path: root/icing/file/posting_list/flash-index-storage.cc

// Copyright (C) 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "icing/file/posting_list/flash-index-storage.h"

#include <sys/types.h>
#include <unistd.h>

#include <algorithm>
#include <cerrno>
#include <cinttypes>
#include <cstdint>
#include <memory>

#include "icing/text_classifier/lib3/utils/base/status.h"
#include "icing/text_classifier/lib3/utils/base/statusor.h"
#include "icing/absl_ports/canonical_errors.h"
#include "icing/absl_ports/str_cat.h"
#include "icing/file/posting_list/index-block.h"
#include "icing/file/posting_list/posting-list-common.h"
#include "icing/legacy/core/icing-string-util.h"
#include "icing/util/logging.h"
#include "icing/util/math-util.h"
#include "icing/util/status-macros.h"

namespace icing {
namespace lib {

libtextclassifier3::StatusOr<FlashIndexStorage> FlashIndexStorage::Create(
    std::string index_filename, const Filesystem* filesystem,
    PostingListSerializer* serializer, bool in_memory) {
  ICING_RETURN_ERROR_IF_NULL(filesystem);
  ICING_RETURN_ERROR_IF_NULL(serializer);

  FlashIndexStorage storage(filesystem, std::move(index_filename), serializer,
                            in_memory);
  if (!storage.Init()) {
    return absl_ports::InternalError(
        "Unable to successfully read header block!");
  }
  return storage;
}

/* static */ libtextclassifier3::StatusOr<int>
FlashIndexStorage::ReadHeaderMagic(const Filesystem* filesystem,
                                   const std::string& index_filename) {
  ICING_RETURN_ERROR_IF_NULL(filesystem);

  if (!filesystem->FileExists(index_filename.c_str())) {
    return absl_ports::NotFoundError("Flash index file doesn't exist");
  }

  ScopedFd sfd(filesystem->OpenForRead(index_filename.c_str()));
  if (!sfd.is_valid()) {
    return absl_ports::InternalError("Fail to open flash index file");
  }

  uint32_t block_size = SelectBlockSize();
  // Read and validate header.
  ICING_ASSIGN_OR_RETURN(HeaderBlock header_block,
                         HeaderBlock::Read(filesystem, sfd.get(), block_size));
  return header_block.header()->magic;
}

FlashIndexStorage::~FlashIndexStorage() {
  if (header_block_ != nullptr) {
    FlushInMemoryFreeList();
    PersistToDisk();
  }
}

/* static */ uint32_t FlashIndexStorage::SelectBlockSize() {
  // This should be close to the flash page size.
  static constexpr uint32_t kMinBlockSize = 4096;

  // Determine a good block size.
  uint32_t page_size = getpagesize();
  uint32_t block_size = std::max(kMinBlockSize, page_size);

  // Align up to the nearest page size.
  return math_util::RoundUpTo(block_size, page_size);
}

bool FlashIndexStorage::Init() {
  storage_sfd_ = ScopedFd(filesystem_->OpenForWrite(index_filename_.c_str()));
  if (!storage_sfd_.is_valid()) {
    return false;
  }

  // Read in or create the header.
  return InitHeader();
}

bool FlashIndexStorage::InitHeader() {
  // Look for an existing file size.
  int64_t file_size = filesystem_->GetFileSize(storage_sfd_.get());
  if (file_size == Filesystem::kBadFileSize) {
    ICING_LOG(ERROR) << "Could not initialize main index. Bad file size.";
    return false;
  }

  if (file_size == 0) {
    if (!CreateHeader()) {
      ICING_LOG(ERROR)
          << "Could not initialize main index. Unable to create header.";
      return false;
    }
  } else {
    if (!OpenHeader(file_size)) {
      ICING_LOG(ERROR)
          << "Could not initialize main index. Unable to open header.";
      return false;
    }
  }
  in_memory_freelists_.resize(header_block_->header()->num_index_block_infos);

  return true;
}

bool FlashIndexStorage::CreateHeader() {
  uint32_t block_size = SelectBlockSize();
  header_block_ = std::make_unique<HeaderBlock>(filesystem_, block_size);
  // Initialize.
  header_block_->header()->magic = HeaderBlock::Header::kMagic;
  header_block_->header()->block_size = block_size;
  header_block_->header()->last_indexed_docid = kInvalidDocumentId;

