From 19b01f94acf711032f1e03421f209f36146f9048 Mon Sep 17 00:00:00 2001
From: Jin Shang <shangjin1997@gmail.com>
Date: Sun, 20 Aug 2023 22:22:08 +0800
Subject: [PATCH] add util test

---
 cpp/src/arrow/array/array_test.cc | 13 +++++++++++++
 cpp/src/arrow/array/util.cc       |  2 +-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/cpp/src/arrow/array/array_test.cc b/cpp/src/arrow/array/array_test.cc
index e9d478f108584..2d33c2777a060 100644
--- a/cpp/src/arrow/array/array_test.cc
+++ b/cpp/src/arrow/array/array_test.cc
@@ -552,6 +552,19 @@ TEST_F(TestArray, TestMakeArrayOfNullUnion) {
       ASSERT_EQ(typed_union.raw_value_offsets()[i], 0);
     }
   }
+
+  // zero-length nested types should have union children with length zero
+  auto list_union = list(dense_union({field("a", utf8()), field("b", int32())}, {0, 1}));
+  ASSERT_OK_AND_ASSIGN(auto list_union_nulls, MakeArrayOfNull(list_union, 0));
+  ASSERT_OK(list_union_nulls->ValidateFull());
+  ASSERT_EQ(list_union_nulls->null_count(), 0);
+  {
+    const auto& typed_list = checked_cast<const ListArray&>(*list_union_nulls);
+    const auto& typed_union = checked_cast<const DenseUnionArray&>(*typed_list.values());
+    // Child field has length 0
+    ASSERT_EQ(typed_union.field(0)->length(), 0);
+    ASSERT_EQ(typed_union.field(0)->null_count(), 0);
+  }
 }
 
 TEST_F(TestArray, TestValidateNullCount) {
diff --git a/cpp/src/arrow/array/util.cc b/cpp/src/arrow/array/util.cc
index 25525e7a67ca0..150a7901a33ff 100644
--- a/cpp/src/arrow/array/util.cc
+++ b/cpp/src/arrow/array/util.cc
@@ -572,7 +572,7 @@ class NullArrayFactory {
     int64_t child_length = length_;
     if (type.mode() == UnionMode::DENSE) {
       // For dense unions, we set the offsets to all zero and create children
-      // with length 1
+      // with length 1 if length_ is non-zero
       out_->buffers.resize(3);
       out_->buffers[2] = buffer_;