path: root/src/rocksdb/table/block_based/block_based_table_reader.cc

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#include "table/block_based/block_based_table_reader.h"
#include <algorithm>
#include <array>
#include <limits>
#include <string>
#include <utility>
#include <vector>

#include "db/dbformat.h"
#include "db/pinned_iterators_manager.h"

#include "file/file_prefetch_buffer.h"
#include "file/random_access_file_reader.h"

#include "rocksdb/cache.h"
#include "rocksdb/comparator.h"
#include "rocksdb/env.h"
#include "rocksdb/file_system.h"
#include "rocksdb/filter_policy.h"
#include "rocksdb/iterator.h"
#include "rocksdb/options.h"
#include "rocksdb/statistics.h"
#include "rocksdb/table.h"
#include "rocksdb/table_properties.h"

#include "table/block_based/block.h"
#include "table/block_based/block_based_filter_block.h"
#include "table/block_based/block_based_table_factory.h"
#include "table/block_based/block_prefix_index.h"
#include "table/block_based/filter_block.h"
#include "table/block_based/full_filter_block.h"
#include "table/block_based/partitioned_filter_block.h"
#include "table/block_fetcher.h"
#include "table/format.h"
#include "table/get_context.h"
#include "table/internal_iterator.h"
#include "table/meta_blocks.h"
#include "table/multiget_context.h"
#include "table/persistent_cache_helper.h"
#include "table/sst_file_writer_collectors.h"
#include "table/two_level_iterator.h"

#include "monitoring/perf_context_imp.h"
#include "test_util/sync_point.h"
#include "util/coding.h"
#include "util/crc32c.h"
#include "util/stop_watch.h"
#include "util/string_util.h"
#include "util/util.h"
#include "util/xxhash.h"

namespace ROCKSDB_NAMESPACE {

extern const uint64_t kBlockBasedTableMagicNumber;
extern const std::string kHashIndexPrefixesBlock;
extern const std::string kHashIndexPrefixesMetadataBlock;

typedef BlockBasedTable::IndexReader IndexReader;

// Found that 256 KB readahead size provides the best performance, based on
// experiments, for auto readahead. Experiment data is in PR #3282.
const size_t BlockBasedTable::kMaxAutoReadaheadSize = 256 * 1024;

BlockBasedTable::~BlockBasedTable() {
  delete rep_;
}

std::atomic<uint64_t> BlockBasedTable::next_cache_key_id_(0);

template <typename TBlocklike>
class BlocklikeTraits;

template <>
class BlocklikeTraits<BlockContents> {
 public:
  static BlockContents* Create(BlockContents&& contents,
                               SequenceNumber /* global_seqno */,
                               size_t /* read_amp_bytes_per_bit */,
                               Statistics* /* statistics */,
                               bool /* using_zstd */,
                               const FilterPolicy* /* filter_policy */) {
    return new BlockContents(std::move(contents));
  }

  static uint32_t GetNumRestarts(const BlockContents& /* contents */) {
    return 0;
  }
};

template <>
class BlocklikeTraits<ParsedFullFilterBlock> {
 public:
  static ParsedFullFilterBlock* Create(BlockContents&& contents,
                                       SequenceNumber /* global_seqno */,
                                       size_t /* read_amp_bytes_per_bit */,
                                       Statistics* /* statistics */,
                                       bool /* using_zstd */,
                                       const FilterPolicy* filter_policy) {
    return new ParsedFullFilterBlock(filter_policy, std::move(contents));
  }

  static uint32_t GetNumRestarts(const ParsedFullFilterBlock& /* block */) {
    return 0;
  }
};

template <>
class BlocklikeTraits<Block> {
 public:
  static Block* Create(BlockContents&& contents, SequenceNumber global_seqno,
                       size_t read_amp_bytes_per_bit, Statistics* statistics,
                       bool /* using_zstd */,
                       const FilterPolicy* /* filter_policy */) {
    return new Block(std::move(contents), global_seqno, read_amp_bytes_per_bit,
                     statistics);
  }

  static uint32_t GetNumRestarts(const Block& block) {
    return block.NumRestarts();
  }
};

template <>
class BlocklikeTraits<UncompressionDict> {
 public:
  static UncompressionDict* Create(BlockContents&& contents,
                                   SequenceNumber /* global_seqno */,
                                   size_t /* read_amp_bytes_per_bit */,
                                   Statistics* /* statistics */,
                                   bool using_zstd,
                                   const FilterPolicy* /* filter_policy */) {
    return new UncompressionDict(contents.data, std::move(contents.allocation),
                                 using_zstd);
  }

  static uint32_t GetNumRestarts(const UncompressionDict& /* dict */) {
    return 0;
  }
};

namespace {
// Read the block identified by "handle" from "file".
// The only relevant option is options.verify_checksums for now.
// On failure return non-OK.
// On success fill *result and return OK - caller owns *result
// @param uncompression_dict Data for presetting the compression library's
//    dictionary.
template <typename TBlocklike>
Status ReadBlockFromFile(
    RandomAccessFileReader* file, FilePrefetchBuffer* prefetch_buffer,
    const Footer& footer, const ReadOptions& options, const BlockHandle& handle,
    std::unique_ptr<TBlocklike>* result, const ImmutableCFOptions& ioptions,
    bool do_uncompress, bool maybe_compressed, BlockType block_type,
    const UncompressionDict& uncompression_dict,
    const PersistentCacheOptions& cache_options, SequenceNumber global_seqno,
    size_t read_amp_bytes_per_bit, MemoryAllocator* memory_allocator,
    bool for_compaction, bool using_zstd, const FilterPolicy* filter_policy) {
  assert(result);

  BlockContents contents;
  BlockFetcher block_fetcher(
      file, prefetch_buffer, footer, options, handle, &contents, ioptions,
      do_uncompress, maybe_compressed, block_type, uncompression_dict,
      cache_options, memory_allocator, nullptr, for_compaction);
  Status s = block_fetcher.ReadBlockContents();
  if (s.ok()) {
    result->reset(BlocklikeTraits<TBlocklike>::Create(
        std::move(contents), global_seqno, read_amp_bytes_per_bit,
        ioptions.statistics, using_zstd, filter_policy));
  }

  return s;
}

inline MemoryAllocator* GetMemoryAllocator(
    const BlockBasedTableOptions& table_options) {
  return table_options.block_cache.get()
             ? table_options.block_cache->memory_allocator()
             : nullptr;
}

inline MemoryAllocator* GetMemoryAllocatorForCompressedBlock(
    const BlockBasedTableOptions& table_options) {
  return table_options.block_cache_compressed.get()
             ? table_options.block_cache_compressed->memory_allocator()
             : nullptr;
}

// Delete the entry resided in the cache.
template <class Entry>
void DeleteCachedEntry(const Slice& /*key*/, void* value) {
  auto entry = reinterpret_cast<Entry*>(value);
  delete entry;
}

// Release the cached entry and decrement its ref count.
void ForceReleaseCachedEntry(void* arg, void* h) {
  Cache* cache = reinterpret_cast<Cache*>(arg);
  Cache::Handle* handle = reinterpret_cast<Cache::Handle*>(h);
  cache->Release(handle, true /* force_erase */);
}

// Release the cached entry and decrement its ref count.
// Do not force erase
void ReleaseCachedEntry(void* arg, void* h) {
  Cache* cache = reinterpret_cast<Cache*>(arg);
  Cache::Handle* handle = reinterpret_cast<Cache::Handle*>(h);
  cache->Release(handle, false /* force_erase */);
}

// For hash based index, return true if prefix_extractor and
// prefix_extractor_block mismatch, false otherwise. This flag will be used
// as total_order_seek via NewIndexIterator
bool PrefixExtractorChanged(const TableProperties* table_properties,
                            const SliceTransform* prefix_extractor) {
  // BlockBasedTableOptions::kHashSearch requires prefix_extractor to be set.
  // Turn off hash index in prefix_extractor is not set; if  prefix_extractor
  // is set but prefix_extractor_block is not set, also disable hash index
  if (prefix_extractor == nullptr || table_properties == nullptr ||
      table_properties->prefix_extractor_name.empty()) {
    return true;
  }

  // prefix_extractor and prefix_extractor_block are both non-empty
  if (table_properties->prefix_extractor_name.compare(
          prefix_extractor->Name()) != 0) {
    return true;
  } else {
    return false;
  }
}

CacheAllocationPtr CopyBufferToHeap(MemoryAllocator* allocator, Slice& buf) {
  CacheAllocationPtr heap_buf;
  heap_buf = AllocateBlock(buf.size(), allocator);
  memcpy(heap_buf.get(), buf.data(), buf.size());
  return heap_buf;
}

}  // namespace

// Encapsulates common functionality for the various index reader
// implementations. Provides access to the index block regardless of whether
// it is owned by the reader or stored in the cache, or whether it is pinned
// in the cache or not.
class BlockBasedTable::IndexReaderCommon : public BlockBasedTable::IndexReader {
 public:
  IndexReaderCommon(const BlockBasedTable* t,
                    CachableEntry<Block>&& index_block)
      : table_(t), index_block_(std::move(index_block)) {
    assert(table_ != nullptr);
  }

 protected:
  static Status ReadIndexBlock(const BlockBasedTable* table,
                               FilePrefetchBuffer* prefetch_buffer,
                               const ReadOptions& read_options, bool use_cache,
                               GetContext* get_context,
                               BlockCacheLookupContext* lookup_context,
                               CachableEntry<Block>* index_block);

  const BlockBasedTable* table() const { return table_; }

  const InternalKeyComparator* internal_comparator() const {
    assert(table_ != nullptr);
    assert(table_->get_rep() != nullptr);

    return &table_->get_rep()->internal_comparator;
  }

  bool index_has_first_key() const {
    assert(table_ != nullptr);
    assert(table_->get_rep() != nullptr);
    return table_->get_rep()->index_has_first_key;
  }

  bool index_key_includes_seq() const {
    assert(table_ != nullptr);
    assert(table_->get_rep() != nullptr);
    return table_->get_rep()->index_key_includes_seq;
  }

  bool index_value_is_full() const {
    assert(table_ != nullptr);
    assert(table_->get_rep() != nullptr);
    return table_->get_rep()->index_value_is_full;
  }

  bool cache_index_blocks() const {
    assert(table_ != nullptr);
    assert(table_->get_rep() != nullptr);
    return table_->get_rep()->table_options.cache_index_and_filter_blocks;
  }

  Status GetOrReadIndexBlock(bool no_io, GetContext* get_context,
                             BlockCacheLookupContext* lookup_context,
                             CachableEntry<Block>* index_block) const;

  size_t ApproximateIndexBlockMemoryUsage() const {
    assert(!index_block_.GetOwnValue() || index_block_.GetValue() != nullptr);
    return index_block_.GetOwnValue()
               ? index_block_.GetValue()->ApproximateMemoryUsage()
               : 0;
  }

 private:
  const BlockBasedTable* table_;
  CachableEntry<Block> index_block_;
};

Status BlockBasedTable::IndexReaderCommon::ReadIndexBlock(
    const BlockBasedTable* table, FilePrefetchBuffer* prefetch_buffer,
    const ReadOptions& read_options, bool use_cache, GetContext* get_context,
    BlockCacheLookupContext* lookup_context,
    CachableEntry<Block>* index_block) {
  PERF_TIMER_GUARD(read_index_block_nanos);

  assert(table != nullptr);
  assert(index_block != nullptr);
  assert(index_block->IsEmpty());

  const Rep* const rep = table->get_rep();
  assert(rep != nullptr);

  const Status s = table->RetrieveBlock(
      prefetch_buffer, read_options, rep->footer.index_handle(),
      UncompressionDict::GetEmptyDict(), index_block, BlockType::kIndex,
      get_context, lookup_context, /* for_compaction */ false, use_cache);

  return s;
}

Status BlockBasedTable::IndexReaderCommon::GetOrReadIndexBlock(
    bool no_io, GetContext* get_context,
    BlockCacheLookupContext* lookup_context,
    CachableEntry<Block>* index_block) const {
  assert(index_block != nullptr);

  if (!index_block_.IsEmpty()) {
    index_block->SetUnownedValue(index_block_.GetValue());
    return Status::OK();
  }

  ReadOptions read_options;
  if (no_io) {
    read_options.read_tier = kBlockCacheTier;
  }

  return ReadIndexBlock(table_, /*prefetch_buffer=*/nullptr, read_options,
                        cache_index_blocks(), get_context, lookup_context,
                        index_block);
}

// Index that allows binary search lookup in a two-level index structure.
class PartitionIndexReader : public BlockBasedTable::IndexReaderCommon {
 public:
  // Read the partition index from the file and create an instance for
  // `PartitionIndexReader`.
  // On success, index_reader will be populated; otherwise it will remain
  // unmodified.
  static Status Create(const BlockBasedTable* table,
                       FilePrefetchBuffer* prefetch_buffer, bool use_cache,
                       bool prefetch, bool pin,
                       BlockCacheLookupContext* lookup_context,
                       std::unique_ptr<IndexReader>* index_reader) {
    assert(table != nullptr);
    assert(table->get_rep());
    assert(!pin || prefetch);
    assert(index_reader != nullptr);

    CachableEntry<Block> index_block;
    if (prefetch || !use_cache) {
      const Status s =
          ReadIndexBlock(table, prefetch_buffer, ReadOptions(), use_cache,
                         /*get_context=*/nullptr, lookup_context, &index_block);
      if (!s.ok()) {
        return s;
      }

      if (use_cache && !pin) {
        index_block.Reset();
      }
    }

    index_reader->reset(
        new PartitionIndexReader(table, std::move(index_block)));

    return Status::OK();
  }

  // return a two-level iterator: first level is on the partition index
  InternalIteratorBase<IndexValue>* NewIterator(
      const ReadOptions& read_options, bool /* disable_prefix_seek */,
      IndexBlockIter* iter, GetContext* get_context,
      BlockCacheLookupContext* lookup_context) override {
    const bool no_io = (read_options.read_tier == kBlockCacheTier);
    CachableEntry<Block> index_block;
    const Status s =
        GetOrReadIndexBlock(no_io, get_context, lookup_context, &index_block);
    if (!s.ok()) {
      if (iter != nullptr) {
        iter->Invalidate(s);
        return iter;
      }

      return NewErrorInternalIterator<IndexValue>(s);
    }

    InternalIteratorBase<IndexValue>* it = nullptr;

    Statistics* kNullStats = nullptr;
    // Filters are already checked before seeking the index
    if (!partition_map_.empty()) {
      // We don't return pinned data from index blocks, so no need
      // to set `block_contents_pinned`.
      it = NewTwoLevelIterator(
          new BlockBasedTable::PartitionedIndexIteratorState(table(),
                                                             &partition_map_),
          index_block.GetValue()->NewIndexIterator(
              internal_comparator(), internal_comparator()->user_comparator(),
              nullptr, kNullStats, true, index_has_first_key(),
              index_key_includes_seq(), index_value_is_full()));
    } else {
      ReadOptions ro;
      ro.fill_cache = read_options.fill_cache;
      // We don't return pinned data from index blocks, so no need
      // to set `block_contents_pinned`.
      it = new BlockBasedTableIterator<IndexBlockIter, IndexValue>(
          table(), ro, *internal_comparator(),
          index_block.GetValue()->NewIndexIterator(
              internal_comparator(), internal_comparator()->user_comparator(),
              nullptr, kNullStats, true, index_has_first_key(),
              index_key_includes_seq(), index_value_is_full()),
          false, true, /* prefix_extractor */ nullptr, BlockType::kIndex,
          lookup_context ? lookup_context->caller
                         : TableReaderCaller::kUncategorized);
    }

    assert(it != nullptr);
    index_block.TransferTo(it);

    return it;

    // TODO(myabandeh): Update TwoLevelIterator to be able to make use of
    // on-stack BlockIter while the state is on heap. Currentlly it assumes
    // the first level iter is always on heap and will attempt to delete it
    // in its destructor.
  }

  void CacheDependencies(bool pin) override {
    // Before read partitions, prefetch them to avoid lots of IOs
    BlockCacheLookupContext lookup_context{TableReaderCaller::kPrefetch};
    const BlockBasedTable::Rep* rep = table()->rep_;
    IndexBlockIter biter;
    BlockHandle handle;
    Statistics* kNullStats = nullptr;

    CachableEntry<Block> index_block;
    Status s = GetOrReadIndexBlock(false /* no_io */, nullptr /* get_context */,
                                   &lookup_context, &index_block);
    if (!s.ok()) {
      ROCKS_LOG_WARN(rep->ioptions.info_log,
                     "Error retrieving top-level index block while trying to "
                     "cache index partitions: %s",
                     s.ToString().c_str());
      return;
    }

    // We don't return pinned data from index blocks, so no need
    // to set `block_contents_pinned`.
    index_block.GetValue()->NewIndexIterator(
        internal_comparator(), internal_comparator()->user_comparator(), &biter,
        kNullStats, true, index_has_first_key(), index_key_includes_seq(),
        index_value_is_full());
    // Index partitions are assumed to be consecuitive. Prefetch them all.
    // Read the first block offset
    biter.SeekToFirst();
    if (!biter.Valid()) {
      // Empty index.
      return;
    }
    handle = biter.value().handle;
    uint64_t prefetch_off = handle.offset();

    // Read the last block's offset
    biter.SeekToLast();
    if (!biter.Valid()) {
      // Empty index.
      return;
    }
    handle = biter.value().handle;
    uint64_t last_off = handle.offset() + block_size(handle);
    uint64_t prefetch_len = last_off - prefetch_off;
    std::unique_ptr<FilePrefetchBuffer> prefetch_buffer;
    rep->CreateFilePrefetchBuffer(0, 0, &prefetch_buffer);
    s = prefetch_buffer->Prefetch(rep->file.get(), prefetch_off,
                                  static_cast<size_t>(prefetch_len));

