Fix IVF extend list resize bug

cpp/include/cuvs/neighbors/common.hpp

@@ -853,6 +853,14 @@ using enable_if_valid_list_t = typename enable_if_valid_list<ListT, T>::type;
  *       `cuvs::neighbors::ivf_pq::helpers::resize_list` which handle type casting internally.
  */
 template <typename ListT>
+CUVS_EXPORT void resize_list(raft::resources const& res,
+                             std::shared_ptr<ListT>& orig_list,  // NOLINT
+                             const typename ListT::spec_type& spec,
+                             typename ListT::size_type new_used_size,
+                             typename ListT::size_type old_logical_size,
+                             typename ListT::size_type old_used_size);
+
+template <typename ListT>
 CUVS_EXPORT void resize_list(raft::resources const& res,
                              std::shared_ptr<ListT>& orig_list,  // NOLINT
                              const typename ListT::spec_type& spec,

cpp/src/neighbors/ivf_flat/ivf_flat_build.cuh

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2022-2026, NVIDIA CORPORATION.
+ * SPDX-FileCopyrightText: Copyright (c) 2022-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 
@@ -286,6 +286,7 @@ void extend(raft::resources const& handle,
                        lists[label],
                        list_device_spec,
                        new_list_sizes[label],
+                       old_list_sizes[label],
                        raft::Pow2<kIndexGroupSize>::roundUp(old_list_sizes[label]));
     }
   }

cpp/src/neighbors/ivf_list.cuh

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2024-2026, NVIDIA CORPORATION.
+ * SPDX-FileCopyrightText: Copyright (c) 2024-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 
@@ -66,13 +66,17 @@ CUVS_EXPORT void resize_list(raft::resources const& res,
                              std::shared_ptr<ListT>& orig_list,  // NOLINT
                              const typename ListT::spec_type& spec,
                              typename ListT::size_type new_used_size,
+                             typename ListT::size_type old_logical_size,
                              typename ListT::size_type old_used_size)
 {
+  // old_logical_size is the previous visible size from this index's list_sizes().
+  // old_used_size is the old allocation copy extent and may include padded slots
+  // required by interleaved list layouts.
   bool skip_resize = false;
   if (orig_list) {
     if (new_used_size <= orig_list->indices.extent(0)) {
-      auto shared_list_size = old_used_size;
-      if (new_used_size <= old_used_size ||
+      auto shared_list_size = old_logical_size;
+      if (new_used_size <= old_logical_size ||
           orig_list->size.compare_exchange_strong(shared_list_size, new_used_size)) {
         // We don't need to resize the list if:
         //  1. The list exists
@@ -104,6 +108,16 @@ CUVS_EXPORT void resize_list(raft::resources const& res,
   new_list.swap(orig_list);
 }
 
+template <typename ListT>
+CUVS_EXPORT void resize_list(raft::resources const& res,
+                             std::shared_ptr<ListT>& orig_list,  // NOLINT
+                             const typename ListT::spec_type& spec,
+                             typename ListT::size_type new_used_size,
+                             typename ListT::size_type old_used_size)
+{
+  resize_list(res, orig_list, spec, new_used_size, old_used_size, old_used_size);
+}
+
 template <typename ListT>
 enable_if_valid_list_t<ListT> serialize_list(const raft::resources& handle,
                                              std::ostream& os,

cpp/src/neighbors/ivf_sq/ivf_sq_build.cuh

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION.
+ * SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 
@@ -358,6 +358,7 @@ void extend_inplace(raft::resources const& handle,
                        lists[label],
                        list_device_spec,
                        new_list_sizes[label],
+                       old_list_sizes[label],
                        raft::Pow2<kIndexGroupSize>::roundUp(old_list_sizes[label]));
     }
   }

cpp/tests/neighbors/ann_ivf_flat/test_float_int64_t.cu

@@ -1,12 +1,18 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2024, NVIDIA CORPORATION.
+ * SPDX-FileCopyrightText: Copyright (c) 2024-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 
 #include <gtest/gtest.h>
 
 #include "../ann_ivf_flat.cuh"
 
+#include <raft/core/resource/cuda_stream.hpp>
+#include <raft/linalg/init.cuh>
+#include <raft/matrix/init.cuh>
+
+#include <optional>
+
 namespace cuvs::neighbors::ivf_flat {
 
 typedef AnnIVFFlatTest<float, float, int64_t> AnnIVFFlatTestF_float;
@@ -19,4 +25,65 @@ TEST_P(AnnIVFFlatTestF_float, AnnIVFFlat)
 
 INSTANTIATE_TEST_CASE_P(AnnIVFFlatTest, AnnIVFFlatTestF_float, ::testing::ValuesIn(inputs));
 
