handle null variadic buffers in C bridge

Author: fenfeng9

cpp/src/arrow/c/bridge.cc

@@ -608,12 +608,8 @@ struct ArrayExporter {
       export_.variadic_buffer_sizes_.resize(variadic_buffers.size());
       size_t i = 0;
       for (const auto& buf : variadic_buffers) {
-        if (buf == nullptr) {
-          return Status::Invalid("Cannot export array of type ", data->type->ToString(),
-                                 ": null variadic buffer at buffer index #", i + 2,
-                                 " (variadic buffer index #", i, ")");
-        }
-        export_.variadic_buffer_sizes_[i++] = buf->size();
+        // The C Data Interface allows null variadic buffer pointers with size 0.
+        export_.variadic_buffer_sizes_[i++] = buf == nullptr ? 0 : buf->size();
       }
       export_.buffers_.back() = export_.variadic_buffer_sizes_.data();
     }

cpp/src/arrow/c/bridge_test.cc

@@ -930,7 +930,8 @@ TEST_F(TestArrayExport, PrimitiveSliced) {
   TestPrimitive(factory);
 }
 
-TEST_F(TestArrayExport, RejectNullVariadicBuffers) {
+TEST_F(TestArrayExport, NullVariadicBuffers) {
+  // GH-49740: _export_to_c segmentation fault for binary_view array.
   for (const auto& type : {binary_view(), utf8_view()}) {
     auto arr =
         MakeArray(ArrayData::Make(type, /*length=*/2,
@@ -942,12 +943,14 @@ TEST_F(TestArrayExport, RejectNullVariadicBuffers) {
                                    nullptr}));
 
     struct ArrowArray c_export;
-    EXPECT_RAISES_WITH_MESSAGE_THAT(
-        Invalid,
-        ::testing::HasSubstr(
-            "Cannot export array of type " + type->ToString() +
-            ": null variadic buffer at buffer index #2 (variadic buffer index #0)"),
-        ExportArray(*arr, &c_export));
+    ASSERT_OK(ExportArray(*arr, &c_export));
+    ArrayExportGuard guard(&c_export);
+
+    ASSERT_EQ(c_export.n_buffers, 4);
+    ASSERT_EQ(c_export.buffers[2], nullptr);
+    ASSERT_NE(c_export.buffers[3], nullptr);
+    const auto* variadic_buffer_sizes = static_cast<const int64_t*>(c_export.buffers[3]);
+    ASSERT_EQ(variadic_buffer_sizes[0], 0);
   }
 }
 
@@ -3017,6 +3020,43 @@ TEST_F(TestArrayImport, String) {
   CheckImport(ArrayFromJSON(large_binary(), "[]"));
 }
 
+TEST_F(TestArrayImport, NullVariadicBuffers) {
+  // The C Data Interface allows null variadic buffer pointers with size 0.
+  // Import normalizes them to non-null zero-size buffers in Arrow C++.
+  std::vector<BinaryViewType::c_type> views = {
+      util::ToInlineBinaryView("hello"),
+      util::ToInlineBinaryView("world"),
+  };
+  constexpr int64_t null_variadic_buffer_sizes[] = {0};
+  const void* null_variadic_buffer[] = {
+      nullptr,
+      views.data(),
+      nullptr,
+      null_variadic_buffer_sizes,
+  };
+
+  for (const auto& type : {binary_view(), utf8_view()}) {
+    FillStringViewLike(/*length=*/2, /*null_count=*/0, /*offset=*/0, null_variadic_buffer,
+                       /*data_buffer_count=*/1);
+
+    ArrayReleaseCallback cb(&c_struct_);
+    ASSERT_OK_AND_ASSIGN(auto array, ImportArray(&c_struct_, type));
+    ASSERT_TRUE(ArrowArrayIsReleased(&c_struct_));
+    Reset();
+
+    ASSERT_OK(array->ValidateFull());
+    AssertArraysEqual(*ArrayFromJSON(type, R"(["hello", "world"])"), *array,
+                      /*verbose=*/true);
+
+    ASSERT_EQ(array->data()->buffers.size(), 3);
+    ASSERT_NE(array->data()->buffers[2], nullptr);
+    ASSERT_EQ(array->data()->buffers[2]->size(), 0);
+    cb.AssertNotCalled();
+    array.reset();
+    cb.AssertCalled();
+  }
+}
+
 TEST_F(TestArrayImport, StringWithOffset) {
   FillStringLike(3, 0, 1, string_buffers_no_nulls1);
   CheckImport(ArrayFromJSON(utf8(), R"(["", "bar", "quux"])"));