    // After prefetch, read the partitions one by one
    biter.SeekToFirst();
    auto ro = ReadOptions();
    for (; biter.Valid(); biter.Next()) {
      handle = biter.value().handle;
      CachableEntry<Block> block;
      // TODO: Support counter batch update for partitioned index and
      // filter blocks
      s = table()->MaybeReadBlockAndLoadToCache(
          prefetch_buffer.get(), ro, handle, UncompressionDict::GetEmptyDict(),
          &block, BlockType::kIndex, /*get_context=*/nullptr, &lookup_context,
          /*contents=*/nullptr);

      assert(s.ok() || block.GetValue() == nullptr);
      if (s.ok() && block.GetValue() != nullptr) {
        if (block.IsCached()) {
          if (pin) {
            partition_map_[handle.offset()] = std::move(block);
          }
        }
      }
    }
  }

  size_t ApproximateMemoryUsage() const override {
    size_t usage = ApproximateIndexBlockMemoryUsage();
#ifdef ROCKSDB_MALLOC_USABLE_SIZE
    usage += malloc_usable_size(const_cast<PartitionIndexReader*>(this));
#else
    usage += sizeof(*this);
#endif  // ROCKSDB_MALLOC_USABLE_SIZE
    // TODO(myabandeh): more accurate estimate of partition_map_ mem usage
    return usage;
  }

 private:
  PartitionIndexReader(const BlockBasedTable* t,
                       CachableEntry<Block>&& index_block)
      : IndexReaderCommon(t, std::move(index_block)) {}

  std::unordered_map<uint64_t, CachableEntry<Block>> partition_map_;
};

// Index that allows binary search lookup for the first key of each block.
// This class can be viewed as a thin wrapper for `Block` class which already
// supports binary search.
class BinarySearchIndexReader : public BlockBasedTable::IndexReaderCommon {
 public:
  // Read index from the file and create an intance for
  // `BinarySearchIndexReader`.
  // On success, index_reader will be populated; otherwise it will remain
  // unmodified.
  static Status Create(const BlockBasedTable* table,
                       FilePrefetchBuffer* prefetch_buffer, bool use_cache,
                       bool prefetch, bool pin,
                       BlockCacheLookupContext* lookup_context,
                       std::unique_ptr<IndexReader>* index_reader) {
    assert(table != nullptr);
    assert(table->get_rep());
    assert(!pin || prefetch);
    assert(index_reader != nullptr);

    CachableEntry<Block> index_block;
    if (prefetch || !use_cache) {
      const Status s =
          ReadIndexBlock(table, prefetch_buffer, ReadOptions(), use_cache,
                         /*get_context=*/nullptr, lookup_context, &index_block);
      if (!s.ok()) {
        return s;
      }

      if (use_cache && !pin) {
        index_block.Reset();
      }
    }

    index_reader->reset(
        new BinarySearchIndexReader(table, std::move(index_block)));

    return Status::OK();
  }

  InternalIteratorBase<IndexValue>* NewIterator(
      const ReadOptions& read_options, bool /* disable_prefix_seek */,
      IndexBlockIter* iter, GetContext* get_context,
      BlockCacheLookupContext* lookup_context) override {
    const bool no_io = (read_options.read_tier == kBlockCacheTier);
    CachableEntry<Block> index_block;
    const Status s =
        GetOrReadIndexBlock(no_io, get_context, lookup_context, &index_block);
    if (!s.ok()) {
      if (iter != nullptr) {
        iter->Invalidate(s);
        return iter;
      }

      return NewErrorInternalIterator<IndexValue>(s);
    }

    Statistics* kNullStats = nullptr;
    // We don't return pinned data from index blocks, so no need
    // to set `block_contents_pinned`.
    auto it = index_block.GetValue()->NewIndexIterator(
        internal_comparator(), internal_comparator()->user_comparator(), iter,
        kNullStats, true, index_has_first_key(), index_key_includes_seq(),
        index_value_is_full());

    assert(it != nullptr);
    index_block.TransferTo(it);

    return it;
  }

  size_t ApproximateMemoryUsage() const override {
    size_t usage = ApproximateIndexBlockMemoryUsage();
#ifdef ROCKSDB_MALLOC_USABLE_SIZE
    usage += malloc_usable_size(const_cast<BinarySearchIndexReader*>(this));
#else
    usage += sizeof(*this);
#endif  // ROCKSDB_MALLOC_USABLE_SIZE
    return usage;
  }

 private:
  BinarySearchIndexReader(const BlockBasedTable* t,
                          CachableEntry<Block>&& index_block)
      : IndexReaderCommon(t, std::move(index_block)) {}
};

// Index that leverages an internal hash table to quicken the lookup for a given
// key.
class HashIndexReader : public BlockBasedTable::IndexReaderCommon {
 public:
  static Status Create(const BlockBasedTable* table,
                       FilePrefetchBuffer* prefetch_buffer,
                       InternalIterator* meta_index_iter, bool use_cache,
                       bool prefetch, bool pin,
                       BlockCacheLookupContext* lookup_context,
                       std::unique_ptr<IndexReader>* index_reader) {
    assert(table != nullptr);
    assert(index_reader != nullptr);
    assert(!pin || prefetch);

    const BlockBasedTable::Rep* rep = table->get_rep();
    assert(rep != nullptr);

    CachableEntry<Block> index_block;
    if (prefetch || !use_cache) {
      const Status s =
          ReadIndexBlock(table, prefetch_buffer, ReadOptions(), use_cache,
                         /*get_context=*/nullptr, lookup_context, &index_block);
      if (!s.ok()) {
        return s;
      }

      if (use_cache && !pin) {
        index_block.Reset();
      }
    }

    // Note, failure to create prefix hash index does not need to be a
    // hard error. We can still fall back to the original binary search index.
    // So, Create will succeed regardless, from this point on.

    index_reader->reset(new HashIndexReader(table, std::move(index_block)));

    // Get prefixes block
    BlockHandle prefixes_handle;
    Status s = FindMetaBlock(meta_index_iter, kHashIndexPrefixesBlock,
                             &prefixes_handle);
    if (!s.ok()) {
      // TODO: log error
      return Status::OK();
    }

    // Get index metadata block
    BlockHandle prefixes_meta_handle;
    s = FindMetaBlock(meta_index_iter, kHashIndexPrefixesMetadataBlock,
                      &prefixes_meta_handle);
    if (!s.ok()) {
      // TODO: log error
      return Status::OK();
    }

    RandomAccessFileReader* const file = rep->file.get();
    const Footer& footer = rep->footer;
    const ImmutableCFOptions& ioptions = rep->ioptions;
    const PersistentCacheOptions& cache_options = rep->persistent_cache_options;
    MemoryAllocator* const memory_allocator =
        GetMemoryAllocator(rep->table_options);

    // Read contents for the blocks
    BlockContents prefixes_contents;
    BlockFetcher prefixes_block_fetcher(
        file, prefetch_buffer, footer, ReadOptions(), prefixes_handle,
        &prefixes_contents, ioptions, true /*decompress*/,
        true /*maybe_compressed*/, BlockType::kHashIndexPrefixes,
        UncompressionDict::GetEmptyDict(), cache_options, memory_allocator);
    s = prefixes_block_fetcher.ReadBlockContents();
    if (!s.ok()) {
      return s;
    }
    BlockContents prefixes_meta_contents;
    BlockFetcher prefixes_meta_block_fetcher(
        file, prefetch_buffer, footer, ReadOptions(), prefixes_meta_handle,
        &prefixes_meta_contents, ioptions, true /*decompress*/,
        true /*maybe_compressed*/, BlockType::kHashIndexMetadata,
        UncompressionDict::GetEmptyDict(), cache_options, memory_allocator);
    s = prefixes_meta_block_fetcher.ReadBlockContents();
    if (!s.ok()) {
      // TODO: log error
      return Status::OK();
    }

    BlockPrefixIndex* prefix_index = nullptr;
    assert(rep->internal_prefix_transform.get() != nullptr);
    s = BlockPrefixIndex::Create(rep->internal_prefix_transform.get(),
                                 prefixes_contents.data,
                                 prefixes_meta_contents.data, &prefix_index);
    // TODO: log error
    if (s.ok()) {
      HashIndexReader* const hash_index_reader =
          static_cast<HashIndexReader*>(index_reader->get());
      hash_index_reader->prefix_index_.reset(prefix_index);
    }

    return Status::OK();
  }

  InternalIteratorBase<IndexValue>* NewIterator(
      const ReadOptions& read_options, bool disable_prefix_seek,
      IndexBlockIter* iter, GetContext* get_context,
      BlockCacheLookupContext* lookup_context) override {
    const bool no_io = (read_options.read_tier == kBlockCacheTier);
    CachableEntry<Block> index_block;
    const Status s =
        GetOrReadIndexBlock(no_io, get_context, lookup_context, &index_block);
    if (!s.ok()) {
      if (iter != nullptr) {
        iter->Invalidate(s);
        return iter;
      }

      return NewErrorInternalIterator<IndexValue>(s);
    }

    Statistics* kNullStats = nullptr;
    const bool total_order_seek =
        read_options.total_order_seek || disable_prefix_seek;
    // We don't return pinned data from index blocks, so no need
    // to set `block_contents_pinned`.
    auto it = index_block.GetValue()->NewIndexIterator(
        internal_comparator(), internal_comparator()->user_comparator(), iter,
        kNullStats, total_order_seek, index_has_first_key(),
        index_key_includes_seq(), index_value_is_full(),
        false /* block_contents_pinned */, prefix_index_.get());

    assert(it != nullptr);
    index_block.TransferTo(it);

    return it;
  }

  size_t ApproximateMemoryUsage() const override {
    size_t usage = ApproximateIndexBlockMemoryUsage();
#ifdef ROCKSDB_MALLOC_USABLE_SIZE
    usage += malloc_usable_size(const_cast<HashIndexReader*>(this));
#else
    if (prefix_index_) {
      usage += prefix_index_->ApproximateMemoryUsage();
    }
    usage += sizeof(*this);
#endif  // ROCKSDB_MALLOC_USABLE_SIZE
    return usage;
  }

 private:
  HashIndexReader(const BlockBasedTable* t, CachableEntry<Block>&& index_block)
      : IndexReaderCommon(t, std::move(index_block)) {}

  std::unique_ptr<BlockPrefixIndex> prefix_index_;
};

void BlockBasedTable::UpdateCacheHitMetrics(BlockType block_type,
                                            GetContext* get_context,
                                            size_t usage) const {
  Statistics* const statistics = rep_->ioptions.statistics;

  PERF_COUNTER_ADD(block_cache_hit_count, 1);
  PERF_COUNTER_BY_LEVEL_ADD(block_cache_hit_count, 1,
                            static_cast<uint32_t>(rep_->level));

  if (get_context) {
    ++get_context->get_context_stats_.num_cache_hit;
    get_context->get_context_stats_.num_cache_bytes_read += usage;
  } else {
    RecordTick(statistics, BLOCK_CACHE_HIT);
    RecordTick(statistics, BLOCK_CACHE_BYTES_READ, usage);
  }

  switch (block_type) {
    case BlockType::kFilter:
      PERF_COUNTER_ADD(block_cache_filter_hit_count, 1);

      if (get_context) {
        ++get_context->get_context_stats_.num_cache_filter_hit;
      } else {
        RecordTick(statistics, BLOCK_CACHE_FILTER_HIT);
      }
      break;

    case BlockType::kCompressionDictionary:
      // TODO: introduce perf counter for compression dictionary hit count
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_compression_dict_hit;
      } else {
        RecordTick(statistics, BLOCK_CACHE_COMPRESSION_DICT_HIT);
      }
      break;

    case BlockType::kIndex:
      PERF_COUNTER_ADD(block_cache_index_hit_count, 1);

      if (get_context) {
        ++get_context->get_context_stats_.num_cache_index_hit;
      } else {
        RecordTick(statistics, BLOCK_CACHE_INDEX_HIT);
      }
      break;

    default:
      // TODO: introduce dedicated tickers/statistics/counters
      // for range tombstones
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_data_hit;
      } else {
        RecordTick(statistics, BLOCK_CACHE_DATA_HIT);
      }
      break;
  }
}

void BlockBasedTable::UpdateCacheMissMetrics(BlockType block_type,
                                             GetContext* get_context) const {
  Statistics* const statistics = rep_->ioptions.statistics;

  // TODO: introduce aggregate (not per-level) block cache miss count
  PERF_COUNTER_BY_LEVEL_ADD(block_cache_miss_count, 1,
                            static_cast<uint32_t>(rep_->level));

  if (get_context) {
    ++get_context->get_context_stats_.num_cache_miss;
  } else {
    RecordTick(statistics, BLOCK_CACHE_MISS);
  }

  // TODO: introduce perf counters for misses per block type
  switch (block_type) {
    case BlockType::kFilter:
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_filter_miss;
      } else {
        RecordTick(statistics, BLOCK_CACHE_FILTER_MISS);
      }
      break;

    case BlockType::kCompressionDictionary:
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_compression_dict_miss;
      } else {
        RecordTick(statistics, BLOCK_CACHE_COMPRESSION_DICT_MISS);
      }
      break;

    case BlockType::kIndex:
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_index_miss;
      } else {
        RecordTick(statistics, BLOCK_CACHE_INDEX_MISS);
      }
      break;

    default:
      // TODO: introduce dedicated tickers/statistics/counters
      // for range tombstones
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_data_miss;
      } else {
        RecordTick(statistics, BLOCK_CACHE_DATA_MISS);
      }
      break;
  }
}

void BlockBasedTable::UpdateCacheInsertionMetrics(BlockType block_type,
                                                  GetContext* get_context,
                                                  size_t usage) const {
  Statistics* const statistics = rep_->ioptions.statistics;

  // TODO: introduce perf counters for block cache insertions
  if (get_context) {
    ++get_context->get_context_stats_.num_cache_add;
    get_context->get_context_stats_.num_cache_bytes_write += usage;
  } else {
    RecordTick(statistics, BLOCK_CACHE_ADD);
    RecordTick(statistics, BLOCK_CACHE_BYTES_WRITE, usage);
  }

  switch (block_type) {
    case BlockType::kFilter:
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_filter_add;
        get_context->get_context_stats_.num_cache_filter_bytes_insert += usage;
      } else {
        RecordTick(statistics, BLOCK_CACHE_FILTER_ADD);
        RecordTick(statistics, BLOCK_CACHE_FILTER_BYTES_INSERT, usage);
      }
      break;

    case BlockType::kCompressionDictionary:
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_compression_dict_add;
        get_context->get_context_stats_
            .num_cache_compression_dict_bytes_insert += usage;
      } else {
        RecordTick(statistics, BLOCK_CACHE_COMPRESSION_DICT_ADD);
        RecordTick(statistics, BLOCK_CACHE_COMPRESSION_DICT_BYTES_INSERT,
                   usage);
      }
      break;

    case BlockType::kIndex:
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_index_add;
        get_context->get_context_stats_.num_cache_index_bytes_insert += usage;
      } else {
        RecordTick(statistics, BLOCK_CACHE_INDEX_ADD);
        RecordTick(statistics, BLOCK_CACHE_INDEX_BYTES_INSERT, usage);
      }
      break;

    default:
      // TODO: introduce dedicated tickers/statistics/counters
      // for range tombstones
      if (get_context) {
        ++get_context->get_context_stats_.num_cache_data_add;
        get_context->get_context_stats_.num_cache_data_bytes_insert += usage;
      } else {
        RecordTick(statistics, BLOCK_CACHE_DATA_ADD);
        RecordTick(statistics, BLOCK_CACHE_DATA_BYTES_INSERT, usage);
      }
      break;
  }
}

Cache::Handle* BlockBasedTable::GetEntryFromCache(
    Cache* block_cache, const Slice& key, BlockType block_type,
    GetContext* get_context) const {
  auto cache_handle = block_cache->Lookup(key, rep_->ioptions.statistics);

  if (cache_handle != nullptr) {
    UpdateCacheHitMetrics(block_type, get_context,
                          block_cache->GetUsage(cache_handle));
  } else {
    UpdateCacheMissMetrics(block_type, get_context);
  }

  return cache_handle;
}

// Helper function to setup the cache key's prefix for the Table.
void BlockBasedTable::SetupCacheKeyPrefix(Rep* rep) {
  assert(kMaxCacheKeyPrefixSize >= 10);
  rep->cache_key_prefix_size = 0;
  rep->compressed_cache_key_prefix_size = 0;
  if (rep->table_options.block_cache != nullptr) {
    GenerateCachePrefix(rep->table_options.block_cache.get(), rep->file->file(),
                        &rep->cache_key_prefix[0], &rep->cache_key_prefix_size);
  }
  if (rep->table_options.persistent_cache != nullptr) {
    GenerateCachePrefix(/*cache=*/nullptr, rep->file->file(),
                        &rep->persistent_cache_key_prefix[0],
                        &rep->persistent_cache_key_prefix_size);
  }
  if (rep->table_options.block_cache_compressed != nullptr) {
    GenerateCachePrefix(rep->table_options.block_cache_compressed.get(),
                        rep->file->file(), &rep->compressed_cache_key_prefix[0],
                        &rep->compressed_cache_key_prefix_size);
  }
}

void BlockBasedTable::GenerateCachePrefix(Cache* cc, FSRandomAccessFile* file,
                                          char* buffer, size_t* size) {
  // generate an id from the file
  *size = file->GetUniqueId(buffer, kMaxCacheKeyPrefixSize);

  // If the prefix wasn't generated or was too long,
  // create one from the cache.
  if (cc != nullptr && *size == 0) {
    char* end = EncodeVarint64(buffer, cc->NewId());
    *size = static_cast<size_t>(end - buffer);
  }
}

void BlockBasedTable::GenerateCachePrefix(Cache* cc, FSWritableFile* file,
                                          char* buffer, size_t* size) {
  // generate an id from the file
  *size = file->GetUniqueId(buffer, kMaxCacheKeyPrefixSize);

  // If the prefix wasn't generated or was too long,
  // create one from the cache.
  if (cc != nullptr && *size == 0) {
    char* end = EncodeVarint64(buffer, cc->NewId());
    *size = static_cast<size_t>(end - buffer);
  }
}

namespace {
// Return True if table_properties has `user_prop_name` has a `true` value
// or it doesn't contain this property (for backward compatible).
bool IsFeatureSupported(const TableProperties& table_properties,
                        const std::string& user_prop_name, Logger* info_log) {
  auto& props = table_properties.user_collected_properties;
  auto pos = props.find(user_prop_name);
  // Older version doesn't have this value set. Skip this check.
  if (pos != props.end()) {
    if (pos->second == kPropFalse) {
      return false;
    } else if (pos->second != kPropTrue) {
      ROCKS_LOG_WARN(info_log, "Property %s has invalidate value %s",
                     user_prop_name.c_str(), pos->second.c_str());
    }
  }
  return true;
}

