go
index 58524a768..2b6c55f17 100644
--- a/arrow/array/union.go
+++ b/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]
diff --git a/arrow/array/union_test.go b/arrow/array/union_test.go
index e20f02473..24c85552f 100644
--- a/arrow/array/union_test.go
+++ b/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{