  // Work down from the largest posting list that fits in
  // block_size. We don't care about locality of blocks because this
  // is a flash index.
  for (uint32_t posting_list_bytes = max_posting_list_bytes();
       posting_list_bytes >= serializer_->GetMinPostingListSize();
       posting_list_bytes /= 2) {
    uint32_t aligned_posting_list_bytes =
        (posting_list_bytes / serializer_->GetDataTypeBytes()) *
        serializer_->GetDataTypeBytes();
    ICING_VLOG(1) << "Block size "
                  << header_block_->header()->num_index_block_infos << ": "
                  << aligned_posting_list_bytes;

    // Initialize free list to empty.
    HeaderBlock::Header::IndexBlockInfo* block_info =
        header_block_->AddIndexBlockInfo();
    if (block_info == nullptr) {
      // This should never happen anyways. Min block size is 4k, so adding these
      // IndexBlockInfos should never exceed the block size.
      return false;
    }
    block_info->posting_list_bytes = aligned_posting_list_bytes;
    block_info->free_list_block_index = kInvalidBlockIndex;
  }

  // Write the header.
  if (!header_block_->Write(storage_sfd_.get())) {
    filesystem_->Truncate(storage_sfd_.get(), 0);
    return false;
  }
  num_blocks_ = 1;
  return true;
}

bool FlashIndexStorage::OpenHeader(int64_t file_size) {
  uint32_t block_size = SelectBlockSize();
  // Read and validate header.
  ICING_ASSIGN_OR_RETURN(
      HeaderBlock read_header,
      HeaderBlock::Read(filesystem_, storage_sfd_.get(), block_size), false);
  if (read_header.header()->magic != HeaderBlock::Header::kMagic) {
    ICING_LOG(ERROR) << "Index header block wrong magic";
    return false;
  }
  if (file_size % read_header.header()->block_size != 0) {
    ICING_LOG(ERROR) << "Index size " << file_size
                     << " not a multiple of block size "
                     << read_header.header()->block_size;
    return false;
  }

  if (file_size < static_cast<int64_t>(read_header.header()->block_size)) {
    ICING_LOG(ERROR) << "Index size " << file_size
                     << " shorter than block size "
                     << read_header.header()->block_size;
    return false;
  }

  if (read_header.header()->block_size % getpagesize() != 0) {
    ICING_LOG(ERROR) << "Block size " << read_header.header()->block_size
                     << " is not a multiple of page size " << getpagesize();
    return false;
  }
  num_blocks_ = file_size / read_header.header()->block_size;
  if (block_size != read_header.header()->block_size) {
    // The block_size changed? That's weird. But the old block_size is still
    // valid (it must be some multiple of the new block_size). So reinitialize
    // with that old block size. Using the old block size means that we can
    // still use the main index, but reads/writes won't be as efficient in terms
    // of flash IO because the 'blocks' that we're reading are actually multiple
    // pages long.
    ICING_LOG(ERROR) << "Block size of existing header ("
                     << read_header.header()->block_size
                     << ") does not match the requested block size ("
                     << block_size << "). Defaulting to existing block size "
                     << read_header.header()->block_size;
    ICING_ASSIGN_OR_RETURN(HeaderBlock read_header,
                           HeaderBlock::Read(filesystem_, storage_sfd_.get(),
                                             read_header.header()->block_size),
                           false);
  }
  header_block_ = std::make_unique<HeaderBlock>(std::move(read_header));

  // Check for memory alignment on posting_list_bytes. See b/29983315.
  // The issue of potential corruption to the header could also be handled by
  // checksumming the header block.
  for (int i = 0; i < header_block_->header()->num_index_block_infos; ++i) {
    int posting_list_bytes =
        header_block_->header()->index_block_infos[i].posting_list_bytes;
    if (posting_list_bytes % serializer_->GetDataTypeBytes() != 0) {
      ICING_LOG(ERROR)
          << "Posting list size misaligned, index " << i << ", size "
          << header_block_->header()->index_block_infos[i].posting_list_bytes
          << ", data_type_bytes " << serializer_->GetDataTypeBytes()
          << ", file_size " << file_size;
      return false;
    }
  }
  return true;
}

bool FlashIndexStorage::PersistToDisk() {
  // First, write header.
  if (!header_block_->Write(storage_sfd_.get())) {
    ICING_LOG(ERROR) << "Write index header failed: " << strerror(errno);
    return false;
  }