+TEST(AnnIVFFlatTest, RepeatedExtendCopyPreservesSharedListWithinCapacity)
+{
+  raft::resources handle;
+  auto stream = raft::resource::get_cuda_stream(handle);
+
+  constexpr int64_t base_rows = 100;
+  constexpr int64_t grow_rows = 20;
+  constexpr int64_t rows      = base_rows + grow_rows;
+  constexpr int64_t dim       = 4;
+
+  auto data = raft::make_device_matrix<float, int64_t>(handle, rows, dim);
+  raft::matrix::fill(handle, data.view(), 0.0f);
+
+  index_params params;
+  params.n_lists                        = 1;
+  params.metric                         = cuvs::distance::DistanceType::L2Expanded;
+  params.add_data_on_build              = false;
+  params.kmeans_trainset_fraction       = 1.0;
+  params.adaptive_centers               = false;
+  params.conservative_memory_allocation = false;
+
+  auto all_data_view =
+    raft::make_device_matrix_view<const float, int64_t>(data.data_handle(), rows, dim);
+  auto empty_index = build(handle, params, all_data_view);
+
+  auto base_data_view =
+    raft::make_device_matrix_view<const float, int64_t>(data.data_handle(), base_rows, dim);
+  auto base_index = extend(handle, base_data_view, std::nullopt, empty_index);
+  raft::resource::sync_stream(handle);
+
+  ASSERT_EQ(base_index.lists()[0]->get_size(), base_rows);
+  ASSERT_EQ(base_index.lists()[0]->indices_capacity(), raft::Pow2<kIndexGroupSize>::roundUp(rows));
+
+  auto indices = raft::make_device_vector<int64_t, int64_t>(handle, grow_rows);
+  raft::linalg::range(indices.data_handle(), base_rows, rows, stream);
+
+  auto grow_data_view = raft::make_device_matrix_view<const float, int64_t>(
+    data.data_handle() + base_rows * dim, grow_rows, dim);
+  auto grow_indices_view =
+    raft::make_device_vector_view<const int64_t, int64_t>(indices.data_handle(), grow_rows);
+  auto first_grown_index =
+    extend(handle,
+           grow_data_view,
+           std::make_optional<raft::device_vector_view<const int64_t, int64_t>>(grow_indices_view),
+           base_index);
+  raft::resource::sync_stream(handle);
+
+  ASSERT_EQ(first_grown_index.lists()[0].get(), base_index.lists()[0].get());
+  ASSERT_EQ(first_grown_index.lists()[0]->get_size(), rows);
+
+  auto second_grown_index =
+    extend(handle,
+           grow_data_view,
+           std::make_optional<raft::device_vector_view<const int64_t, int64_t>>(grow_indices_view),
+           base_index);
+  raft::resource::sync_stream(handle);
+
+  EXPECT_NE(first_grown_index.lists()[0].get(), second_grown_index.lists()[0].get());
+  EXPECT_EQ(second_grown_index.lists()[0]->get_size(), rows);
+}
+
 }  // namespace cuvs::neighbors::ivf_flat

cpp/tests/neighbors/ann_ivf_sq/test_float_int64_t.cu

@@ -1,17 +1,73 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION.
+ * SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 
 #include <gtest/gtest.h>
 
 #include "../ann_ivf_sq.cuh"
 
+#include <raft/core/resource/cuda_stream.hpp>
+#include <raft/linalg/init.cuh>
+#include <raft/matrix/init.cuh>
+
+#include <optional>
+
 namespace cuvs::neighbors::ivf_sq {
 
 typedef AnnIVFSQTest<float, float, int64_t> AnnIVFSQTestF_float;
 TEST_P(AnnIVFSQTestF_float, AnnIVFSQ) { this->testAll(); }
 
 INSTANTIATE_TEST_CASE_P(AnnIVFSQTest, AnnIVFSQTestF_float, ::testing::ValuesIn(inputs));
 
+TEST(AnnIVFSQTest, ExtendInPlaceUpdatesListSizeWithinCapacity)
+{
+  raft::resources handle;
+  auto stream = raft::resource::get_cuda_stream(handle);
+
+  constexpr int64_t base_rows = 100;
+  constexpr int64_t grow_rows = 20;
+  constexpr int64_t rows      = base_rows + grow_rows;
+  constexpr int64_t dim       = 4;
+
+  auto data = raft::make_device_matrix<float, int64_t>(handle, rows, dim);
+  raft::matrix::fill(handle, data.view(), 0.0f);
+
+  index_params params;
+  params.n_lists                        = 1;
+  params.metric                         = cuvs::distance::DistanceType::L2Expanded;
+  params.add_data_on_build              = false;
+  params.max_train_points_per_cluster   = 256;
+  params.conservative_memory_allocation = false;
+
+  auto all_data_view =
+    raft::make_device_matrix_view<const float, int64_t>(data.data_handle(), rows, dim);
+  auto index = build(handle, params, all_data_view);
+
+  auto base_data_view =
+    raft::make_device_matrix_view<const float, int64_t>(data.data_handle(), base_rows, dim);
+  extend(handle, base_data_view, std::nullopt, &index);
+  raft::resource::sync_stream(handle);
+
+  ASSERT_EQ(index.lists()[0]->get_size(), base_rows);
+  ASSERT_EQ(index.lists()[0]->indices_capacity(), raft::Pow2<kIndexGroupSize>::roundUp(rows));
+  auto* base_list = index.lists()[0].get();
+
+  auto indices = raft::make_device_vector<int64_t, int64_t>(handle, grow_rows);
+  raft::linalg::range(indices.data_handle(), base_rows, rows, stream);
+
+  auto grow_data_view = raft::make_device_matrix_view<const float, int64_t>(
+    data.data_handle() + base_rows * dim, grow_rows, dim);
+  auto grow_indices_view =
+    raft::make_device_vector_view<const int64_t, int64_t>(indices.data_handle(), grow_rows);
+  extend(handle,
+         grow_data_view,
+         std::make_optional<raft::device_vector_view<const int64_t, int64_t>>(grow_indices_view),
+         &index);
+  raft::resource::sync_stream(handle);
+
+  EXPECT_EQ(index.lists()[0].get(), base_list);
+  EXPECT_EQ(index.lists()[0]->get_size(), rows);
+}
+
 }  // namespace cuvs::neighbors::ivf_sq