// Caller has to ensure seqno is not nullptr.
Status GetGlobalSequenceNumber(const TableProperties& table_properties,
                               SequenceNumber largest_seqno,
                               SequenceNumber* seqno) {
  const auto& props = table_properties.user_collected_properties;
  const auto version_pos = props.find(ExternalSstFilePropertyNames::kVersion);
  const auto seqno_pos = props.find(ExternalSstFilePropertyNames::kGlobalSeqno);

  *seqno = kDisableGlobalSequenceNumber;
  if (version_pos == props.end()) {
    if (seqno_pos != props.end()) {
      std::array<char, 200> msg_buf;
      // This is not an external sst file, global_seqno is not supported.
      snprintf(
          msg_buf.data(), msg_buf.max_size(),
          "A non-external sst file have global seqno property with value %s",
          seqno_pos->second.c_str());
      return Status::Corruption(msg_buf.data());
    }
    return Status::OK();
  }

  uint32_t version = DecodeFixed32(version_pos->second.c_str());
  if (version < 2) {
    if (seqno_pos != props.end() || version != 1) {
      std::array<char, 200> msg_buf;
      // This is a v1 external sst file, global_seqno is not supported.
      snprintf(msg_buf.data(), msg_buf.max_size(),
               "An external sst file with version %u have global seqno "
               "property with value %s",
               version, seqno_pos->second.c_str());
      return Status::Corruption(msg_buf.data());
    }
    return Status::OK();
  }

  // Since we have a plan to deprecate global_seqno, we do not return failure
  // if seqno_pos == props.end(). We rely on version_pos to detect whether the
  // SST is external.
  SequenceNumber global_seqno(0);
  if (seqno_pos != props.end()) {
    global_seqno = DecodeFixed64(seqno_pos->second.c_str());
  }
  // SstTableReader open table reader with kMaxSequenceNumber as largest_seqno
  // to denote it is unknown.
  if (largest_seqno < kMaxSequenceNumber) {
    if (global_seqno == 0) {
      global_seqno = largest_seqno;
    }
    if (global_seqno != largest_seqno) {
      std::array<char, 200> msg_buf;
      snprintf(
          msg_buf.data(), msg_buf.max_size(),
          "An external sst file with version %u have global seqno property "
          "with value %s, while largest seqno in the file is %llu",
          version, seqno_pos->second.c_str(),
          static_cast<unsigned long long>(largest_seqno));
      return Status::Corruption(msg_buf.data());
    }
  }
  *seqno = global_seqno;

  if (global_seqno > kMaxSequenceNumber) {
    std::array<char, 200> msg_buf;
    snprintf(msg_buf.data(), msg_buf.max_size(),
             "An external sst file with version %u have global seqno property "
             "with value %llu, which is greater than kMaxSequenceNumber",
             version, static_cast<unsigned long long>(global_seqno));
    return Status::Corruption(msg_buf.data());
  }

  return Status::OK();
}
}  // namespace

Slice BlockBasedTable::GetCacheKey(const char* cache_key_prefix,
                                   size_t cache_key_prefix_size,
                                   const BlockHandle& handle, char* cache_key) {
  assert(cache_key != nullptr);
  assert(cache_key_prefix_size != 0);
  assert(cache_key_prefix_size <= kMaxCacheKeyPrefixSize);
  memcpy(cache_key, cache_key_prefix, cache_key_prefix_size);
  char* end =
      EncodeVarint64(cache_key + cache_key_prefix_size, handle.offset());
  return Slice(cache_key, static_cast<size_t>(end - cache_key));
}

Status BlockBasedTable::Open(
    const ImmutableCFOptions& ioptions, const EnvOptions& env_options,
    const BlockBasedTableOptions& table_options,
    const InternalKeyComparator& internal_comparator,
    std::unique_ptr<RandomAccessFileReader>&& file, uint64_t file_size,
    std::unique_ptr<TableReader>* table_reader,
    const SliceTransform* prefix_extractor,
    const bool prefetch_index_and_filter_in_cache, const bool skip_filters,
    const int level, const bool immortal_table,
    const SequenceNumber largest_seqno, TailPrefetchStats* tail_prefetch_stats,
    BlockCacheTracer* const block_cache_tracer) {
  table_reader->reset();

  Status s;
  Footer footer;
  std::unique_ptr<FilePrefetchBuffer> prefetch_buffer;

  // prefetch both index and filters, down to all partitions
  const bool prefetch_all = prefetch_index_and_filter_in_cache || level == 0;
  const bool preload_all = !table_options.cache_index_and_filter_blocks;

  if (!ioptions.allow_mmap_reads) {
    s = PrefetchTail(file.get(), file_size, tail_prefetch_stats, prefetch_all,
                     preload_all, &prefetch_buffer);
  } else {
    // Should not prefetch for mmap mode.
    prefetch_buffer.reset(new FilePrefetchBuffer(
        nullptr, 0, 0, false /* enable */, true /* track_min_offset */));
  }

  // Read in the following order:
  //    1. Footer
  //    2. [metaindex block]
  //    3. [meta block: properties]
  //    4. [meta block: range deletion tombstone]
  //    5. [meta block: compression dictionary]
  //    6. [meta block: index]
  //    7. [meta block: filter]
  s = ReadFooterFromFile(file.get(), prefetch_buffer.get(), file_size, &footer,
                         kBlockBasedTableMagicNumber);
  if (!s.ok()) {
    return s;
  }
  if (!BlockBasedTableSupportedVersion(footer.version())) {
    return Status::Corruption(
        "Unknown Footer version. Maybe this file was created with newer "
        "version of RocksDB?");
  }

  // We've successfully read the footer. We are ready to serve requests.
  // Better not mutate rep_ after the creation. eg. internal_prefix_transform
  // raw pointer will be used to create HashIndexReader, whose reset may
  // access a dangling pointer.
  BlockCacheLookupContext lookup_context{TableReaderCaller::kPrefetch};
  Rep* rep = new BlockBasedTable::Rep(ioptions, env_options, table_options,
                                      internal_comparator, skip_filters, level,
                                      immortal_table);
  rep->file = std::move(file);
  rep->footer = footer;
  rep->hash_index_allow_collision = table_options.hash_index_allow_collision;
  // We need to wrap data with internal_prefix_transform to make sure it can
  // handle prefix correctly.
  if (prefix_extractor != nullptr) {
    rep->internal_prefix_transform.reset(
        new InternalKeySliceTransform(prefix_extractor));
  }
  SetupCacheKeyPrefix(rep);
  std::unique_ptr<BlockBasedTable> new_table(
      new BlockBasedTable(rep, block_cache_tracer));

  // page cache options
  rep->persistent_cache_options =
      PersistentCacheOptions(rep->table_options.persistent_cache,
                             std::string(rep->persistent_cache_key_prefix,
                                         rep->persistent_cache_key_prefix_size),
                             rep->ioptions.statistics);

  // Meta-blocks are not dictionary compressed. Explicitly set the dictionary
  // handle to null, otherwise it may be seen as uninitialized during the below
  // meta-block reads.
  rep->compression_dict_handle = BlockHandle::NullBlockHandle();

  // Read metaindex
  std::unique_ptr<Block> metaindex;
  std::unique_ptr<InternalIterator> metaindex_iter;
  s = new_table->ReadMetaIndexBlock(prefetch_buffer.get(), &metaindex,
                                    &metaindex_iter);
  if (!s.ok()) {
    return s;
  }

  // Populates table_properties and some fields that depend on it,
  // such as index_type.
  s = new_table->ReadPropertiesBlock(prefetch_buffer.get(),
                                     metaindex_iter.get(), largest_seqno);
  if (!s.ok()) {
    return s;
  }
  s = new_table->ReadRangeDelBlock(prefetch_buffer.get(), metaindex_iter.get(),
                                   internal_comparator, &lookup_context);
  if (!s.ok()) {
    return s;
  }
  s = new_table->PrefetchIndexAndFilterBlocks(
      prefetch_buffer.get(), metaindex_iter.get(), new_table.get(),
      prefetch_all, table_options, level, &lookup_context);

  if (s.ok()) {
    // Update tail prefetch stats
    assert(prefetch_buffer.get() != nullptr);
    if (tail_prefetch_stats != nullptr) {
      assert(prefetch_buffer->min_offset_read() < file_size);
      tail_prefetch_stats->RecordEffectiveSize(
          static_cast<size_t>(file_size) - prefetch_buffer->min_offset_read());
    }

    *table_reader = std::move(new_table);
  }

  return s;
}

Status BlockBasedTable::PrefetchTail(
    RandomAccessFileReader* file, uint64_t file_size,
    TailPrefetchStats* tail_prefetch_stats, const bool prefetch_all,
    const bool preload_all,
    std::unique_ptr<FilePrefetchBuffer>* prefetch_buffer) {
  size_t tail_prefetch_size = 0;
  if (tail_prefetch_stats != nullptr) {
    // Multiple threads may get a 0 (no history) when running in parallel,
    // but it will get cleared after the first of them finishes.
    tail_prefetch_size = tail_prefetch_stats->GetSuggestedPrefetchSize();
  }
  if (tail_prefetch_size == 0) {
    // Before read footer, readahead backwards to prefetch data. Do more
    // readahead if we're going to read index/filter.
    // TODO: This may incorrectly select small readahead in case partitioned
    // index/filter is enabled and top-level partition pinning is enabled.
    // That's because we need to issue readahead before we read the properties,
    // at which point we don't yet know the index type.
    tail_prefetch_size = prefetch_all || preload_all ? 512 * 1024 : 4 * 1024;
  }
  size_t prefetch_off;
  size_t prefetch_len;
  if (file_size < tail_prefetch_size) {
    prefetch_off = 0;
    prefetch_len = static_cast<size_t>(file_size);
  } else {
    prefetch_off = static_cast<size_t>(file_size - tail_prefetch_size);
    prefetch_len = tail_prefetch_size;
  }
  TEST_SYNC_POINT_CALLBACK("BlockBasedTable::Open::TailPrefetchLen",
                           &tail_prefetch_size);
  Status s;
  // TODO should not have this special logic in the future.
  if (!file->use_direct_io()) {
    prefetch_buffer->reset(new FilePrefetchBuffer(
        nullptr, 0, 0, false /* enable */, true /* track_min_offset */));
    s = file->Prefetch(prefetch_off, prefetch_len);
  } else {
    prefetch_buffer->reset(new FilePrefetchBuffer(
        nullptr, 0, 0, true /* enable */, true /* track_min_offset */));
    s = (*prefetch_buffer)->Prefetch(file, prefetch_off, prefetch_len);
  }
  return s;
}

Status VerifyChecksum(const ChecksumType type, const char* buf, size_t len,
                      uint32_t expected) {
  Status s;
  uint32_t actual = 0;
  switch (type) {
    case kNoChecksum:
      break;
    case kCRC32c:
      expected = crc32c::Unmask(expected);
      actual = crc32c::Value(buf, len);
      break;
    case kxxHash:
      actual = XXH32(buf, static_cast<int>(len), 0);
      break;
    case kxxHash64:
      actual = static_cast<uint32_t>(XXH64(buf, static_cast<int>(len), 0) &
                                     uint64_t{0xffffffff});
      break;
    default:
      s = Status::Corruption("unknown checksum type");
  }
  if (s.ok() && actual != expected) {
    s = Status::Corruption("properties block checksum mismatched");
  }
  return s;
}

Status BlockBasedTable::TryReadPropertiesWithGlobalSeqno(
    FilePrefetchBuffer* prefetch_buffer, const Slice& handle_value,
    TableProperties** table_properties) {
  assert(table_properties != nullptr);
  // If this is an external SST file ingested with write_global_seqno set to
  // true, then we expect the checksum mismatch because checksum was written
  // by SstFileWriter, but its global seqno in the properties block may have
  // been changed during ingestion. In this case, we read the properties
  // block, copy it to a memory buffer, change the global seqno to its
  // original value, i.e. 0, and verify the checksum again.
  BlockHandle props_block_handle;
  CacheAllocationPtr tmp_buf;
  Status s = ReadProperties(handle_value, rep_->file.get(), prefetch_buffer,
                            rep_->footer, rep_->ioptions, table_properties,
                            false /* verify_checksum */, &props_block_handle,
                            &tmp_buf, false /* compression_type_missing */,
                            nullptr /* memory_allocator */);
  if (s.ok() && tmp_buf) {
    const auto seqno_pos_iter =
        (*table_properties)
            ->properties_offsets.find(
                ExternalSstFilePropertyNames::kGlobalSeqno);
    size_t block_size = static_cast<size_t>(props_block_handle.size());
    if (seqno_pos_iter != (*table_properties)->properties_offsets.end()) {
      uint64_t global_seqno_offset = seqno_pos_iter->second;
      EncodeFixed64(
          tmp_buf.get() + global_seqno_offset - props_block_handle.offset(), 0);
    }
    uint32_t value = DecodeFixed32(tmp_buf.get() + block_size + 1);
    s = ROCKSDB_NAMESPACE::VerifyChecksum(rep_->footer.checksum(),
                                          tmp_buf.get(), block_size + 1, value);
  }
  return s;
}

Status BlockBasedTable::ReadPropertiesBlock(
    FilePrefetchBuffer* prefetch_buffer, InternalIterator* meta_iter,
    const SequenceNumber largest_seqno) {
  bool found_properties_block = true;
  Status s;
  s = SeekToPropertiesBlock(meta_iter, &found_properties_block);

  if (!s.ok()) {
    ROCKS_LOG_WARN(rep_->ioptions.info_log,
                   "Error when seeking to properties block from file: %s",
                   s.ToString().c_str());
  } else if (found_properties_block) {
    s = meta_iter->status();
    TableProperties* table_properties = nullptr;
    if (s.ok()) {
      s = ReadProperties(
          meta_iter->value(), rep_->file.get(), prefetch_buffer, rep_->footer,
          rep_->ioptions, &table_properties, true /* verify_checksum */,
          nullptr /* ret_block_handle */, nullptr /* ret_block_contents */,
          false /* compression_type_missing */, nullptr /* memory_allocator */);
    }

    if (s.IsCorruption()) {
      s = TryReadPropertiesWithGlobalSeqno(prefetch_buffer, meta_iter->value(),
                                           &table_properties);
    }
    std::unique_ptr<TableProperties> props_guard;
    if (table_properties != nullptr) {
      props_guard.reset(table_properties);
    }

    if (!s.ok()) {
      ROCKS_LOG_WARN(rep_->ioptions.info_log,
                     "Encountered error while reading data from properties "
                     "block %s",
                     s.ToString().c_str());
    } else {
      assert(table_properties != nullptr);
      rep_->table_properties.reset(props_guard.release());
      rep_->blocks_maybe_compressed =
          rep_->table_properties->compression_name !=
          CompressionTypeToString(kNoCompression);
      rep_->blocks_definitely_zstd_compressed =
          (rep_->table_properties->compression_name ==
               CompressionTypeToString(kZSTD) ||
           rep_->table_properties->compression_name ==
               CompressionTypeToString(kZSTDNotFinalCompression));
    }
  } else {
    ROCKS_LOG_ERROR(rep_->ioptions.info_log,
                    "Cannot find Properties block from file.");
  }
#ifndef ROCKSDB_LITE
  if (rep_->table_properties) {
    ParseSliceTransform(rep_->table_properties->prefix_extractor_name,
                        &(rep_->table_prefix_extractor));
  }
#endif  // ROCKSDB_LITE

  // Read the table properties, if provided.
  if (rep_->table_properties) {
    rep_->whole_key_filtering &=
        IsFeatureSupported(*(rep_->table_properties),
                           BlockBasedTablePropertyNames::kWholeKeyFiltering,
                           rep_->ioptions.info_log);
    rep_->prefix_filtering &=
        IsFeatureSupported(*(rep_->table_properties),
                           BlockBasedTablePropertyNames::kPrefixFiltering,
                           rep_->ioptions.info_log);

    rep_->index_key_includes_seq =
        rep_->table_properties->index_key_is_user_key == 0;
    rep_->index_value_is_full =
        rep_->table_properties->index_value_is_delta_encoded == 0;

    // Update index_type with the true type.
    // If table properties don't contain index type, we assume that the table
    // is in very old format and has kBinarySearch index type.
    auto& props = rep_->table_properties->user_collected_properties;
    auto pos = props.find(BlockBasedTablePropertyNames::kIndexType);
    if (pos != props.end()) {
      rep_->index_type = static_cast<BlockBasedTableOptions::IndexType>(
          DecodeFixed32(pos->second.c_str()));
    }

    rep_->index_has_first_key =
        rep_->index_type == BlockBasedTableOptions::kBinarySearchWithFirstKey;

    s = GetGlobalSequenceNumber(*(rep_->table_properties), largest_seqno,
                                &(rep_->global_seqno));
    if (!s.ok()) {
      ROCKS_LOG_ERROR(rep_->ioptions.info_log, "%s", s.ToString().c_str());
    }
  }
  return s;
}

Status BlockBasedTable::ReadRangeDelBlock(
    FilePrefetchBuffer* prefetch_buffer, InternalIterator* meta_iter,
    const InternalKeyComparator& internal_comparator,
    BlockCacheLookupContext* lookup_context) {
  Status s;
  bool found_range_del_block;
  BlockHandle range_del_handle;
  s = SeekToRangeDelBlock(meta_iter, &found_range_del_block, &range_del_handle);
  if (!s.ok()) {
    ROCKS_LOG_WARN(
        rep_->ioptions.info_log,
        "Error when seeking to range delete tombstones block from file: %s",
        s.ToString().c_str());
  } else if (found_range_del_block && !range_del_handle.IsNull()) {
    ReadOptions read_options;
    std::unique_ptr<InternalIterator> iter(NewDataBlockIterator<DataBlockIter>(
        read_options, range_del_handle,
        /*input_iter=*/nullptr, BlockType::kRangeDeletion,
        /*get_context=*/nullptr, lookup_context, Status(), prefetch_buffer));
    assert(iter != nullptr);
    s = iter->status();
    if (!s.ok()) {
      ROCKS_LOG_WARN(
          rep_->ioptions.info_log,
          "Encountered error while reading data from range del block %s",
          s.ToString().c_str());
    } else {
      rep_->fragmented_range_dels =
          std::make_shared<FragmentedRangeTombstoneList>(std::move(iter),
                                                         internal_comparator);
    }
  }
  return s;
}

Status BlockBasedTable::PrefetchIndexAndFilterBlocks(
    FilePrefetchBuffer* prefetch_buffer, InternalIterator* meta_iter,
    BlockBasedTable* new_table, bool prefetch_all,
    const BlockBasedTableOptions& table_options, const int level,
    BlockCacheLookupContext* lookup_context) {
  Status s;