  // Then sync.
  return filesystem_->DataSync(storage_sfd_.get());
}

libtextclassifier3::Status FlashIndexStorage::Reset() {
  // Reset in-memory members to default values.
  num_blocks_ = 0;
  header_block_.reset();
  storage_sfd_.reset();
  in_memory_freelists_.clear();

  // Delete the underlying file.
  if (!filesystem_->DeleteFile(index_filename_.c_str())) {
    return absl_ports::InternalError(
        absl_ports::StrCat("Unable to delete file: ", index_filename_));
  }

  // Re-initialize.
  if (!Init()) {
    return absl_ports::InternalError(
        "Unable to successfully read header block!");
  }
  return libtextclassifier3::Status::OK;
}

libtextclassifier3::StatusOr<PostingListHolder>
FlashIndexStorage::GetPostingList(PostingListIdentifier id) const {
  ICING_ASSIGN_OR_RETURN(IndexBlock block, GetIndexBlock(id.block_index()));
  ICING_ASSIGN_OR_RETURN(
      IndexBlock::PostingListAndBlockInfo pl_block_info,
      block.GetAllocatedPostingList(id.posting_list_index()));
  return PostingListHolder(std::move(pl_block_info.posting_list_used), id,
                           pl_block_info.next_block_index);
}

libtextclassifier3::StatusOr<IndexBlock> FlashIndexStorage::GetIndexBlock(
    uint32_t block_index) const {
  if (block_index >= num_blocks_) {
    return absl_ports::OutOfRangeError(IcingStringUtil::StringPrintf(
        "Unable to create an index block at index %" PRIu32
        " when only %d blocks have been allocated.",
        block_index, num_blocks_));
  }
  off_t offset = static_cast<off_t>(block_index) * block_size();
  return IndexBlock::CreateFromPreexistingIndexBlockRegion(
      filesystem_, serializer_, storage_sfd_.get(), offset, block_size());
}

libtextclassifier3::StatusOr<IndexBlock> FlashIndexStorage::CreateIndexBlock(
    uint32_t block_index, uint32_t posting_list_size) const {
  if (block_index >= num_blocks_) {
    return absl_ports::OutOfRangeError(IcingStringUtil::StringPrintf(
        "Unable to create an index block at index %" PRIu32
        " when only %d blocks have been allocated.",
        block_index, num_blocks_));
  }
  off_t offset = static_cast<off_t>(block_index) * block_size();
  return IndexBlock::CreateFromUninitializedRegion(
      filesystem_, serializer_, storage_sfd_.get(), offset, block_size(),
      posting_list_size);
}

int FlashIndexStorage::FindBestIndexBlockInfo(
    uint32_t posting_list_bytes) const {
  int i = header_block_->header()->num_index_block_infos - 1;
  for (; i >= 0; i--) {
    if (header_block_->header()->index_block_infos[i].posting_list_bytes >=
        posting_list_bytes) {
      return i;
    }
  }
  return i;
}

libtextclassifier3::StatusOr<PostingListHolder>
FlashIndexStorage::GetPostingListFromInMemoryFreeList(int block_info_index) {
  // Get something from in memory free list.
  ICING_ASSIGN_OR_RETURN(PostingListIdentifier posting_list_id,
                         in_memory_freelists_[block_info_index].TryPop());
  // Remember, posting lists stored on the in-memory free list were never
  // actually freed. So it will still contain a valid PostingListUsed. First, we
  // need to free this posting list.
  ICING_ASSIGN_OR_RETURN(IndexBlock block,
                         GetIndexBlock(posting_list_id.block_index()));
  ICING_RETURN_IF_ERROR(
      block.FreePostingList(posting_list_id.posting_list_index()));

  // Now, we can allocate a posting list from the same index block. It may not
  // be the same posting list that was just freed, but that's okay.
  ICING_ASSIGN_OR_RETURN(IndexBlock::PostingListAndBlockInfo pl_block_info,
                         block.AllocatePostingList());
  posting_list_id = PostingListIdentifier(
      posting_list_id.block_index(), pl_block_info.posting_list_index,
      posting_list_id.posting_list_index_bits());

  return PostingListHolder(std::move(pl_block_info.posting_list_used),
                           posting_list_id, pl_block_info.next_block_index);
}

libtextclassifier3::StatusOr<PostingListHolder>
FlashIndexStorage::GetPostingListFromOnDiskFreeList(int block_info_index) {
  // Get something from the free list.
  uint32_t block_index = header_block_->header()
                             ->index_block_infos[block_info_index]
                             .free_list_block_index;
  if (block_index == kInvalidBlockIndex) {
    return absl_ports::NotFoundError("No available entry in free list.");
  }

