Fix IVF extend list resize bug

[qwertyforce]

Description

Fixes a bug in IVF-Flat and IVF-SQ extend() where a list could receive new vectors but keep its old logical list::size.

The issue happens when extending a list that already has enough allocated capacity. Interleaved IVF list storage may be padded, so the old copy extent can be larger than the old logical size.

Example:

old logical size: 100
old padded copy size: 128
new logical size: 120

Previously, resize_list() received only the padded value, 128, and used it both for copying old storage and for deciding whether list::size needed to be updated. It checked 120 <= 128, skipped the update, and left list::size == 100 even though new vectors were inserted up to 120.

This stale size can break the shared-list / copy-on-write logic in later extends and corrupt search results.

Fix

Pass separate old sizes to ivf::resize_list():

old_logical_size: used for the list::size update/CAS decision
old_used_size:    used for copying old storage, may be padded

The existing overload is kept for call sites where both sizes are the same.

Tests

Added regression tests for:

IVF-Flat repeated extend from a shared base index with enough existing capacity
IVF-SQ in-place extend with enough existing capacity

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[achirkin]

old_used_size is the old logical size. old real size is checked via extends.

[qwertyforce]

old_used_size is the old logical size. old real size is checked via extends.

in ivf-flat extend, resize_list() receives raft::Pow2<kIndexGroupSize>::roundUp(old_list_sizes[label]) as an argument, right?

[qwertyforce]

here is poc

// ivf_flat_extend_size_poc.cpp

#include <cuvs/neighbors/ivf_flat.hpp>

#include <raft/core/copy.cuh>
#include <raft/core/device_mdarray.hpp>
#include <raft/core/device_mdspan.hpp>
#include <raft/core/resource/cuda_stream.hpp>
#include <raft/core/resources.hpp>

#include <cstdint>
#include <iostream>
#include <numeric>
#include <optional>
#include <vector>

namespace ivf_flat = cuvs::neighbors::ivf_flat;

template <typename IndexT>
uint32_t visible_list_size(raft::resources const& handle, IndexT& index)
{
  auto stream = raft::resource::get_cuda_stream(handle);
  uint32_t h = 0;
  raft::copy(&h, index.list_sizes().data_handle(), 1, stream);
  raft::resource::sync_stream(handle);
  return h;
}

template <typename IndexT>
void print_sizes(char const* tag, raft::resources const& handle, IndexT& index)
{
  std::cout << tag << "\n"
            << "  index.list_sizes()[0] = " << visible_list_size(handle, index) << "\n"
            << "  list::size            = " << index.lists()[0]->get_size() << "\n"
            << "  capacity              = " << index.lists()[0]->indices_capacity() << "\n";
}

int main()
{
  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;

  std::vector<float> h_data(rows * dim);
  for (int64_t r = 0; r < rows; ++r) {
    for (int64_t c = 0; c < dim; ++c) {
      h_data[r * dim + c] = static_cast<float>(r + c);
    }
  }

  auto data = raft::make_device_matrix<float, int64_t>(handle, rows, dim);
  raft::copy(data.data_handle(), h_data.data(), h_data.size(), stream);

  ivf_flat::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_view = raft::make_device_matrix_view<const float, int64_t, raft::row_major>(
    data.data_handle(), rows, dim);

  auto empty_index = ivf_flat::build(handle, params, all_view);

  auto base_view = raft::make_device_matrix_view<const float, int64_t, raft::row_major>(
    data.data_handle(), base_rows, dim);

  auto base_index = ivf_flat::extend(handle, base_view, std::nullopt, empty_index);
  raft::resource::sync_stream(handle);

  print_sizes("after base extend", handle, base_index);

  std::vector<int64_t> h_ids(grow_rows);
  std::iota(h_ids.begin(), h_ids.end(), base_rows);

  auto ids = raft::make_device_vector<int64_t, int64_t>(handle, grow_rows);
  raft::copy(ids.data_handle(), h_ids.data(), h_ids.size(), stream);

  auto grow_view = raft::make_device_matrix_view<const float, int64_t, raft::row_major>(
    data.data_handle() + base_rows * dim, grow_rows, dim);

  auto ids_view =
    raft::make_device_vector_view<const int64_t, int64_t>(ids.data_handle(), grow_rows);

  auto grown_index = ivf_flat::extend(
    handle,
    grow_view,
    std::make_optional<raft::device_vector_view<const int64_t, int64_t>>(ids_view),
    base_index);

  raft::resource::sync_stream(handle);

  print_sizes("after grow extend", handle, grown_index);

  auto visible  = visible_list_size(handle, grown_index);
  auto internal = grown_index.lists()[0]->get_size();

  if (visible == rows && internal == base_rows) {
    std::cout << "\nBUG: index-visible size is " << visible
              << " but internal list::size stayed " << internal << "\n";
    return 1;
  }

  std::cout << "\nNo mismatch observed\n";
  return 0;
}

after base extend
index.list_sizes()[0] = 100
list::size = 100
capacity = 128

after grow extend
index.list_sizes()[0] = 120
list::size = 100
capacity = 128

BUG: index-visible size is 120 but internal list::size stayed 100

[achirkin]

Ok, sorry for closing this prematurely, this does seem legit. I'm not sure though if this internal get_size() is used where it could cause any problems. I'll look into this in the following days.

[qwertyforce]

Thank you very much! I forgot to mention how i even stumbled upon this - i got very strange Illegal memory access errors (i am using python wrapper for multiple extends + search + torch for calculating the vectors) and believe that this bug is related (IMA was in calc_chunk_indices_kernel)
But I couldnt distill my production code to a small reproducible poc (the IMA is also non-deterministic), so all i can do is either the one poc i sent, or https://gist.github.com/qwertyforce/65d9bb2d108701240fc9ef1bc952e685 (tldr: if you create 2 new indexes by extend from the base one (not inplace), child_b data can leak into child_a. child_a is not modified after child_b is created, but child_a search results change)