  // Find filter handle and filter type
  if (rep_->filter_policy) {
    for (auto filter_type :
         {Rep::FilterType::kFullFilter, Rep::FilterType::kPartitionedFilter,
          Rep::FilterType::kBlockFilter}) {
      std::string prefix;
      switch (filter_type) {
        case Rep::FilterType::kFullFilter:
          prefix = kFullFilterBlockPrefix;
          break;
        case Rep::FilterType::kPartitionedFilter:
          prefix = kPartitionedFilterBlockPrefix;
          break;
        case Rep::FilterType::kBlockFilter:
          prefix = kFilterBlockPrefix;
          break;
        default:
          assert(0);
      }
      std::string filter_block_key = prefix;
      filter_block_key.append(rep_->filter_policy->Name());
      if (FindMetaBlock(meta_iter, filter_block_key, &rep_->filter_handle)
              .ok()) {
        rep_->filter_type = filter_type;
        break;
      }
    }
  }

  // Find compression dictionary handle
  bool found_compression_dict = false;
  s = SeekToCompressionDictBlock(meta_iter, &found_compression_dict,
                                 &rep_->compression_dict_handle);
  if (!s.ok()) {
    return s;
  }

  BlockBasedTableOptions::IndexType index_type = rep_->index_type;

  const bool use_cache = table_options.cache_index_and_filter_blocks;

  // pin both index and filters, down to all partitions
  const bool pin_all =
      rep_->table_options.pin_l0_filter_and_index_blocks_in_cache && level == 0;

  // prefetch the first level of index
  const bool prefetch_index =
      prefetch_all ||
      (table_options.pin_top_level_index_and_filter &&
       index_type == BlockBasedTableOptions::kTwoLevelIndexSearch);
  // pin the first level of index
  const bool pin_index =
      pin_all || (table_options.pin_top_level_index_and_filter &&
                  index_type == BlockBasedTableOptions::kTwoLevelIndexSearch);

  std::unique_ptr<IndexReader> index_reader;
  s = new_table->CreateIndexReader(prefetch_buffer, meta_iter, use_cache,
                                   prefetch_index, pin_index, lookup_context,
                                   &index_reader);
  if (!s.ok()) {
    return s;
  }

  rep_->index_reader = std::move(index_reader);

  // The partitions of partitioned index are always stored in cache. They
  // are hence follow the configuration for pin and prefetch regardless of
  // the value of cache_index_and_filter_blocks
  if (prefetch_all) {
    rep_->index_reader->CacheDependencies(pin_all);
  }

  // prefetch the first level of filter
  const bool prefetch_filter =
      prefetch_all ||
      (table_options.pin_top_level_index_and_filter &&
       rep_->filter_type == Rep::FilterType::kPartitionedFilter);
  // Partition fitlers cannot be enabled without partition indexes
  assert(!prefetch_filter || prefetch_index);
  // pin the first level of filter
  const bool pin_filter =
      pin_all || (table_options.pin_top_level_index_and_filter &&
                  rep_->filter_type == Rep::FilterType::kPartitionedFilter);

  if (rep_->filter_policy) {
    auto filter = new_table->CreateFilterBlockReader(
        prefetch_buffer, use_cache, prefetch_filter, pin_filter,
        lookup_context);
    if (filter) {
      // Refer to the comment above about paritioned indexes always being cached
      if (prefetch_all) {
        filter->CacheDependencies(pin_all);
      }

      rep_->filter = std::move(filter);
    }
  }

  if (!rep_->compression_dict_handle.IsNull()) {
    std::unique_ptr<UncompressionDictReader> uncompression_dict_reader;
    s = UncompressionDictReader::Create(this, prefetch_buffer, use_cache,
                                        prefetch_all, pin_all, lookup_context,
                                        &uncompression_dict_reader);
    if (!s.ok()) {
      return s;
    }

    rep_->uncompression_dict_reader = std::move(uncompression_dict_reader);
  }

  assert(s.ok());
  return s;
}

void BlockBasedTable::SetupForCompaction() {
  switch (rep_->ioptions.access_hint_on_compaction_start) {
    case Options::NONE:
      break;
    case Options::NORMAL:
      rep_->file->file()->Hint(FSRandomAccessFile::kNormal);
      break;
    case Options::SEQUENTIAL:
      rep_->file->file()->Hint(FSRandomAccessFile::kSequential);
      break;
    case Options::WILLNEED:
      rep_->file->file()->Hint(FSRandomAccessFile::kWillNeed);
      break;
    default:
      assert(false);
  }
}

std::shared_ptr<const TableProperties> BlockBasedTable::GetTableProperties()
    const {
  return rep_->table_properties;
}

size_t BlockBasedTable::ApproximateMemoryUsage() const {
  size_t usage = 0;
  if (rep_->filter) {
    usage += rep_->filter->ApproximateMemoryUsage();
  }
  if (rep_->index_reader) {
    usage += rep_->index_reader->ApproximateMemoryUsage();
  }
  if (rep_->uncompression_dict_reader) {
    usage += rep_->uncompression_dict_reader->ApproximateMemoryUsage();
  }
  return usage;
}

// Load the meta-index-block from the file. On success, return the loaded
// metaindex
// block and its iterator.
Status BlockBasedTable::ReadMetaIndexBlock(
    FilePrefetchBuffer* prefetch_buffer,
    std::unique_ptr<Block>* metaindex_block,
    std::unique_ptr<InternalIterator>* iter) {
  // TODO(sanjay): Skip this if footer.metaindex_handle() size indicates
  // it is an empty block.
  std::unique_ptr<Block> metaindex;
  Status s = ReadBlockFromFile(
      rep_->file.get(), prefetch_buffer, rep_->footer, ReadOptions(),
      rep_->footer.metaindex_handle(), &metaindex, rep_->ioptions,
      true /* decompress */, true /*maybe_compressed*/, BlockType::kMetaIndex,
      UncompressionDict::GetEmptyDict(), rep_->persistent_cache_options,
      kDisableGlobalSequenceNumber, 0 /* read_amp_bytes_per_bit */,
      GetMemoryAllocator(rep_->table_options), false /* for_compaction */,
      rep_->blocks_definitely_zstd_compressed, nullptr /* filter_policy */);

  if (!s.ok()) {
    ROCKS_LOG_ERROR(rep_->ioptions.info_log,
                    "Encountered error while reading data from properties"
                    " block %s",
                    s.ToString().c_str());
    return s;
  }

  *metaindex_block = std::move(metaindex);
  // meta block uses bytewise comparator.
  iter->reset(metaindex_block->get()->NewDataIterator(BytewiseComparator(),
                                                      BytewiseComparator()));
  return Status::OK();
}

template <typename TBlocklike>
Status BlockBasedTable::GetDataBlockFromCache(
    const Slice& block_cache_key, const Slice& compressed_block_cache_key,
    Cache* block_cache, Cache* block_cache_compressed,
    const ReadOptions& read_options, CachableEntry<TBlocklike>* block,
    const UncompressionDict& uncompression_dict, BlockType block_type,
    GetContext* get_context) const {
  const size_t read_amp_bytes_per_bit =
      block_type == BlockType::kData
          ? rep_->table_options.read_amp_bytes_per_bit
          : 0;
  assert(block);
  assert(block->IsEmpty());

  Status s;
  BlockContents* compressed_block = nullptr;
  Cache::Handle* block_cache_compressed_handle = nullptr;

  // Lookup uncompressed cache first
  if (block_cache != nullptr) {
    auto cache_handle = GetEntryFromCache(block_cache, block_cache_key,
                                          block_type, get_context);
    if (cache_handle != nullptr) {
      block->SetCachedValue(
          reinterpret_cast<TBlocklike*>(block_cache->Value(cache_handle)),
          block_cache, cache_handle);
      return s;
    }
  }

  // If not found, search from the compressed block cache.
  assert(block->IsEmpty());

  if (block_cache_compressed == nullptr) {
    return s;
  }

  assert(!compressed_block_cache_key.empty());
  block_cache_compressed_handle =
      block_cache_compressed->Lookup(compressed_block_cache_key);

  Statistics* statistics = rep_->ioptions.statistics;

  // if we found in the compressed cache, then uncompress and insert into
  // uncompressed cache
  if (block_cache_compressed_handle == nullptr) {
    RecordTick(statistics, BLOCK_CACHE_COMPRESSED_MISS);
    return s;
  }

  // found compressed block
  RecordTick(statistics, BLOCK_CACHE_COMPRESSED_HIT);
  compressed_block = reinterpret_cast<BlockContents*>(
      block_cache_compressed->Value(block_cache_compressed_handle));
  CompressionType compression_type = compressed_block->get_compression_type();
  assert(compression_type != kNoCompression);

  // Retrieve the uncompressed contents into a new buffer
  BlockContents contents;
  UncompressionContext context(compression_type);
  UncompressionInfo info(context, uncompression_dict, compression_type);
  s = UncompressBlockContents(
      info, compressed_block->data.data(), compressed_block->data.size(),
      &contents, rep_->table_options.format_version, rep_->ioptions,
      GetMemoryAllocator(rep_->table_options));

  // Insert uncompressed block into block cache
  if (s.ok()) {
    std::unique_ptr<TBlocklike> block_holder(
        BlocklikeTraits<TBlocklike>::Create(
            std::move(contents), rep_->get_global_seqno(block_type),
            read_amp_bytes_per_bit, statistics,
            rep_->blocks_definitely_zstd_compressed,
            rep_->table_options.filter_policy.get()));  // uncompressed block

    if (block_cache != nullptr && block_holder->own_bytes() &&
        read_options.fill_cache) {
      size_t charge = block_holder->ApproximateMemoryUsage();
      Cache::Handle* cache_handle = nullptr;
      s = block_cache->Insert(block_cache_key, block_holder.get(), charge,
                              &DeleteCachedEntry<TBlocklike>, &cache_handle);
      if (s.ok()) {
        assert(cache_handle != nullptr);
        block->SetCachedValue(block_holder.release(), block_cache,
                              cache_handle);

        UpdateCacheInsertionMetrics(block_type, get_context, charge);
      } else {
        RecordTick(statistics, BLOCK_CACHE_ADD_FAILURES);
      }
    } else {
      block->SetOwnedValue(block_holder.release());
    }
  }

  // Release hold on compressed cache entry
  block_cache_compressed->Release(block_cache_compressed_handle);
  return s;
}

template <typename TBlocklike>
Status BlockBasedTable::PutDataBlockToCache(
    const Slice& block_cache_key, const Slice& compressed_block_cache_key,
    Cache* block_cache, Cache* block_cache_compressed,
    CachableEntry<TBlocklike>* cached_block, BlockContents* raw_block_contents,
    CompressionType raw_block_comp_type,
    const UncompressionDict& uncompression_dict, SequenceNumber seq_no,
    MemoryAllocator* memory_allocator, BlockType block_type,
    GetContext* get_context) const {
  const ImmutableCFOptions& ioptions = rep_->ioptions;
  const uint32_t format_version = rep_->table_options.format_version;
  const size_t read_amp_bytes_per_bit =
      block_type == BlockType::kData
          ? rep_->table_options.read_amp_bytes_per_bit
          : 0;
  const Cache::Priority priority =
      rep_->table_options.cache_index_and_filter_blocks_with_high_priority &&
              (block_type == BlockType::kFilter ||
               block_type == BlockType::kCompressionDictionary ||
               block_type == BlockType::kIndex)
          ? Cache::Priority::HIGH
          : Cache::Priority::LOW;
  assert(cached_block);
  assert(cached_block->IsEmpty());

  Status s;
  Statistics* statistics = ioptions.statistics;

  std::unique_ptr<TBlocklike> block_holder;
  if (raw_block_comp_type != kNoCompression) {
    // Retrieve the uncompressed contents into a new buffer
    BlockContents uncompressed_block_contents;
    UncompressionContext context(raw_block_comp_type);
    UncompressionInfo info(context, uncompression_dict, raw_block_comp_type);
    s = UncompressBlockContents(info, raw_block_contents->data.data(),
                                raw_block_contents->data.size(),
                                &uncompressed_block_contents, format_version,
                                ioptions, memory_allocator);
    if (!s.ok()) {
      return s;
    }

    block_holder.reset(BlocklikeTraits<TBlocklike>::Create(
        std::move(uncompressed_block_contents), seq_no, read_amp_bytes_per_bit,
        statistics, rep_->blocks_definitely_zstd_compressed,
        rep_->table_options.filter_policy.get()));
  } else {
    block_holder.reset(BlocklikeTraits<TBlocklike>::Create(
        std::move(*raw_block_contents), seq_no, read_amp_bytes_per_bit,
        statistics, rep_->blocks_definitely_zstd_compressed,
        rep_->table_options.filter_policy.get()));
  }

  // Insert compressed block into compressed block cache.
  // Release the hold on the compressed cache entry immediately.
  if (block_cache_compressed != nullptr &&
      raw_block_comp_type != kNoCompression && raw_block_contents != nullptr &&
      raw_block_contents->own_bytes()) {
#ifndef NDEBUG
    assert(raw_block_contents->is_raw_block);
#endif  // NDEBUG

    // We cannot directly put raw_block_contents because this could point to
    // an object in the stack.
    BlockContents* block_cont_for_comp_cache =
        new BlockContents(std::move(*raw_block_contents));
    s = block_cache_compressed->Insert(
        compressed_block_cache_key, block_cont_for_comp_cache,
        block_cont_for_comp_cache->ApproximateMemoryUsage(),
        &DeleteCachedEntry<BlockContents>);
    if (s.ok()) {
      // Avoid the following code to delete this cached block.
      RecordTick(statistics, BLOCK_CACHE_COMPRESSED_ADD);
    } else {
      RecordTick(statistics, BLOCK_CACHE_COMPRESSED_ADD_FAILURES);
      delete block_cont_for_comp_cache;
    }
  }

  // insert into uncompressed block cache
  if (block_cache != nullptr && block_holder->own_bytes()) {
    size_t charge = block_holder->ApproximateMemoryUsage();
    Cache::Handle* cache_handle = nullptr;
    s = block_cache->Insert(block_cache_key, block_holder.get(), charge,
                            &DeleteCachedEntry<TBlocklike>, &cache_handle,
                            priority);
    if (s.ok()) {
      assert(cache_handle != nullptr);
      cached_block->SetCachedValue(block_holder.release(), block_cache,
                                   cache_handle);

      UpdateCacheInsertionMetrics(block_type, get_context, charge);
    } else {
      RecordTick(statistics, BLOCK_CACHE_ADD_FAILURES);
    }
  } else {
    cached_block->SetOwnedValue(block_holder.release());
  }

  return s;
}

std::unique_ptr<FilterBlockReader> BlockBasedTable::CreateFilterBlockReader(
    FilePrefetchBuffer* prefetch_buffer, bool use_cache, bool prefetch,
    bool pin, BlockCacheLookupContext* lookup_context) {
  auto& rep = rep_;
  auto filter_type = rep->filter_type;
  if (filter_type == Rep::FilterType::kNoFilter) {
    return std::unique_ptr<FilterBlockReader>();
  }

  assert(rep->filter_policy);

  switch (filter_type) {
    case Rep::FilterType::kPartitionedFilter:
      return PartitionedFilterBlockReader::Create(
          this, prefetch_buffer, use_cache, prefetch, pin, lookup_context);

    case Rep::FilterType::kBlockFilter:
      return BlockBasedFilterBlockReader::Create(
          this, prefetch_buffer, use_cache, prefetch, pin, lookup_context);

    case Rep::FilterType::kFullFilter:
      return FullFilterBlockReader::Create(this, prefetch_buffer, use_cache,
                                           prefetch, pin, lookup_context);

    default:
      // filter_type is either kNoFilter (exited the function at the first if),
      // or it must be covered in this switch block
      assert(false);
      return std::unique_ptr<FilterBlockReader>();
  }
}

// disable_prefix_seek should be set to true when prefix_extractor found in SST
// differs from the one in mutable_cf_options and index type is HashBasedIndex
InternalIteratorBase<IndexValue>* BlockBasedTable::NewIndexIterator(
    const ReadOptions& read_options, bool disable_prefix_seek,
    IndexBlockIter* input_iter, GetContext* get_context,
    BlockCacheLookupContext* lookup_context) const {
  assert(rep_ != nullptr);
  assert(rep_->index_reader != nullptr);

  // We don't return pinned data from index blocks, so no need
  // to set `block_contents_pinned`.
  return rep_->index_reader->NewIterator(read_options, disable_prefix_seek,
                                         input_iter, get_context,
                                         lookup_context);
}

// Convert an index iterator value (i.e., an encoded BlockHandle)
// into an iterator over the contents of the corresponding block.
// If input_iter is null, new a iterator
// If input_iter is not null, update this iter and return it
template <typename TBlockIter>
TBlockIter* BlockBasedTable::NewDataBlockIterator(
    const ReadOptions& ro, const BlockHandle& handle, TBlockIter* input_iter,
    BlockType block_type, GetContext* get_context,
    BlockCacheLookupContext* lookup_context, Status s,
    FilePrefetchBuffer* prefetch_buffer, bool for_compaction) const {
  PERF_TIMER_GUARD(new_table_block_iter_nanos);

  TBlockIter* iter = input_iter != nullptr ? input_iter : new TBlockIter;
  if (!s.ok()) {
    iter->Invalidate(s);
    return iter;
  }

  CachableEntry<UncompressionDict> uncompression_dict;
  if (rep_->uncompression_dict_reader) {
    const bool no_io = (ro.read_tier == kBlockCacheTier);
    s = rep_->uncompression_dict_reader->GetOrReadUncompressionDictionary(
        prefetch_buffer, no_io, get_context, lookup_context,
        &uncompression_dict);
    if (!s.ok()) {
      iter->Invalidate(s);
      return iter;
    }
  }

  const UncompressionDict& dict = uncompression_dict.GetValue()
                                      ? *uncompression_dict.GetValue()
                                      : UncompressionDict::GetEmptyDict();

  CachableEntry<Block> block;
  s = RetrieveBlock(prefetch_buffer, ro, handle, dict, &block, block_type,
                    get_context, lookup_context, for_compaction,
                    /* use_cache */ true);

  if (!s.ok()) {
    assert(block.IsEmpty());
    iter->Invalidate(s);
    return iter;
  }

  assert(block.GetValue() != nullptr);

