From 61a32cc351dec508ae87e29b9607ca98e939d947 Mon Sep 17 00:00:00 2001
From: Alexander Taepper <alexander.taepper@gmail.com>
Date: Thu, 22 Oct 2026 11:00:05 +0200
Subject: [PATCH] fix extension array variadic buffer detection

---
 cpp/src/arrow/array/array_test.cc | 20 ++++++++++++++++++++
 cpp/src/arrow/array/data.cc       |  9 +++++++--
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/cpp/src/arrow/array/array_test.cc b/cpp/src/arrow/array/array_test.cc
index 2cdf94bfab95..55ac106e6907 100644
--- a/cpp/src/arrow/array/array_test.cc
+++ b/cpp/src/arrow/array/array_test.cc
@@ -1139,6 +1139,26 @@ TEST_F(TestArray, TestBinaryViewAppendArraySlice) {
   AssertArraysEqual(*src, *dst);
 }
 
+// GH-54924: ArraySpan::HasVariadicBuffers must look through extension types.
+TEST_F(TestArray, TestBinaryViewExtensionArraySpanRoundtrip) {
+  auto storage = ArrayFromJSON(
+      binary_view(),
+      R"([null, "short", "long string that is not inlined", "another somewhat long string"])");
+  auto extension_type = binary_view_extension_type();
+  auto extension_array = ExtensionType::WrapArray(extension_type, storage);
+  ASSERT_OK(extension_array->ValidateFull());
+
+  ArraySpan span(*extension_array->data());
+  ASSERT_TRUE(span.HasVariadicBuffers());
+  ASSERT_EQ(span.GetVariadicBuffers().size(), storage->data()->buffers.size() - 2);
+
+  auto roundtripped = span.ToArray();
+  ASSERT_OK(roundtripped->ValidateFull());
+  AssertArraysEqual(*extension_array, *roundtripped);
+  ASSERT_EQ(roundtripped->data()->buffers.size(),
+            extension_array->data()->buffers.size());
+}
+
 TEST_F(TestArray, ValidateBuffersPrimitive) {
   auto empty_buffer = std::make_shared<Buffer>("");
   auto null_buffer = Buffer::FromString("\xff");
diff --git a/cpp/src/arrow/array/data.cc b/cpp/src/arrow/array/data.cc
index edeb1c311906..5e34758c0bcc 100644
--- a/cpp/src/arrow/array/data.cc
+++ b/cpp/src/arrow/array/data.cc
@@ -321,7 +321,7 @@ void ArraySpan::SetMembers(const ArrayData& data) {
     this->buffers[i] = {};
   }
 
-  if (type_id == Type::STRING_VIEW || type_id == Type::BINARY_VIEW) {
+  if (HasVariadicBuffers()) {
     // store the span of data buffers in the third buffer
     this->buffers[2] = internal::PackVariadicBuffers(std::span(data.buffers).subspan(2));
   }
@@ -687,7 +687,12 @@ std::span<const std::shared_ptr<Buffer>> ArraySpan::GetVariadicBuffers() const {
 }
 
 bool ArraySpan::HasVariadicBuffers() const {
-  return type->id() == Type::BINARY_VIEW || type->id() == Type::STRING_VIEW;
+  Type::type type_id = type->id();
+  if (type_id == Type::EXTENSION) {
+    const auto* ext_type = checked_cast<const ExtensionType*>(type);
+    type_id = ext_type->storage_type()->id();
+  }
+  return type_id == Type::BINARY_VIEW || type_id == Type::STRING_VIEW;
 }
 
 std::shared_ptr<Array> ArraySpan::ToArray() const {