  // Get the index block
  ICING_ASSIGN_OR_RETURN(IndexBlock block, GetIndexBlock(block_index));
  ICING_ASSIGN_OR_RETURN(IndexBlock::PostingListAndBlockInfo pl_block_info,
                         block.AllocatePostingList());
  PostingListIdentifier posting_list_id =
      PostingListIdentifier(block_index, pl_block_info.posting_list_index,
                            block.posting_list_index_bits());
  if (!pl_block_info.has_free_posting_lists) {
    ICING_RETURN_IF_ERROR(
        RemoveFromOnDiskFreeList(block_index, block_info_index, &block));
  }

  return PostingListHolder(std::move(pl_block_info.posting_list_used),
                           posting_list_id, pl_block_info.next_block_index);
}

libtextclassifier3::StatusOr<PostingListHolder>
FlashIndexStorage::AllocateNewPostingList(int block_info_index) {
  uint32_t block_index = GrowIndex();
  if (block_index == kInvalidBlockIndex) {
    return absl_ports::ResourceExhaustedError(
        "Unable to grow the index further!");
  }
  ICING_ASSIGN_OR_RETURN(
      IndexBlock block,
      CreateIndexBlock(block_index, header_block_->header()
                                        ->index_block_infos[block_info_index]
                                        .posting_list_bytes));
  ICING_ASSIGN_OR_RETURN(IndexBlock::PostingListAndBlockInfo pl_block_info,
                         block.AllocatePostingList());
  PostingListIdentifier posting_list_id =
      PostingListIdentifier(block_index, pl_block_info.posting_list_index,
                            block.posting_list_index_bits());
  if (pl_block_info.has_free_posting_lists) {
    AddToOnDiskFreeList(block_index, block_info_index, &block);
  }

  return PostingListHolder(std::move(pl_block_info.posting_list_used),
                           posting_list_id, pl_block_info.next_block_index);
}

libtextclassifier3::StatusOr<PostingListHolder>
FlashIndexStorage::AllocatePostingList(uint32_t min_posting_list_bytes) {
  int max_pl_size = max_posting_list_bytes();
  if (min_posting_list_bytes > max_pl_size) {
    return absl_ports::InvalidArgumentError(IcingStringUtil::StringPrintf(
        "Requested posting list size %d exceeds max posting list size %d",
        min_posting_list_bytes, max_pl_size));
  }
  int best_block_info_index = FindBestIndexBlockInfo(min_posting_list_bytes);

  auto holder_or = GetPostingListFromInMemoryFreeList(best_block_info_index);
  if (holder_or.ok()) {
    return std::move(holder_or).ValueOrDie();
  }

  // Nothing in memory. Look for something in the block file.
  holder_or = GetPostingListFromOnDiskFreeList(best_block_info_index);
  if (holder_or.ok()) {
    return std::move(holder_or).ValueOrDie();
  }

  return AllocateNewPostingList(best_block_info_index);
}

libtextclassifier3::StatusOr<PostingListHolder>
FlashIndexStorage::AllocateAndChainMaxSizePostingList(
    uint32_t prev_block_index) {
  uint32_t max_pl_size = max_posting_list_bytes();
  int best_block_info_index = FindBestIndexBlockInfo(max_pl_size);

  auto holder_or = GetPostingListFromInMemoryFreeList(best_block_info_index);
  if (!holder_or.ok()) {
    // Nothing in memory. Look for something in the block file.
    holder_or = GetPostingListFromOnDiskFreeList(best_block_info_index);
  }

  if (!holder_or.ok()) {
    // Nothing in memory or block file. Allocate new block and posting list.
    holder_or = AllocateNewPostingList(best_block_info_index);
  }