  // Block contents are pinned and it is still pinned after the iterator
  // is destroyed as long as cleanup functions are moved to another object,
  // when:
  // 1. block cache handle is set to be released in cleanup function, or
  // 2. it's pointing to immortal source. If own_bytes is true then we are
  //    not reading data from the original source, whether immortal or not.
  //    Otherwise, the block is pinned iff the source is immortal.
  const bool block_contents_pinned =
      block.IsCached() ||
      (!block.GetValue()->own_bytes() && rep_->immortal_table);
  iter = InitBlockIterator<TBlockIter>(rep_, block.GetValue(), iter,
                                       block_contents_pinned);

  if (!block.IsCached()) {
    if (!ro.fill_cache && rep_->cache_key_prefix_size != 0) {
      // insert a dummy record to block cache to track the memory usage
      Cache* const block_cache = rep_->table_options.block_cache.get();
      Cache::Handle* cache_handle = nullptr;
      // There are two other types of cache keys: 1) SST cache key added in
      // `MaybeReadBlockAndLoadToCache` 2) dummy cache key added in
      // `write_buffer_manager`. Use longer prefix (41 bytes) to differentiate
      // from SST cache key(31 bytes), and use non-zero prefix to
      // differentiate from `write_buffer_manager`
      const size_t kExtraCacheKeyPrefix = kMaxVarint64Length * 4 + 1;
      char cache_key[kExtraCacheKeyPrefix + kMaxVarint64Length];
      // Prefix: use rep_->cache_key_prefix padded by 0s
      memset(cache_key, 0, kExtraCacheKeyPrefix + kMaxVarint64Length);
      assert(rep_->cache_key_prefix_size != 0);
      assert(rep_->cache_key_prefix_size <= kExtraCacheKeyPrefix);
      memcpy(cache_key, rep_->cache_key_prefix, rep_->cache_key_prefix_size);
      char* end = EncodeVarint64(cache_key + kExtraCacheKeyPrefix,
                                 next_cache_key_id_++);
      assert(end - cache_key <=
             static_cast<int>(kExtraCacheKeyPrefix + kMaxVarint64Length));
      const Slice unique_key(cache_key, static_cast<size_t>(end - cache_key));
      s = block_cache->Insert(unique_key, nullptr,
                              block.GetValue()->ApproximateMemoryUsage(),
                              nullptr, &cache_handle);

      if (s.ok()) {
        assert(cache_handle != nullptr);
        iter->RegisterCleanup(&ForceReleaseCachedEntry, block_cache,
                              cache_handle);
      }
    }
  } else {
    iter->SetCacheHandle(block.GetCacheHandle());
  }

  block.TransferTo(iter);

  return iter;
}

template <>
DataBlockIter* BlockBasedTable::InitBlockIterator<DataBlockIter>(
    const Rep* rep, Block* block, DataBlockIter* input_iter,
    bool block_contents_pinned) {
  return block->NewDataIterator(
      &rep->internal_comparator, rep->internal_comparator.user_comparator(),
      input_iter, rep->ioptions.statistics, block_contents_pinned);
}

template <>
IndexBlockIter* BlockBasedTable::InitBlockIterator<IndexBlockIter>(
    const Rep* rep, Block* block, IndexBlockIter* input_iter,
    bool block_contents_pinned) {
  return block->NewIndexIterator(
      &rep->internal_comparator, rep->internal_comparator.user_comparator(),
      input_iter, rep->ioptions.statistics, /* total_order_seek */ true,
      rep->index_has_first_key, rep->index_key_includes_seq,
      rep->index_value_is_full, block_contents_pinned);
}

// Convert an uncompressed data block (i.e CachableEntry<Block>)
// into an iterator over the contents of the corresponding block.
// If input_iter is null, new a iterator
// If input_iter is not null, update this iter and return it
template <typename TBlockIter>
TBlockIter* BlockBasedTable::NewDataBlockIterator(const ReadOptions& ro,
                                                  CachableEntry<Block>& block,
                                                  TBlockIter* input_iter,
                                                  Status s) const {
  PERF_TIMER_GUARD(new_table_block_iter_nanos);

  TBlockIter* iter = input_iter != nullptr ? input_iter : new TBlockIter;
  if (!s.ok()) {
    iter->Invalidate(s);
    return iter;
  }

  assert(block.GetValue() != nullptr);
  // Block contents are pinned and it is still pinned after the iterator
  // is destroyed as long as cleanup functions are moved to another object,
  // when:
  // 1. block cache handle is set to be released in cleanup function, or
  // 2. it's pointing to immortal source. If own_bytes is true then we are
  //    not reading data from the original source, whether immortal or not.
  //    Otherwise, the block is pinned iff the source is immortal.
  const bool block_contents_pinned =
      block.IsCached() ||
      (!block.GetValue()->own_bytes() && rep_->immortal_table);
  iter = InitBlockIterator<TBlockIter>(rep_, block.GetValue(), iter,
                                       block_contents_pinned);

  if (!block.IsCached()) {
    if (!ro.fill_cache && rep_->cache_key_prefix_size != 0) {
      // insert a dummy record to block cache to track the memory usage
      Cache* const block_cache = rep_->table_options.block_cache.get();
      Cache::Handle* cache_handle = nullptr;
      // There are two other types of cache keys: 1) SST cache key added in
      // `MaybeReadBlockAndLoadToCache` 2) dummy cache key added in
      // `write_buffer_manager`. Use longer prefix (41 bytes) to differentiate
      // from SST cache key(31 bytes), and use non-zero prefix to
      // differentiate from `write_buffer_manager`
      const size_t kExtraCacheKeyPrefix = kMaxVarint64Length * 4 + 1;
      char cache_key[kExtraCacheKeyPrefix + kMaxVarint64Length];
      // Prefix: use rep_->cache_key_prefix padded by 0s
      memset(cache_key, 0, kExtraCacheKeyPrefix + kMaxVarint64Length);
      assert(rep_->cache_key_prefix_size != 0);
      assert(rep_->cache_key_prefix_size <= kExtraCacheKeyPrefix);
      memcpy(cache_key, rep_->cache_key_prefix, rep_->cache_key_prefix_size);
      char* end = EncodeVarint64(cache_key + kExtraCacheKeyPrefix,
                                 next_cache_key_id_++);
      assert(end - cache_key <=
             static_cast<int>(kExtraCacheKeyPrefix + kMaxVarint64Length));
      const Slice unique_key(cache_key, static_cast<size_t>(end - cache_key));
      s = block_cache->Insert(unique_key, nullptr,
                              block.GetValue()->ApproximateMemoryUsage(),
                              nullptr, &cache_handle);
      if (s.ok()) {
        assert(cache_handle != nullptr);
        iter->RegisterCleanup(&ForceReleaseCachedEntry, block_cache,
                              cache_handle);
      }
    }
  } else {
    iter->SetCacheHandle(block.GetCacheHandle());
  }

  block.TransferTo(iter);
  return iter;
}

// If contents is nullptr, this function looks up the block caches for the
// data block referenced by handle, and read the block from disk if necessary.
// If contents is non-null, it skips the cache lookup and disk read, since
// the caller has already read it. In both cases, if ro.fill_cache is true,
// it inserts the block into the block cache.
template <typename TBlocklike>
Status BlockBasedTable::MaybeReadBlockAndLoadToCache(
    FilePrefetchBuffer* prefetch_buffer, const ReadOptions& ro,
    const BlockHandle& handle, const UncompressionDict& uncompression_dict,
    CachableEntry<TBlocklike>* block_entry, BlockType block_type,
    GetContext* get_context, BlockCacheLookupContext* lookup_context,
    BlockContents* contents) const {
  assert(block_entry != nullptr);
  const bool no_io = (ro.read_tier == kBlockCacheTier);
  Cache* block_cache = rep_->table_options.block_cache.get();
  // No point to cache compressed blocks if it never goes away
  Cache* block_cache_compressed =
      rep_->immortal_table ? nullptr
                           : rep_->table_options.block_cache_compressed.get();

  // First, try to get the block from the cache
  //
  // If either block cache is enabled, we'll try to read from it.
  Status s;
  char cache_key[kMaxCacheKeyPrefixSize + kMaxVarint64Length];
  char compressed_cache_key[kMaxCacheKeyPrefixSize + kMaxVarint64Length];
  Slice key /* key to the block cache */;
  Slice ckey /* key to the compressed block cache */;
  bool is_cache_hit = false;
  if (block_cache != nullptr || block_cache_compressed != nullptr) {
    // create key for block cache
    if (block_cache != nullptr) {
      key = GetCacheKey(rep_->cache_key_prefix, rep_->cache_key_prefix_size,
                        handle, cache_key);
    }

    if (block_cache_compressed != nullptr) {
      ckey = GetCacheKey(rep_->compressed_cache_key_prefix,
                         rep_->compressed_cache_key_prefix_size, handle,
                         compressed_cache_key);
    }

    if (!contents) {
      s = GetDataBlockFromCache(key, ckey, block_cache, block_cache_compressed,
                                ro, block_entry, uncompression_dict, block_type,
                                get_context);
      if (block_entry->GetValue()) {
        // TODO(haoyu): Differentiate cache hit on uncompressed block cache and
        // compressed block cache.
        is_cache_hit = true;
      }
    }

    // Can't find the block from the cache. If I/O is allowed, read from the
    // file.
    if (block_entry->GetValue() == nullptr && !no_io && ro.fill_cache) {
      Statistics* statistics = rep_->ioptions.statistics;
      const bool maybe_compressed =
          block_type != BlockType::kFilter &&
          block_type != BlockType::kCompressionDictionary &&
          rep_->blocks_maybe_compressed;
      const bool do_uncompress = maybe_compressed && !block_cache_compressed;
      CompressionType raw_block_comp_type;
      BlockContents raw_block_contents;
      if (!contents) {
        StopWatch sw(rep_->ioptions.env, statistics, READ_BLOCK_GET_MICROS);
        BlockFetcher block_fetcher(
            rep_->file.get(), prefetch_buffer, rep_->footer, ro, handle,
            &raw_block_contents, rep_->ioptions, do_uncompress,
            maybe_compressed, block_type, uncompression_dict,
            rep_->persistent_cache_options,
            GetMemoryAllocator(rep_->table_options),
            GetMemoryAllocatorForCompressedBlock(rep_->table_options));
        s = block_fetcher.ReadBlockContents();
        raw_block_comp_type = block_fetcher.get_compression_type();
        contents = &raw_block_contents;
      } else {
        raw_block_comp_type = contents->get_compression_type();
      }

      if (s.ok()) {
        SequenceNumber seq_no = rep_->get_global_seqno(block_type);
        // If filling cache is allowed and a cache is configured, try to put the
        // block to the cache.
        s = PutDataBlockToCache(
            key, ckey, block_cache, block_cache_compressed, block_entry,
            contents, raw_block_comp_type, uncompression_dict, seq_no,
            GetMemoryAllocator(rep_->table_options), block_type, get_context);
      }
    }
  }

  // Fill lookup_context.
  if (block_cache_tracer_ && block_cache_tracer_->is_tracing_enabled() &&
      lookup_context) {
    size_t usage = 0;
    uint64_t nkeys = 0;
    if (block_entry->GetValue()) {
      // Approximate the number of keys in the block using restarts.
      nkeys =
          rep_->table_options.block_restart_interval *
          BlocklikeTraits<TBlocklike>::GetNumRestarts(*block_entry->GetValue());
      usage = block_entry->GetValue()->ApproximateMemoryUsage();
    }
    TraceType trace_block_type = TraceType::kTraceMax;
    switch (block_type) {
      case BlockType::kData:
        trace_block_type = TraceType::kBlockTraceDataBlock;
        break;
      case BlockType::kFilter:
        trace_block_type = TraceType::kBlockTraceFilterBlock;
        break;
      case BlockType::kCompressionDictionary:
        trace_block_type = TraceType::kBlockTraceUncompressionDictBlock;
        break;
      case BlockType::kRangeDeletion:
        trace_block_type = TraceType::kBlockTraceRangeDeletionBlock;
        break;
      case BlockType::kIndex:
        trace_block_type = TraceType::kBlockTraceIndexBlock;
        break;
      default:
        // This cannot happen.
        assert(false);
        break;
    }
    bool no_insert = no_io || !ro.fill_cache;
    if (BlockCacheTraceHelper::IsGetOrMultiGetOnDataBlock(
            trace_block_type, lookup_context->caller)) {
      // Defer logging the access to Get() and MultiGet() to trace additional
      // information, e.g., referenced_key_exist_in_block.

      // Make a copy of the block key here since it will be logged later.
      lookup_context->FillLookupContext(
          is_cache_hit, no_insert, trace_block_type,
          /*block_size=*/usage, /*block_key=*/key.ToString(), nkeys);
    } else {
      // Avoid making copy of block_key and cf_name when constructing the access
      // record.
      BlockCacheTraceRecord access_record(
          rep_->ioptions.env->NowMicros(),
          /*block_key=*/"", trace_block_type,
          /*block_size=*/usage, rep_->cf_id_for_tracing(),
          /*cf_name=*/"", rep_->level_for_tracing(),
          rep_->sst_number_for_tracing(), lookup_context->caller, is_cache_hit,
          no_insert, lookup_context->get_id,
          lookup_context->get_from_user_specified_snapshot,
          /*referenced_key=*/"");
      block_cache_tracer_->WriteBlockAccess(access_record, key,
                                            rep_->cf_name_for_tracing(),
                                            lookup_context->referenced_key);
    }
  }

  assert(s.ok() || block_entry->GetValue() == nullptr);
  return s;
}

// This function reads multiple data blocks from disk using Env::MultiRead()
// and optionally inserts them into the block cache. It uses the scratch
// buffer provided by the caller, which is contiguous. If scratch is a nullptr
// it allocates a separate buffer for each block. Typically, if the blocks
// need to be uncompressed and there is no compressed block cache, callers
// can allocate a temporary scratch buffer in order to minimize memory
// allocations.
// If options.fill_cache is true, it inserts the blocks into cache. If its
// false and scratch is non-null and the blocks are uncompressed, it copies
// the buffers to heap. In any case, the CachableEntry<Block> returned will
// own the data bytes.
// If compression is enabled and also there is no compressed block cache,
// the adjacent blocks are read out in one IO (combined read)
// batch - A MultiGetRange with only those keys with unique data blocks not
//         found in cache
// handles - A vector of block handles. Some of them me be NULL handles
// scratch - An optional contiguous buffer to read compressed blocks into
void BlockBasedTable::RetrieveMultipleBlocks(
    const ReadOptions& options, const MultiGetRange* batch,
    const autovector<BlockHandle, MultiGetContext::MAX_BATCH_SIZE>* handles,
    autovector<Status, MultiGetContext::MAX_BATCH_SIZE>* statuses,
    autovector<CachableEntry<Block>, MultiGetContext::MAX_BATCH_SIZE>* results,
    char* scratch, const UncompressionDict& uncompression_dict) const {
  RandomAccessFileReader* file = rep_->file.get();
  const Footer& footer = rep_->footer;
  const ImmutableCFOptions& ioptions = rep_->ioptions;
  SequenceNumber global_seqno = rep_->get_global_seqno(BlockType::kData);
  size_t read_amp_bytes_per_bit = rep_->table_options.read_amp_bytes_per_bit;
  MemoryAllocator* memory_allocator = GetMemoryAllocator(rep_->table_options);

  if (file->use_direct_io() || ioptions.allow_mmap_reads) {
    size_t idx_in_batch = 0;
    for (auto mget_iter = batch->begin(); mget_iter != batch->end();
         ++mget_iter, ++idx_in_batch) {
      BlockCacheLookupContext lookup_data_block_context(
          TableReaderCaller::kUserMultiGet);
      const BlockHandle& handle = (*handles)[idx_in_batch];
      if (handle.IsNull()) {
        continue;
      }

      (*statuses)[idx_in_batch] =
          RetrieveBlock(nullptr, options, handle, uncompression_dict,
                        &(*results)[idx_in_batch], BlockType::kData,
                        mget_iter->get_context, &lookup_data_block_context,
                        /* for_compaction */ false, /* use_cache */ true);
    }
    return;
  }

  autovector<FSReadRequest, MultiGetContext::MAX_BATCH_SIZE> read_reqs;
  size_t buf_offset = 0;
  size_t idx_in_batch = 0;

  uint64_t prev_offset = 0;
  size_t prev_len = 0;
  autovector<size_t, MultiGetContext::MAX_BATCH_SIZE> req_idx_for_block;
  autovector<size_t, MultiGetContext::MAX_BATCH_SIZE> req_offset_for_block;
  for (auto mget_iter = batch->begin(); mget_iter != batch->end();
       ++mget_iter, ++idx_in_batch) {
    const BlockHandle& handle = (*handles)[idx_in_batch];
    if (handle.IsNull()) {
      continue;
    }

    size_t prev_end = static_cast<size_t>(prev_offset) + prev_len;

    // If current block is adjacent to the previous one, at the same time,
    // compression is enabled and there is no compressed cache, we combine
    // the two block read as one.
    if (scratch != nullptr && prev_end == handle.offset()) {
      req_offset_for_block.emplace_back(prev_len);
      prev_len += block_size(handle);
    } else {
      // No compression or current block and previous one is not adjacent:
      // Step 1, create a new request for previous blocks
      if (prev_len != 0) {
        FSReadRequest req;
        req.offset = prev_offset;
        req.len = prev_len;
        if (scratch == nullptr) {
          req.scratch = new char[req.len];
        } else {
          req.scratch = scratch + buf_offset;
          buf_offset += req.len;
        }
        req.status = IOStatus::OK();
        read_reqs.emplace_back(req);
      }

      // Step 2, remeber the previous block info
      prev_offset = handle.offset();
      prev_len = block_size(handle);
      req_offset_for_block.emplace_back(0);
    }
    req_idx_for_block.emplace_back(read_reqs.size());
  }
  // Handle the last block and process the pending last request
  if (prev_len != 0) {
    FSReadRequest req;
    req.offset = prev_offset;
    req.len = prev_len;
    if (scratch == nullptr) {
      req.scratch = new char[req.len];
    } else {
      req.scratch = scratch + buf_offset;
    }
    req.status = IOStatus::OK();
    read_reqs.emplace_back(req);
  }

  file->MultiRead(&read_reqs[0], read_reqs.size());

  idx_in_batch = 0;
  size_t valid_batch_idx = 0;
  for (auto mget_iter = batch->begin(); mget_iter != batch->end();
       ++mget_iter, ++idx_in_batch) {
    const BlockHandle& handle = (*handles)[idx_in_batch];

    if (handle.IsNull()) {
      continue;
    }

    assert(valid_batch_idx < req_idx_for_block.size());
    assert(valid_batch_idx < req_offset_for_block.size());
    assert(req_idx_for_block[valid_batch_idx] < read_reqs.size());
    size_t& req_idx = req_idx_for_block[valid_batch_idx];
    size_t& req_offset = req_offset_for_block[valid_batch_idx];
    valid_batch_idx++;
    FSReadRequest& req = read_reqs[req_idx];
    Status s = req.status;
    if (s.ok()) {
      if (req.result.size() != req.len) {
        s = Status::Corruption(
            "truncated block read from " + rep_->file->file_name() +
            " offset " + ToString(handle.offset()) + ", expected " +
            ToString(req.len) + " bytes, got " + ToString(req.result.size()));
      }
    }

    BlockContents raw_block_contents;
    size_t cur_read_end = req_offset + block_size(handle);
    if (cur_read_end > req.result.size()) {
      s = Status::Corruption(
          "truncated block read from " + rep_->file->file_name() + " offset " +
          ToString(handle.offset()) + ", expected " + ToString(req.len) +
          " bytes, got " + ToString(req.result.size()));
    }

    bool blocks_share_read_buffer = (req.result.size() != block_size(handle));
    if (s.ok()) {
      if (scratch == nullptr && !blocks_share_read_buffer) {
        // We allocated a buffer for this block. Give ownership of it to
        // BlockContents so it can free the memory
        assert(req.result.data() == req.scratch);
        std::unique_ptr<char[]> raw_block(req.scratch + req_offset);
        raw_block_contents = BlockContents(std::move(raw_block), handle.size());
      } else {
        // We used the scratch buffer which are shared by the blocks.
        // raw_block_contents does not have the ownership.
        raw_block_contents =
            BlockContents(Slice(req.scratch + req_offset, handle.size()));
      }

#ifndef NDEBUG
      raw_block_contents.is_raw_block = true;
#endif
      if (options.verify_checksums) {
        PERF_TIMER_GUARD(block_checksum_time);
        const char* data = req.result.data();
        uint32_t expected =
            DecodeFixed32(data + req_offset + handle.size() + 1);
        // Since the scratch might be shared. the offset of the data block in
        // the buffer might not be 0. req.result.data() only point to the
        // begin address of each read request, we need to add the offset
        // in each read request. Checksum is stored in the block trailer,
        // which is handle.size() + 1.
        s = ROCKSDB_NAMESPACE::VerifyChecksum(footer.checksum(),
                                              req.result.data() + req_offset,
                                              handle.size() + 1, expected);
        TEST_SYNC_POINT_CALLBACK("RetrieveMultipleBlocks:VerifyChecksum", &s);
      }
    }

    if (s.ok()) {
      // It handles a rare case: compression is set and these is no compressed
      // cache (enable combined read). In this case, the scratch != nullptr.
      // At the same time, some blocks are actually not compressed,
      // since its compression space saving is smaller than the threshold. In
      // this case, if the block shares the scratch memory, we need to copy it
      // to the heap such that it can be added to the regular block cache.
      CompressionType compression_type =
          raw_block_contents.get_compression_type();
      if (scratch != nullptr && compression_type == kNoCompression) {
        Slice raw = Slice(req.scratch + req_offset, block_size(handle));
        raw_block_contents = BlockContents(
            CopyBufferToHeap(GetMemoryAllocator(rep_->table_options), raw),
            handle.size());
#ifndef NDEBUG
        raw_block_contents.is_raw_block = true;
#endif
      }
    }

    if (s.ok()) {
      if (options.fill_cache) {
        BlockCacheLookupContext lookup_data_block_context(
            TableReaderCaller::kUserMultiGet);
        CachableEntry<Block>* block_entry = &(*results)[idx_in_batch];
        // MaybeReadBlockAndLoadToCache will insert into the block caches if
        // necessary. Since we're passing the raw block contents, it will
        // avoid looking up the block cache
        s = MaybeReadBlockAndLoadToCache(
            nullptr, options, handle, uncompression_dict, block_entry,
            BlockType::kData, mget_iter->get_context,
            &lookup_data_block_context, &raw_block_contents);

        // block_entry value could be null if no block cache is present, i.e
        // BlockBasedTableOptions::no_block_cache is true and no compressed
        // block cache is configured. In that case, fall
        // through and set up the block explicitly
        if (block_entry->GetValue() != nullptr) {
          continue;
        }
      }

      CompressionType compression_type =
          raw_block_contents.get_compression_type();
      BlockContents contents;
      if (compression_type != kNoCompression) {
        UncompressionContext context(compression_type);
        UncompressionInfo info(context, uncompression_dict, compression_type);
        s = UncompressBlockContents(info, req.result.data() + req_offset,
                                    handle.size(), &contents, footer.version(),
                                    rep_->ioptions, memory_allocator);
      } else {
        // There are two cases here: 1) caller uses the scratch buffer; 2) we
        // use the requst buffer. If scratch buffer is used, we ensure that
        // all raw blocks are copyed to the heap as single blocks. If scratch
        // buffer is not used, we also have no combined read, so the raw
        // block can be used directly.
        contents = std::move(raw_block_contents);
      }
      if (s.ok()) {
        (*results)[idx_in_batch].SetOwnedValue(
            new Block(std::move(contents), global_seqno, read_amp_bytes_per_bit,
                      ioptions.statistics));
      }
    }
    (*statuses)[idx_in_batch] = s;
  }
}

template <typename TBlocklike>
Status BlockBasedTable::RetrieveBlock(
    FilePrefetchBuffer* prefetch_buffer, const ReadOptions& ro,
    const BlockHandle& handle, const UncompressionDict& uncompression_dict,
    CachableEntry<TBlocklike>* block_entry, BlockType block_type,
    GetContext* get_context, BlockCacheLookupContext* lookup_context,
    bool for_compaction, bool use_cache) const {
  assert(block_entry);
  assert(block_entry->IsEmpty());

  Status s;
  if (use_cache) {
    s = MaybeReadBlockAndLoadToCache(prefetch_buffer, ro, handle,
                                     uncompression_dict, block_entry,
                                     block_type, get_context, lookup_context,
                                     /*contents=*/nullptr);

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

    if (block_entry->GetValue() != nullptr) {
      assert(s.ok());
      return s;
    }
  }

  assert(block_entry->IsEmpty());

  const bool no_io = ro.read_tier == kBlockCacheTier;
  if (no_io) {
    return Status::Incomplete("no blocking io");
  }

  const bool maybe_compressed =
      block_type != BlockType::kFilter &&
      block_type != BlockType::kCompressionDictionary &&
      rep_->blocks_maybe_compressed;
  const bool do_uncompress = maybe_compressed;
  std::unique_ptr<TBlocklike> block;

  {
    StopWatch sw(rep_->ioptions.env, rep_->ioptions.statistics,
                 READ_BLOCK_GET_MICROS);
    s = ReadBlockFromFile(
        rep_->file.get(), prefetch_buffer, rep_->footer, ro, handle, &block,
        rep_->ioptions, do_uncompress, maybe_compressed, block_type,
        uncompression_dict, rep_->persistent_cache_options,
        rep_->get_global_seqno(block_type),
        block_type == BlockType::kData
            ? rep_->table_options.read_amp_bytes_per_bit
            : 0,
        GetMemoryAllocator(rep_->table_options), for_compaction,
        rep_->blocks_definitely_zstd_compressed,
        rep_->table_options.filter_policy.get());
  }

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

  block_entry->SetOwnedValue(block.release());

  assert(s.ok());
  return s;
}

// Explicitly instantiate templates for both "blocklike" types we use.
// This makes it possible to keep the template definitions in the .cc file.
template Status BlockBasedTable::RetrieveBlock<BlockContents>(
    FilePrefetchBuffer* prefetch_buffer, const ReadOptions& ro,
    const BlockHandle& handle, const UncompressionDict& uncompression_dict,
    CachableEntry<BlockContents>* block_entry, BlockType block_type,
    GetContext* get_context, BlockCacheLookupContext* lookup_context,
    bool for_compaction, bool use_cache) const;

template Status BlockBasedTable::RetrieveBlock<ParsedFullFilterBlock>(
    FilePrefetchBuffer* prefetch_buffer, const ReadOptions& ro,
    const BlockHandle& handle, const UncompressionDict& uncompression_dict,
    CachableEntry<ParsedFullFilterBlock>* block_entry, BlockType block_type,
    GetContext* get_context, BlockCacheLookupContext* lookup_context,
    bool for_compaction, bool use_cache) const;

template Status BlockBasedTable::RetrieveBlock<Block>(
    FilePrefetchBuffer* prefetch_buffer, const ReadOptions& ro,
    const BlockHandle& handle, const UncompressionDict& uncompression_dict,
    CachableEntry<Block>* block_entry, BlockType block_type,
    GetContext* get_context, BlockCacheLookupContext* lookup_context,
    bool for_compaction, bool use_cache) const;

template Status BlockBasedTable::RetrieveBlock<UncompressionDict>(
    FilePrefetchBuffer* prefetch_buffer, const ReadOptions& ro,
    const BlockHandle& handle, const UncompressionDict& uncompression_dict,
    CachableEntry<UncompressionDict>* block_entry, BlockType block_type,
    GetContext* get_context, BlockCacheLookupContext* lookup_context,
    bool for_compaction, bool use_cache) const;

BlockBasedTable::PartitionedIndexIteratorState::PartitionedIndexIteratorState(
    const BlockBasedTable* table,
    std::unordered_map<uint64_t, CachableEntry<Block>>* block_map)
    : table_(table), block_map_(block_map) {}

InternalIteratorBase<IndexValue>*
BlockBasedTable::PartitionedIndexIteratorState::NewSecondaryIterator(
    const BlockHandle& handle) {
  // Return a block iterator on the index partition
  auto block = block_map_->find(handle.offset());
  // This is a possible scenario since block cache might not have had space
  // for the partition
  if (block != block_map_->end()) {
    const Rep* rep = table_->get_rep();
    assert(rep);

    Statistics* kNullStats = nullptr;
    // We don't return pinned data from index blocks, so no need
    // to set `block_contents_pinned`.
    return block->second.GetValue()->NewIndexIterator(
        &rep->internal_comparator, rep->internal_comparator.user_comparator(),
        nullptr, kNullStats, true, rep->index_has_first_key,
        rep->index_key_includes_seq, rep->index_value_is_full);
  }
  // Create an empty iterator
  return new IndexBlockIter();
}

// This will be broken if the user specifies an unusual implementation
// of Options.comparator, or if the user specifies an unusual
// definition of prefixes in BlockBasedTableOptions.filter_policy.
// In particular, we require the following three properties:
//
// 1) key.starts_with(prefix(key))
// 2) Compare(prefix(key), key) <= 0.
// 3) If Compare(key1, key2) <= 0, then Compare(prefix(key1), prefix(key2)) <= 0
//
// Otherwise, this method guarantees no I/O will be incurred.
//
// REQUIRES: this method shouldn't be called while the DB lock is held.
bool BlockBasedTable::PrefixMayMatch(
    const Slice& internal_key, const ReadOptions& read_options,
    const SliceTransform* options_prefix_extractor,
    const bool need_upper_bound_check,
    BlockCacheLookupContext* lookup_context) const {
  if (!rep_->filter_policy) {
    return true;
  }

  const SliceTransform* prefix_extractor;

  if (rep_->table_prefix_extractor == nullptr) {
    if (need_upper_bound_check) {
      return true;
    }
    prefix_extractor = options_prefix_extractor;
  } else {
    prefix_extractor = rep_->table_prefix_extractor.get();
  }
  auto user_key = ExtractUserKey(internal_key);
  if (!prefix_extractor->InDomain(user_key)) {
    return true;
  }

  bool may_match = true;
  Status s;

  // First, try check with full filter
  FilterBlockReader* const filter = rep_->filter.get();
  bool filter_checked = true;
  if (filter != nullptr) {
    if (!filter->IsBlockBased()) {
      const Slice* const const_ikey_ptr = &internal_key;
      may_match = filter->RangeMayExist(
          read_options.iterate_upper_bound, user_key, prefix_extractor,
          rep_->internal_comparator.user_comparator(), const_ikey_ptr,
          &filter_checked, need_upper_bound_check, lookup_context);
    } else {
      // if prefix_extractor changed for block based filter, skip filter
      if (need_upper_bound_check) {
        return true;
      }
      auto prefix = prefix_extractor->Transform(user_key);
      InternalKey internal_key_prefix(prefix, kMaxSequenceNumber, kTypeValue);
      auto internal_prefix = internal_key_prefix.Encode();

      // To prevent any io operation in this method, we set `read_tier` to make
      // sure we always read index or filter only when they have already been
      // loaded to memory.
      ReadOptions no_io_read_options;
      no_io_read_options.read_tier = kBlockCacheTier;

      // Then, try find it within each block
      // we already know prefix_extractor and prefix_extractor_name must match
      // because `CheckPrefixMayMatch` first checks `check_filter_ == true`
      std::unique_ptr<InternalIteratorBase<IndexValue>> iiter(NewIndexIterator(
          no_io_read_options,
          /*need_upper_bound_check=*/false, /*input_iter=*/nullptr,
          /*get_context=*/nullptr, lookup_context));
      iiter->Seek(internal_prefix);

      if (!iiter->Valid()) {
        // we're past end of file
        // if it's incomplete, it means that we avoided I/O
        // and we're not really sure that we're past the end
        // of the file
        may_match = iiter->status().IsIncomplete();
      } else if ((rep_->index_key_includes_seq ? ExtractUserKey(iiter->key())
                                               : iiter->key())
                     .starts_with(ExtractUserKey(internal_prefix))) {
        // we need to check for this subtle case because our only
        // guarantee is that "the key is a string >= last key in that data
        // block" according to the doc/table_format.txt spec.
        //
        // Suppose iiter->key() starts with the desired prefix; it is not
        // necessarily the case that the corresponding data block will
        // contain the prefix, since iiter->key() need not be in the
        // block.  However, the next data block may contain the prefix, so
        // we return true to play it safe.
        may_match = true;
      } else if (filter->IsBlockBased()) {
        // iiter->key() does NOT start with the desired prefix.  Because
        // Seek() finds the first key that is >= the seek target, this
        // means that iiter->key() > prefix.  Thus, any data blocks coming
        // after the data block corresponding to iiter->key() cannot
        // possibly contain the key.  Thus, the corresponding data block
        // is the only on could potentially contain the prefix.
        BlockHandle handle = iiter->value().handle;
        may_match = filter->PrefixMayMatch(
            prefix, prefix_extractor, handle.offset(), /*no_io=*/false,
            /*const_key_ptr=*/nullptr, /*get_context=*/nullptr, lookup_context);
      }
    }
  }

  if (filter_checked) {
    Statistics* statistics = rep_->ioptions.statistics;
    RecordTick(statistics, BLOOM_FILTER_PREFIX_CHECKED);
    if (!may_match) {
      RecordTick(statistics, BLOOM_FILTER_PREFIX_USEFUL);
    }
  }

  return may_match;
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::Seek(const Slice& target) {
  SeekImpl(&target);
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::SeekToFirst() {
  SeekImpl(nullptr);
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::SeekImpl(
    const Slice* target) {
  is_out_of_bound_ = false;
  is_at_first_key_from_index_ = false;
  if (target && !CheckPrefixMayMatch(*target, IterDirection::kForward)) {
    ResetDataIter();
    return;
  }

  bool need_seek_index = true;
  if (block_iter_points_to_real_block_ && block_iter_.Valid()) {
    // Reseek.
    prev_block_offset_ = index_iter_->value().handle.offset();

    if (target) {
      // We can avoid an index seek if:
      // 1. The new seek key is larger than the current key
      // 2. The new seek key is within the upper bound of the block
      // Since we don't necessarily know the internal key for either
      // the current key or the upper bound, we check user keys and
      // exclude the equality case. Considering internal keys can
      // improve for the boundary cases, but it would complicate the
      // code.
      if (user_comparator_.Compare(ExtractUserKey(*target),
                                   block_iter_.user_key()) > 0 &&
          user_comparator_.Compare(ExtractUserKey(*target),
                                   index_iter_->user_key()) < 0) {
        need_seek_index = false;
      }
    }
  }

  if (need_seek_index) {
    if (target) {
      index_iter_->Seek(*target);
    } else {
      index_iter_->SeekToFirst();
    }

    if (!index_iter_->Valid()) {
      ResetDataIter();
      return;
    }
  }

  IndexValue v = index_iter_->value();
  const bool same_block = block_iter_points_to_real_block_ &&
                          v.handle.offset() == prev_block_offset_;

  // TODO(kolmike): Remove the != kBlockCacheTier condition.
  if (!v.first_internal_key.empty() && !same_block &&
      (!target || icomp_.Compare(*target, v.first_internal_key) <= 0) &&
      read_options_.read_tier != kBlockCacheTier) {
    // Index contains the first key of the block, and it's >= target.
    // We can defer reading the block.
    is_at_first_key_from_index_ = true;
    // ResetDataIter() will invalidate block_iter_. Thus, there is no need to
    // call CheckDataBlockWithinUpperBound() to check for iterate_upper_bound
    // as that will be done later when the data block is actually read.
    ResetDataIter();
  } else {
    // Need to use the data block.
    if (!same_block) {
      InitDataBlock();
    } else {
      // When the user does a reseek, the iterate_upper_bound might have
      // changed. CheckDataBlockWithinUpperBound() needs to be called
      // explicitly if the reseek ends up in the same data block.
      // If the reseek ends up in a different block, InitDataBlock() will do
      // the iterator upper bound check.
      CheckDataBlockWithinUpperBound();
    }

    if (target) {
      block_iter_.Seek(*target);
    } else {
      block_iter_.SeekToFirst();
    }
    FindKeyForward();
  }

  CheckOutOfBound();

  if (target) {
    assert(!Valid() || ((block_type_ == BlockType::kIndex &&
                         !table_->get_rep()->index_key_includes_seq)
                            ? (user_comparator_.Compare(ExtractUserKey(*target),
                                                        key()) <= 0)
                            : (icomp_.Compare(*target, key()) <= 0)));
  }
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::SeekForPrev(
    const Slice& target) {
  is_out_of_bound_ = false;
  is_at_first_key_from_index_ = false;
  // For now totally disable prefix seek in auto prefix mode because we don't
  // have logic
  if (!CheckPrefixMayMatch(target, IterDirection::kBackward)) {
    ResetDataIter();
    return;
  }