  if (!holder_or.ok()) {
    return holder_or;
  }

  PostingListHolder holder = std::move(holder_or).ValueOrDie();
  ICING_ASSIGN_OR_RETURN(IndexBlock block,
                         GetIndexBlock(holder.id.block_index()));
  ICING_RETURN_IF_ERROR(block.SetNextBlockIndex(prev_block_index));
  holder.next_block_index = prev_block_index;
  return holder;
}

void FlashIndexStorage::AddToOnDiskFreeList(uint32_t block_index,
                                            int block_info_index,
                                            IndexBlock* index_block) {
  libtextclassifier3::Status status =
      index_block->SetNextBlockIndex(header_block_->header()
                                         ->index_block_infos[block_info_index]
                                         .free_list_block_index);
  if (!status.ok()) {
    // If an error occurs, then simply skip this block. It just prevents us from
    // allocating posting lists from this free block in the future and thus
    // wastes at most one block, but the entire storage (including the
    // FlashIndexStorage header) is still valid. Therefore, we can swallow
    // errors here.
    ICING_VLOG(1) << "Fail to set next block index to chain blocks with free "
                     "lists on disk: "
                  << status.error_message();
    return;
  }

  header_block_->header()
      ->index_block_infos[block_info_index]
      .free_list_block_index = block_index;
}

libtextclassifier3::Status FlashIndexStorage::RemoveFromOnDiskFreeList(
    uint32_t block_index, int block_info_index, IndexBlock* index_block) {
  // Cannot be used anymore. Move free ptr to the next block.
  ICING_ASSIGN_OR_RETURN(uint32_t next_block_index,
                         index_block->GetNextBlockIndex());
  ICING_RETURN_IF_ERROR(index_block->SetNextBlockIndex(kInvalidBlockIndex));
  header_block_->header()
      ->index_block_infos[block_info_index]
      .free_list_block_index = next_block_index;
  return libtextclassifier3::Status::OK;
}

libtextclassifier3::Status FlashIndexStorage::FreePostingList(
    PostingListHolder&& holder) {
  ICING_ASSIGN_OR_RETURN(IndexBlock block,
                         GetIndexBlock(holder.id.block_index()));

  uint32_t posting_list_bytes = block.posting_list_bytes();
  int best_block_info_index = FindBestIndexBlockInfo(posting_list_bytes);

  // It *should* be guaranteed elsewhere that FindBestIndexBlockInfo will not
  // return a value in >= in_memory_freelists_, but check regardless. If it
  // doesn't fit for some reason, then put it in the Header free list instead.
  if (has_in_memory_freelists_ &&
      best_block_info_index < in_memory_freelists_.size()) {
    in_memory_freelists_[best_block_info_index].Push(holder.id);
  } else {
    ICING_ASSIGN_OR_RETURN(bool was_not_full, block.HasFreePostingLists());
    ICING_RETURN_IF_ERROR(
        block.FreePostingList(holder.id.posting_list_index()));
    // If this block was not already full, then it is already in the free list.
    if (!was_not_full) {
      AddToOnDiskFreeList(holder.id.block_index(), best_block_info_index,
                          &block);
    }
  }
  return libtextclassifier3::Status::OK;
}

libtextclassifier3::Status FlashIndexStorage::WritePostingListToDisk(
    const PostingListHolder& holder) {
  ICING_ASSIGN_OR_RETURN(IndexBlock block,
                         GetIndexBlock(holder.id.block_index()));
  return block.WritePostingListToDisk(holder.posting_list,
                                      holder.id.posting_list_index());
}

int FlashIndexStorage::GrowIndex() {
  if (num_blocks_ >= kMaxBlockIndex) {
    ICING_VLOG(1) << "Reached max block index " << kMaxBlockIndex;
    return kInvalidBlockIndex;
  }

  // Grow the index file.
  if (!filesystem_->Grow(
          storage_sfd_.get(),
          static_cast<uint64_t>(num_blocks_ + 1) * block_size())) {
    ICING_VLOG(1) << "Error growing index file: " << strerror(errno);
    return kInvalidBlockIndex;
  }