  SavePrevIndexValue();

  // Call Seek() rather than SeekForPrev() in the index block, because the
  // target data block will likely to contain the position for `target`, the
  // same as Seek(), rather than than before.
  // For example, if we have three data blocks, each containing two keys:
  //   [2, 4]  [6, 8] [10, 12]
  //  (the keys in the index block would be [4, 8, 12])
  // and the user calls SeekForPrev(7), we need to go to the second block,
  // just like if they call Seek(7).
  // The only case where the block is difference is when they seek to a position
  // in the boundary. For example, if they SeekForPrev(5), we should go to the
  // first block, rather than the second. However, we don't have the information
  // to distinguish the two unless we read the second block. In this case, we'll
  // end up with reading two blocks.
  index_iter_->Seek(target);

  if (!index_iter_->Valid()) {
    auto seek_status = index_iter_->status();
    // Check for IO error
    if (!seek_status.IsNotFound() && !seek_status.ok()) {
      ResetDataIter();
      return;
    }

    // With prefix index, Seek() returns NotFound if the prefix doesn't exist
    if (seek_status.IsNotFound()) {
      // Any key less than the target is fine for prefix seek
      ResetDataIter();
      return;
    } else {
      index_iter_->SeekToLast();
    }
    // Check for IO error
    if (!index_iter_->Valid()) {
      ResetDataIter();
      return;
    }
  }

  InitDataBlock();

  block_iter_.SeekForPrev(target);

  FindKeyBackward();
  CheckDataBlockWithinUpperBound();
  assert(!block_iter_.Valid() ||
         icomp_.Compare(target, block_iter_.key()) >= 0);
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::SeekToLast() {
  is_out_of_bound_ = false;
  is_at_first_key_from_index_ = false;
  SavePrevIndexValue();
  index_iter_->SeekToLast();
  if (!index_iter_->Valid()) {
    ResetDataIter();
    return;
  }
  InitDataBlock();
  block_iter_.SeekToLast();
  FindKeyBackward();
  CheckDataBlockWithinUpperBound();
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::Next() {
  if (is_at_first_key_from_index_ && !MaterializeCurrentBlock()) {
    return;
  }
  assert(block_iter_points_to_real_block_);
  block_iter_.Next();
  FindKeyForward();
  CheckOutOfBound();
}

template <class TBlockIter, typename TValue>
bool BlockBasedTableIterator<TBlockIter, TValue>::NextAndGetResult(
    IterateResult* result) {
  Next();
  bool is_valid = Valid();
  if (is_valid) {
    result->key = key();
    result->may_be_out_of_upper_bound = MayBeOutOfUpperBound();
  }
  return is_valid;
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::Prev() {
  if (is_at_first_key_from_index_) {
    is_at_first_key_from_index_ = false;

    index_iter_->Prev();
    if (!index_iter_->Valid()) {
      return;
    }

    InitDataBlock();
    block_iter_.SeekToLast();
  } else {
    assert(block_iter_points_to_real_block_);
    block_iter_.Prev();
  }

  FindKeyBackward();
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::InitDataBlock() {
  BlockHandle data_block_handle = index_iter_->value().handle;
  if (!block_iter_points_to_real_block_ ||
      data_block_handle.offset() != prev_block_offset_ ||
      // if previous attempt of reading the block missed cache, try again
      block_iter_.status().IsIncomplete()) {
    if (block_iter_points_to_real_block_) {
      ResetDataIter();
    }
    auto* rep = table_->get_rep();

    // Prefetch additional data for range scans (iterators). Enabled only for
    // user reads.
    // Implicit auto readahead:
    //   Enabled after 2 sequential IOs when ReadOptions.readahead_size == 0.
    // Explicit user requested readahead:
    //   Enabled from the very first IO when ReadOptions.readahead_size is set.
    if (lookup_context_.caller != TableReaderCaller::kCompaction) {
      if (read_options_.readahead_size == 0) {
        // Implicit auto readahead
        num_file_reads_++;
        if (num_file_reads_ >
            BlockBasedTable::kMinNumFileReadsToStartAutoReadahead) {
          if (!rep->file->use_direct_io() &&
              (data_block_handle.offset() +
                   static_cast<size_t>(block_size(data_block_handle)) >
               readahead_limit_)) {
            // Buffered I/O
            // Discarding the return status of Prefetch calls intentionally, as
            // we can fallback to reading from disk if Prefetch fails.
            rep->file->Prefetch(data_block_handle.offset(), readahead_size_);
            readahead_limit_ = static_cast<size_t>(data_block_handle.offset() +
                                                   readahead_size_);
            // Keep exponentially increasing readahead size until
            // kMaxAutoReadaheadSize.
            readahead_size_ = std::min(BlockBasedTable::kMaxAutoReadaheadSize,
                                       readahead_size_ * 2);
          } else if (rep->file->use_direct_io() && !prefetch_buffer_) {
            // Direct I/O
            // Let FilePrefetchBuffer take care of the readahead.
            rep->CreateFilePrefetchBuffer(
                BlockBasedTable::kInitAutoReadaheadSize,
                BlockBasedTable::kMaxAutoReadaheadSize, &prefetch_buffer_);
          }
        }
      } else if (!prefetch_buffer_) {
        // Explicit user requested readahead
        // The actual condition is:
        // if (read_options_.readahead_size != 0 && !prefetch_buffer_)
        rep->CreateFilePrefetchBuffer(read_options_.readahead_size,
                                      read_options_.readahead_size,
                                      &prefetch_buffer_);
      }
    } else if (!prefetch_buffer_) {
      rep->CreateFilePrefetchBuffer(compaction_readahead_size_,
                                    compaction_readahead_size_,
                                    &prefetch_buffer_);
    }

    Status s;
    table_->NewDataBlockIterator<TBlockIter>(
        read_options_, data_block_handle, &block_iter_, block_type_,
        /*get_context=*/nullptr, &lookup_context_, s, prefetch_buffer_.get(),
        /*for_compaction=*/lookup_context_.caller ==
            TableReaderCaller::kCompaction);
    block_iter_points_to_real_block_ = true;
    CheckDataBlockWithinUpperBound();
  }
}

template <class TBlockIter, typename TValue>
bool BlockBasedTableIterator<TBlockIter, TValue>::MaterializeCurrentBlock() {
  assert(is_at_first_key_from_index_);
  assert(!block_iter_points_to_real_block_);
  assert(index_iter_->Valid());

  is_at_first_key_from_index_ = false;
  InitDataBlock();
  assert(block_iter_points_to_real_block_);
  block_iter_.SeekToFirst();

  if (!block_iter_.Valid() ||
      icomp_.Compare(block_iter_.key(),
                     index_iter_->value().first_internal_key) != 0) {
    // Uh oh.
    block_iter_.Invalidate(Status::Corruption(
        "first key in index doesn't match first key in block"));
    return false;
  }

  return true;
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::FindKeyForward() {
  // This method's code is kept short to make it likely to be inlined.

  assert(!is_out_of_bound_);
  assert(block_iter_points_to_real_block_);

  if (!block_iter_.Valid()) {
    // This is the only call site of FindBlockForward(), but it's extracted into
    // a separate method to keep FindKeyForward() short and likely to be
    // inlined. When transitioning to a different block, we call
    // FindBlockForward(), which is much longer and is probably not inlined.
    FindBlockForward();
  } else {
    // This is the fast path that avoids a function call.
  }
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::FindBlockForward() {
  // TODO the while loop inherits from two-level-iterator. We don't know
  // whether a block can be empty so it can be replaced by an "if".
  do {
    if (!block_iter_.status().ok()) {
      return;
    }
    // Whether next data block is out of upper bound, if there is one.
    const bool next_block_is_out_of_bound =
        read_options_.iterate_upper_bound != nullptr &&
        block_iter_points_to_real_block_ && !data_block_within_upper_bound_;
    assert(!next_block_is_out_of_bound ||
           user_comparator_.Compare(*read_options_.iterate_upper_bound,
                                    index_iter_->user_key()) <= 0);
    ResetDataIter();
    index_iter_->Next();
    if (next_block_is_out_of_bound) {
      // The next block is out of bound. No need to read it.
      TEST_SYNC_POINT_CALLBACK("BlockBasedTableIterator:out_of_bound", nullptr);
      // We need to make sure this is not the last data block before setting
      // is_out_of_bound_, since the index key for the last data block can be
      // larger than smallest key of the next file on the same level.
      if (index_iter_->Valid()) {
        is_out_of_bound_ = true;
      }
      return;
    }

    if (!index_iter_->Valid()) {
      return;
    }

    IndexValue v = index_iter_->value();

    // TODO(kolmike): Remove the != kBlockCacheTier condition.
    if (!v.first_internal_key.empty() &&
        read_options_.read_tier != kBlockCacheTier) {
      // Index contains the first key of the block. Defer reading the block.
      is_at_first_key_from_index_ = true;
      return;
    }

    InitDataBlock();
    block_iter_.SeekToFirst();
  } while (!block_iter_.Valid());
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::FindKeyBackward() {
  while (!block_iter_.Valid()) {
    if (!block_iter_.status().ok()) {
      return;
    }

    ResetDataIter();
    index_iter_->Prev();

    if (index_iter_->Valid()) {
      InitDataBlock();
      block_iter_.SeekToLast();
    } else {
      return;
    }
  }

  // We could have check lower bound here too, but we opt not to do it for
  // code simplicity.
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter, TValue>::CheckOutOfBound() {
  if (read_options_.iterate_upper_bound != nullptr && Valid()) {
    is_out_of_bound_ = user_comparator_.Compare(
                           *read_options_.iterate_upper_bound, user_key()) <= 0;
  }
}

template <class TBlockIter, typename TValue>
void BlockBasedTableIterator<TBlockIter,
                             TValue>::CheckDataBlockWithinUpperBound() {
  if (read_options_.iterate_upper_bound != nullptr &&
      block_iter_points_to_real_block_) {
    data_block_within_upper_bound_ =
        (user_comparator_.Compare(*read_options_.iterate_upper_bound,
                                  index_iter_->user_key()) > 0);
  }
}

InternalIterator* BlockBasedTable::NewIterator(
    const ReadOptions& read_options, const SliceTransform* prefix_extractor,
    Arena* arena, bool skip_filters, TableReaderCaller caller,
    size_t compaction_readahead_size) {
  BlockCacheLookupContext lookup_context{caller};
  bool need_upper_bound_check =
      read_options.auto_prefix_mode ||
      PrefixExtractorChanged(rep_->table_properties.get(), prefix_extractor);
  if (arena == nullptr) {
    return new BlockBasedTableIterator<DataBlockIter>(
        this, read_options, rep_->internal_comparator,
        NewIndexIterator(
            read_options,
            need_upper_bound_check &&
                rep_->index_type == BlockBasedTableOptions::kHashSearch,
            /*input_iter=*/nullptr, /*get_context=*/nullptr, &lookup_context),
        !skip_filters && !read_options.total_order_seek &&
            prefix_extractor != nullptr,
        need_upper_bound_check, prefix_extractor, BlockType::kData, caller,
        compaction_readahead_size);
  } else {
    auto* mem =
        arena->AllocateAligned(sizeof(BlockBasedTableIterator<DataBlockIter>));
    return new (mem) BlockBasedTableIterator<DataBlockIter>(
        this, read_options, rep_->internal_comparator,
        NewIndexIterator(
            read_options,
            need_upper_bound_check &&
                rep_->index_type == BlockBasedTableOptions::kHashSearch,
            /*input_iter=*/nullptr, /*get_context=*/nullptr, &lookup_context),
        !skip_filters && !read_options.total_order_seek &&
            prefix_extractor != nullptr,
        need_upper_bound_check, prefix_extractor, BlockType::kData, caller,
        compaction_readahead_size);
  }
}

FragmentedRangeTombstoneIterator* BlockBasedTable::NewRangeTombstoneIterator(
    const ReadOptions& read_options) {
  if (rep_->fragmented_range_dels == nullptr) {
    return nullptr;
  }
  SequenceNumber snapshot = kMaxSequenceNumber;
  if (read_options.snapshot != nullptr) {
    snapshot = read_options.snapshot->GetSequenceNumber();
  }
  return new FragmentedRangeTombstoneIterator(
      rep_->fragmented_range_dels, rep_->internal_comparator, snapshot);
}

bool BlockBasedTable::FullFilterKeyMayMatch(
    const ReadOptions& read_options, FilterBlockReader* filter,
    const Slice& internal_key, const bool no_io,
    const SliceTransform* prefix_extractor, GetContext* get_context,
    BlockCacheLookupContext* lookup_context) const {
  if (filter == nullptr || filter->IsBlockBased()) {
    return true;
  }
  Slice user_key = ExtractUserKey(internal_key);
  const Slice* const const_ikey_ptr = &internal_key;
  bool may_match = true;
  if (rep_->whole_key_filtering) {
    size_t ts_sz =
        rep_->internal_comparator.user_comparator()->timestamp_size();
    Slice user_key_without_ts = StripTimestampFromUserKey(user_key, ts_sz);
    may_match =
        filter->KeyMayMatch(user_key_without_ts, prefix_extractor, kNotValid,
                            no_io, const_ikey_ptr, get_context, lookup_context);
  } else if (!read_options.total_order_seek && prefix_extractor &&
             rep_->table_properties->prefix_extractor_name.compare(
                 prefix_extractor->Name()) == 0 &&
             prefix_extractor->InDomain(user_key) &&
             !filter->PrefixMayMatch(prefix_extractor->Transform(user_key),
                                     prefix_extractor, kNotValid, no_io,
                                     const_ikey_ptr, get_context,
                                     lookup_context)) {
    may_match = false;
  }
  if (may_match) {
    RecordTick(rep_->ioptions.statistics, BLOOM_FILTER_FULL_POSITIVE);
    PERF_COUNTER_BY_LEVEL_ADD(bloom_filter_full_positive, 1, rep_->level);
  }
  return may_match;
}

void BlockBasedTable::FullFilterKeysMayMatch(
    const ReadOptions& read_options, FilterBlockReader* filter,
    MultiGetRange* range, const bool no_io,
    const SliceTransform* prefix_extractor,
    BlockCacheLookupContext* lookup_context) const {
  if (filter == nullptr || filter->IsBlockBased()) {
    return;
  }
  if (rep_->whole_key_filtering) {
    filter->KeysMayMatch(range, prefix_extractor, kNotValid, no_io,
                         lookup_context);
  } else if (!read_options.total_order_seek && prefix_extractor &&
             rep_->table_properties->prefix_extractor_name.compare(
                 prefix_extractor->Name()) == 0) {
    filter->PrefixesMayMatch(range, prefix_extractor, kNotValid, false,
                             lookup_context);
  }
}

Status BlockBasedTable::Get(const ReadOptions& read_options, const Slice& key,
                            GetContext* get_context,
                            const SliceTransform* prefix_extractor,
                            bool skip_filters) {
  assert(key.size() >= 8);  // key must be internal key
  assert(get_context != nullptr);
  Status s;
  const bool no_io = read_options.read_tier == kBlockCacheTier;

  FilterBlockReader* const filter =
      !skip_filters ? rep_->filter.get() : nullptr;

  // First check the full filter
  // If full filter not useful, Then go into each block
  uint64_t tracing_get_id = get_context->get_tracing_get_id();
  BlockCacheLookupContext lookup_context{
      TableReaderCaller::kUserGet, tracing_get_id,
      /*get_from_user_specified_snapshot=*/read_options.snapshot != nullptr};
  if (block_cache_tracer_ && block_cache_tracer_->is_tracing_enabled()) {
    // Trace the key since it contains both user key and sequence number.
    lookup_context.referenced_key = key.ToString();
    lookup_context.get_from_user_specified_snapshot =
        read_options.snapshot != nullptr;
  }
  const bool may_match =
      FullFilterKeyMayMatch(read_options, filter, key, no_io, prefix_extractor,
                            get_context, &lookup_context);
  if (!may_match) {
    RecordTick(rep_->ioptions.statistics, BLOOM_FILTER_USEFUL);
    PERF_COUNTER_BY_LEVEL_ADD(bloom_filter_useful, 1, rep_->level);
  } else {
    IndexBlockIter iiter_on_stack;
    // if prefix_extractor found in block differs from options, disable
    // BlockPrefixIndex. Only do this check when index_type is kHashSearch.
    bool need_upper_bound_check = false;
    if (rep_->index_type == BlockBasedTableOptions::kHashSearch) {
      need_upper_bound_check = PrefixExtractorChanged(
          rep_->table_properties.get(), prefix_extractor);
    }
    auto iiter =
        NewIndexIterator(read_options, need_upper_bound_check, &iiter_on_stack,
                         get_context, &lookup_context);
    std::unique_ptr<InternalIteratorBase<IndexValue>> iiter_unique_ptr;
    if (iiter != &iiter_on_stack) {
      iiter_unique_ptr.reset(iiter);
    }

    size_t ts_sz =
        rep_->internal_comparator.user_comparator()->timestamp_size();
    bool matched = false;  // if such user key mathced a key in SST
    bool done = false;
    for (iiter->Seek(key); iiter->Valid() && !done; iiter->Next()) {
      IndexValue v = iiter->value();

      bool not_exist_in_filter =
          filter != nullptr && filter->IsBlockBased() == true &&
          !filter->KeyMayMatch(ExtractUserKeyAndStripTimestamp(key, ts_sz),
                               prefix_extractor, v.handle.offset(), no_io,
                               /*const_ikey_ptr=*/nullptr, get_context,
                               &lookup_context);

      if (not_exist_in_filter) {
        // Not found
        // TODO: think about interaction with Merge. If a user key cannot
        // cross one data block, we should be fine.
        RecordTick(rep_->ioptions.statistics, BLOOM_FILTER_USEFUL);
        PERF_COUNTER_BY_LEVEL_ADD(bloom_filter_useful, 1, rep_->level);
        break;
      }

      if (!v.first_internal_key.empty() && !skip_filters &&
          UserComparatorWrapper(rep_->internal_comparator.user_comparator())
                  .Compare(ExtractUserKey(key),
                           ExtractUserKey(v.first_internal_key)) < 0) {
        // The requested key falls between highest key in previous block and
        // lowest key in current block.
        break;
      }