  return num_blocks_++;
}

libtextclassifier3::Status FlashIndexStorage::FlushInMemoryFreeList() {
  for (int i = 0; i < in_memory_freelists_.size(); ++i) {
    FreeList& freelist = in_memory_freelists_.at(i);
    auto freelist_elt_or = freelist.TryPop();
    while (freelist_elt_or.ok()) {
      PostingListIdentifier freelist_elt = freelist_elt_or.ValueOrDie();
      // Remember, posting lists stored on the in-memory free list were never
      // actually freed. So it will still contain a valid PostingListUsed.
      // First, we need to free this posting list.
      auto block_or = GetIndexBlock(freelist_elt.block_index());
      if (!block_or.ok()) {
        // Can't read the block. Nothing to do here. This posting list will have
        // to leak. Just proceed to the next freelist element.
        freelist_elt_or = freelist.TryPop();
        continue;
      }
      IndexBlock block = std::move(block_or).ValueOrDie();
      ICING_ASSIGN_OR_RETURN(bool was_not_full, block.HasFreePostingLists());
      ICING_RETURN_IF_ERROR(
          block.FreePostingList(freelist_elt.posting_list_index()));
      // If this block was not already full, then it is already in the free
      // list.
      if (!was_not_full) {
        AddToOnDiskFreeList(freelist_elt.block_index(), /*block_info_index=*/i,
                            &block);
      }
      freelist_elt_or = freelist.TryPop();
    }
  }
  return libtextclassifier3::Status::OK;
}

void FlashIndexStorage::GetDebugInfo(DebugInfoVerbosity::Code verbosity,
                                     std::string* out) const {
  // Dump and check integrity of the index block free lists.
  out->append("Free lists:\n");
  for (size_t i = 0; i < header_block_->header()->num_index_block_infos; ++i) {
    // TODO(tjbarron) Port over StringAppendFormat to migrate off of this legacy
    // util.
    IcingStringUtil::SStringAppendF(
        out, 100, "Posting list bytes %u: ",
        header_block_->header()->index_block_infos[i].posting_list_bytes);
    uint32_t block_index =
        header_block_->header()->index_block_infos[i].free_list_block_index;
    int count = 0;
    while (block_index != kInvalidBlockIndex) {
      auto block_or = GetIndexBlock(block_index);
      IcingStringUtil::SStringAppendF(out, 100, "%u ", block_index);
      ++count;

      block_index = kInvalidBlockIndex;
      if (block_or.ok()) {
        auto block_index_or = block_or.ValueOrDie().GetNextBlockIndex();
        if (block_index_or.ok()) {
          block_index = block_index_or.ValueOrDie();
        }
      }
    }
    IcingStringUtil::SStringAppendF(out, 100, "(count=%d)\n", count);
  }

  out->append("In memory free lists:\n");
  if (in_memory_freelists_.size() ==
      header_block_->header()->num_index_block_infos) {
    for (size_t i = 0; i < in_memory_freelists_.size(); ++i) {
      IcingStringUtil::SStringAppendF(
          out, 100, "Posting list bytes %u %s\n",
          header_block_->header()->index_block_infos[i].posting_list_bytes,
          in_memory_freelists_.at(i).DebugString().c_str());
    }
  } else {
    IcingStringUtil::SStringAppendF(
        out, 100,
        "In memory free list size %zu doesn't match index block infos size "
        "%d\n",
        in_memory_freelists_.size(),
        header_block_->header()->num_index_block_infos);
  }
}

// FreeList.
void FlashIndexStorage::FreeList::Push(PostingListIdentifier id) {
  if (free_list_.size() >= kMaxSize) {
    ICING_LOG(WARNING)
        << "Freelist for posting lists of size (block_size / "
        << (1u << id.posting_list_index_bits())
        << ") has reached max size. Dropping freed posting list [block_index:"
        << id.block_index()
        << ", posting_list_index:" << id.posting_list_index() << "]";
    ++num_dropped_free_list_entries_;
    return;
  }

  free_list_.push_back(id);
  free_list_size_high_watermark_ = std::max(
      free_list_size_high_watermark_, static_cast<int>(free_list_.size()));
}

libtextclassifier3::StatusOr<PostingListIdentifier>
FlashIndexStorage::FreeList::TryPop() {
  if (free_list_.empty()) {
    return absl_ports::NotFoundError("No available entry in free list.");
  }

  PostingListIdentifier id = free_list_.back();
  free_list_.pop_back();
  return id;
}

std::string FlashIndexStorage::FreeList::DebugString() const {
  return IcingStringUtil::StringPrintf(
      "size %zu max %d dropped %d", free_list_.size(),
      free_list_size_high_watermark_, num_dropped_free_list_entries_);
}

}  // namespace lib
}  // namespace icing