      BlockCacheLookupContext lookup_data_block_context{
          TableReaderCaller::kUserGet, tracing_get_id,
          /*get_from_user_specified_snapshot=*/read_options.snapshot !=
              nullptr};
      bool does_referenced_key_exist = false;
      DataBlockIter biter;
      uint64_t referenced_data_size = 0;
      NewDataBlockIterator<DataBlockIter>(
          read_options, v.handle, &biter, BlockType::kData, get_context,
          &lookup_data_block_context,
          /*s=*/Status(), /*prefetch_buffer*/ nullptr);

      if (no_io && biter.status().IsIncomplete()) {
        // couldn't get block from block_cache
        // Update Saver.state to Found because we are only looking for
        // whether we can guarantee the key is not there when "no_io" is set
        get_context->MarkKeyMayExist();
        break;
      }
      if (!biter.status().ok()) {
        s = biter.status();
        break;
      }

      bool may_exist = biter.SeekForGet(key);
      // If user-specified timestamp is supported, we cannot end the search
      // just because hash index lookup indicates the key+ts does not exist.
      if (!may_exist && ts_sz == 0) {
        // HashSeek cannot find the key this block and the the iter is not
        // the end of the block, i.e. cannot be in the following blocks
        // either. In this case, the seek_key cannot be found, so we break
        // from the top level for-loop.
        done = true;
      } else {
        // Call the *saver function on each entry/block until it returns false
        for (; biter.Valid(); biter.Next()) {
          ParsedInternalKey parsed_key;
          if (!ParseInternalKey(biter.key(), &parsed_key)) {
            s = Status::Corruption(Slice());
          }

          if (!get_context->SaveValue(
                  parsed_key, biter.value(), &matched,
                  biter.IsValuePinned() ? &biter : nullptr)) {
            if (get_context->State() == GetContext::GetState::kFound) {
              does_referenced_key_exist = true;
              referenced_data_size = biter.key().size() + biter.value().size();
            }
            done = true;
            break;
          }
        }
        s = biter.status();
      }
      // Write the block cache access record.
      if (block_cache_tracer_ && block_cache_tracer_->is_tracing_enabled()) {
        // Avoid making copy of block_key, cf_name, and referenced_key when
        // constructing the access record.
        Slice referenced_key;
        if (does_referenced_key_exist) {
          referenced_key = biter.key();
        } else {
          referenced_key = key;
        }
        BlockCacheTraceRecord access_record(
            rep_->ioptions.env->NowMicros(),
            /*block_key=*/"", lookup_data_block_context.block_type,
            lookup_data_block_context.block_size, rep_->cf_id_for_tracing(),
            /*cf_name=*/"", rep_->level_for_tracing(),
            rep_->sst_number_for_tracing(), lookup_data_block_context.caller,
            lookup_data_block_context.is_cache_hit,
            lookup_data_block_context.no_insert,
            lookup_data_block_context.get_id,
            lookup_data_block_context.get_from_user_specified_snapshot,
            /*referenced_key=*/"", referenced_data_size,
            lookup_data_block_context.num_keys_in_block,
            does_referenced_key_exist);
        block_cache_tracer_->WriteBlockAccess(
            access_record, lookup_data_block_context.block_key,
            rep_->cf_name_for_tracing(), referenced_key);
      }

      if (done) {
        // Avoid the extra Next which is expensive in two-level indexes
        break;
      }
    }
    if (matched && filter != nullptr && !filter->IsBlockBased()) {
      RecordTick(rep_->ioptions.statistics, BLOOM_FILTER_FULL_TRUE_POSITIVE);
      PERF_COUNTER_BY_LEVEL_ADD(bloom_filter_full_true_positive, 1,
                                rep_->level);
    }
    if (s.ok() && !iiter->status().IsNotFound()) {
      s = iiter->status();
    }
  }

  return s;
}

using MultiGetRange = MultiGetContext::Range;
void BlockBasedTable::MultiGet(const ReadOptions& read_options,
                               const MultiGetRange* mget_range,
                               const SliceTransform* prefix_extractor,
                               bool skip_filters) {
  FilterBlockReader* const filter =
      !skip_filters ? rep_->filter.get() : nullptr;
  MultiGetRange sst_file_range(*mget_range, mget_range->begin(),
                               mget_range->end());

  // First check the full filter
  // If full filter not useful, Then go into each block
  const bool no_io = read_options.read_tier == kBlockCacheTier;
  uint64_t tracing_mget_id = BlockCacheTraceHelper::kReservedGetId;
  if (!sst_file_range.empty() && sst_file_range.begin()->get_context) {
    tracing_mget_id = sst_file_range.begin()->get_context->get_tracing_get_id();
  }
  BlockCacheLookupContext lookup_context{
      TableReaderCaller::kUserMultiGet, tracing_mget_id,
      /*get_from_user_specified_snapshot=*/read_options.snapshot != nullptr};
  FullFilterKeysMayMatch(read_options, filter, &sst_file_range, no_io,
                         prefix_extractor, &lookup_context);

  if (skip_filters || !sst_file_range.empty()) {
    IndexBlockIter iiter_on_stack;
    // if prefix_extractor found in block differs from options, disable
    // BlockPrefixIndex. Only do this check when index_type is kHashSearch.
    bool need_upper_bound_check = false;
    if (rep_->index_type == BlockBasedTableOptions::kHashSearch) {
      need_upper_bound_check = PrefixExtractorChanged(
          rep_->table_properties.get(), prefix_extractor);
    }
    auto iiter =
        NewIndexIterator(read_options, need_upper_bound_check, &iiter_on_stack,
                         sst_file_range.begin()->get_context, &lookup_context);
    std::unique_ptr<InternalIteratorBase<IndexValue>> iiter_unique_ptr;
    if (iiter != &iiter_on_stack) {
      iiter_unique_ptr.reset(iiter);
    }

    uint64_t offset = std::numeric_limits<uint64_t>::max();
    autovector<BlockHandle, MultiGetContext::MAX_BATCH_SIZE> block_handles;
    autovector<CachableEntry<Block>, MultiGetContext::MAX_BATCH_SIZE> results;
    autovector<Status, MultiGetContext::MAX_BATCH_SIZE> statuses;
    char stack_buf[kMultiGetReadStackBufSize];
    std::unique_ptr<char[]> block_buf;
    {
      MultiGetRange data_block_range(sst_file_range, sst_file_range.begin(),
                                     sst_file_range.end());

      CachableEntry<UncompressionDict> uncompression_dict;
      Status uncompression_dict_status;
      if (rep_->uncompression_dict_reader) {
        uncompression_dict_status =
            rep_->uncompression_dict_reader->GetOrReadUncompressionDictionary(
                nullptr /* prefetch_buffer */, no_io,
                sst_file_range.begin()->get_context, &lookup_context,
                &uncompression_dict);
      }

      const UncompressionDict& dict = uncompression_dict.GetValue()
                                          ? *uncompression_dict.GetValue()
                                          : UncompressionDict::GetEmptyDict();

      size_t total_len = 0;
      ReadOptions ro = read_options;
      ro.read_tier = kBlockCacheTier;

      for (auto miter = data_block_range.begin();
           miter != data_block_range.end(); ++miter) {
        const Slice& key = miter->ikey;
        iiter->Seek(miter->ikey);

        IndexValue v;
        if (iiter->Valid()) {
          v = iiter->value();
        }
        if (!iiter->Valid() ||
            (!v.first_internal_key.empty() && !skip_filters &&
             UserComparatorWrapper(rep_->internal_comparator.user_comparator())
                     .Compare(ExtractUserKey(key),
                              ExtractUserKey(v.first_internal_key)) < 0)) {
          // The requested key falls between highest key in previous block and
          // lowest key in current block.
          *(miter->s) = iiter->status();
          data_block_range.SkipKey(miter);
          sst_file_range.SkipKey(miter);
          continue;
        }

        if (!uncompression_dict_status.ok()) {
          *(miter->s) = uncompression_dict_status;
          data_block_range.SkipKey(miter);
          sst_file_range.SkipKey(miter);
          continue;
        }

        statuses.emplace_back();
        results.emplace_back();
        if (v.handle.offset() == offset) {
          // We're going to reuse the block for this key later on. No need to
          // look it up now. Place a null handle
          block_handles.emplace_back(BlockHandle::NullBlockHandle());
          continue;
        }
        // Lookup the cache for the given data block referenced by an index
        // iterator value (i.e BlockHandle). If it exists in the cache,
        // initialize block to the contents of the data block.
        offset = v.handle.offset();
        BlockHandle handle = v.handle;
        BlockCacheLookupContext lookup_data_block_context(
            TableReaderCaller::kUserMultiGet);
        Status s = RetrieveBlock(
            nullptr, ro, handle, dict, &(results.back()), BlockType::kData,
            miter->get_context, &lookup_data_block_context,
            /* for_compaction */ false, /* use_cache */ true);
        if (s.IsIncomplete()) {
          s = Status::OK();
        }
        if (s.ok() && !results.back().IsEmpty()) {
          // Found it in the cache. Add NULL handle to indicate there is
          // nothing to read from disk
          block_handles.emplace_back(BlockHandle::NullBlockHandle());
        } else {
          block_handles.emplace_back(handle);
          total_len += block_size(handle);
        }
      }

      if (total_len) {
        char* scratch = nullptr;
        // If the blocks need to be uncompressed and we don't need the
        // compressed blocks, then we can use a contiguous block of
        // memory to read in all the blocks as it will be temporary
        // storage
        // 1. If blocks are compressed and compressed block cache is there,
        //    alloc heap bufs
        // 2. If blocks are uncompressed, alloc heap bufs
        // 3. If blocks are compressed and no compressed block cache, use
        //    stack buf
        if (rep_->table_options.block_cache_compressed == nullptr &&
            rep_->blocks_maybe_compressed) {
          if (total_len <= kMultiGetReadStackBufSize) {
            scratch = stack_buf;
          } else {
            scratch = new char[total_len];
            block_buf.reset(scratch);
          }
        }
        RetrieveMultipleBlocks(read_options, &data_block_range, &block_handles,
                               &statuses, &results, scratch, dict);
      }
    }

    DataBlockIter first_biter;
    DataBlockIter next_biter;
    size_t idx_in_batch = 0;
    for (auto miter = sst_file_range.begin(); miter != sst_file_range.end();
         ++miter) {
      Status s;
      GetContext* get_context = miter->get_context;
      const Slice& key = miter->ikey;
      bool matched = false;  // if such user key matched a key in SST
      bool done = false;
      bool first_block = true;
      do {
        DataBlockIter* biter = nullptr;
        bool reusing_block = true;
        uint64_t referenced_data_size = 0;
        bool does_referenced_key_exist = false;
        BlockCacheLookupContext lookup_data_block_context(
            TableReaderCaller::kUserMultiGet, tracing_mget_id,
            /*get_from_user_specified_snapshot=*/read_options.snapshot !=
                nullptr);
        if (first_block) {
          if (!block_handles[idx_in_batch].IsNull() ||
              !results[idx_in_batch].IsEmpty()) {
            first_biter.Invalidate(Status::OK());
            NewDataBlockIterator<DataBlockIter>(
                read_options, results[idx_in_batch], &first_biter,
                statuses[idx_in_batch]);
            reusing_block = false;
          }
          biter = &first_biter;
          idx_in_batch++;
        } else {
          IndexValue v = iiter->value();
          if (!v.first_internal_key.empty() && !skip_filters &&
              UserComparatorWrapper(rep_->internal_comparator.user_comparator())
                      .Compare(ExtractUserKey(key),
                               ExtractUserKey(v.first_internal_key)) < 0) {
            // The requested key falls between highest key in previous block and
            // lowest key in current block.
            break;
          }

          next_biter.Invalidate(Status::OK());
          NewDataBlockIterator<DataBlockIter>(
              read_options, iiter->value().handle, &next_biter,
              BlockType::kData, get_context, &lookup_data_block_context,
              Status(), nullptr);
          biter = &next_biter;
          reusing_block = false;
        }

        if (read_options.read_tier == kBlockCacheTier &&
            biter->status().IsIncomplete()) {
          // couldn't get block from block_cache
          // Update Saver.state to Found because we are only looking for
          // whether we can guarantee the key is not there when "no_io" is set
          get_context->MarkKeyMayExist();
          break;
        }
        if (!biter->status().ok()) {
          s = biter->status();
          break;
        }

        bool may_exist = biter->SeekForGet(key);
        if (!may_exist) {
          // HashSeek cannot find the key this block and the the iter is not
          // the end of the block, i.e. cannot be in the following blocks
          // either. In this case, the seek_key cannot be found, so we break
          // from the top level for-loop.
          break;
        }

        // Call the *saver function on each entry/block until it returns false
        for (; biter->Valid(); biter->Next()) {
          ParsedInternalKey parsed_key;
          Cleanable dummy;
          Cleanable* value_pinner = nullptr;
          if (!ParseInternalKey(biter->key(), &parsed_key)) {
            s = Status::Corruption(Slice());
          }
          if (biter->IsValuePinned()) {
            if (reusing_block) {
              Cache* block_cache = rep_->table_options.block_cache.get();
              assert(biter->cache_handle() != nullptr);
              block_cache->Ref(biter->cache_handle());
              dummy.RegisterCleanup(&ReleaseCachedEntry, block_cache,
                                    biter->cache_handle());
              value_pinner = &dummy;
            } else {
              value_pinner = biter;
            }
          }
          if (!get_context->SaveValue(parsed_key, biter->value(), &matched,
                                      value_pinner)) {
            if (get_context->State() == GetContext::GetState::kFound) {
              does_referenced_key_exist = true;
              referenced_data_size =
                  biter->key().size() + biter->value().size();
            }
            done = true;
            break;
          }
          s = biter->status();
        }
        // Write the block cache access.
        if (block_cache_tracer_ && block_cache_tracer_->is_tracing_enabled()) {
          // Avoid making copy of block_key, cf_name, and referenced_key when
          // constructing the access record.
          Slice referenced_key;
          if (does_referenced_key_exist) {
            referenced_key = biter->key();
          } else {
            referenced_key = key;
          }
          BlockCacheTraceRecord access_record(
              rep_->ioptions.env->NowMicros(),
              /*block_key=*/"", lookup_data_block_context.block_type,
              lookup_data_block_context.block_size, rep_->cf_id_for_tracing(),
              /*cf_name=*/"", rep_->level_for_tracing(),
              rep_->sst_number_for_tracing(), lookup_data_block_context.caller,
              lookup_data_block_context.is_cache_hit,
              lookup_data_block_context.no_insert,
              lookup_data_block_context.get_id,
              lookup_data_block_context.get_from_user_specified_snapshot,
              /*referenced_key=*/"", referenced_data_size,
              lookup_data_block_context.num_keys_in_block,
              does_referenced_key_exist);
          block_cache_tracer_->WriteBlockAccess(
              access_record, lookup_data_block_context.block_key,
              rep_->cf_name_for_tracing(), referenced_key);
        }
        s = biter->status();
        if (done) {
          // Avoid the extra Next which is expensive in two-level indexes
          break;
        }
        if (first_block) {
          iiter->Seek(key);
        }
        first_block = false;
        iiter->Next();
      } while (iiter->Valid());

      if (matched && filter != nullptr && !filter->IsBlockBased()) {
        RecordTick(rep_->ioptions.statistics, BLOOM_FILTER_FULL_TRUE_POSITIVE);
        PERF_COUNTER_BY_LEVEL_ADD(bloom_filter_full_true_positive, 1,
                                  rep_->level);
      }
      if (s.ok()) {
        s = iiter->status();
      }
      *(miter->s) = s;
    }
  }
}

Status BlockBasedTable::Prefetch(const Slice* const begin,
                                 const Slice* const end) {
  auto& comparator = rep_->internal_comparator;
  UserComparatorWrapper user_comparator(comparator.user_comparator());
  // pre-condition
  if (begin && end && comparator.Compare(*begin, *end) > 0) {
    return Status::InvalidArgument(*begin, *end);
  }
  BlockCacheLookupContext lookup_context{TableReaderCaller::kPrefetch};
  IndexBlockIter iiter_on_stack;
  auto iiter = NewIndexIterator(ReadOptions(), /*need_upper_bound_check=*/false,
                                &iiter_on_stack, /*get_context=*/nullptr,
                                &lookup_context);
  std::unique_ptr<InternalIteratorBase<IndexValue>> iiter_unique_ptr;
  if (iiter != &iiter_on_stack) {
    iiter_unique_ptr = std::unique_ptr<InternalIteratorBase<IndexValue>>(iiter);
  }

  if (!iiter->status().ok()) {
    // error opening index iterator
    return iiter->status();
  }

  // indicates if we are on the last page that need to be pre-fetched
  bool prefetching_boundary_page = false;

  for (begin ? iiter->Seek(*begin) : iiter->SeekToFirst(); iiter->Valid();
       iiter->Next()) {
    BlockHandle block_handle = iiter->value().handle;
    const bool is_user_key = !rep_->index_key_includes_seq;
    if (end &&
        ((!is_user_key && comparator.Compare(iiter->key(), *end) >= 0) ||
         (is_user_key &&
          user_comparator.Compare(iiter->key(), ExtractUserKey(*end)) >= 0))) {
      if (prefetching_boundary_page) {
        break;
      }

      // The index entry represents the last key in the data block.
      // We should load this page into memory as well, but no more
      prefetching_boundary_page = true;
    }

    // Load the block specified by the block_handle into the block cache
    DataBlockIter biter;

    NewDataBlockIterator<DataBlockIter>(
        ReadOptions(), block_handle, &biter, /*type=*/BlockType::kData,
        /*get_context=*/nullptr, &lookup_context, Status(),
        /*prefetch_buffer=*/nullptr);

    if (!biter.status().ok()) {
      // there was an unexpected error while pre-fetching
      return biter.status();
    }
  }

  return Status::OK();
}

Status BlockBasedTable::VerifyChecksum(const ReadOptions& read_options,
                                       TableReaderCaller caller) {
  Status s;
  // Check Meta blocks
  std::unique_ptr<Block> metaindex;
  std::unique_ptr<InternalIterator> metaindex_iter;
  s = ReadMetaIndexBlock(nullptr /* prefetch buffer */, &metaindex,
                         &metaindex_iter);
  if (s.ok()) {
    s = VerifyChecksumInMetaBlocks(metaindex_iter.get());
    if (!s.ok()) {
      return s;
    }
  } else {
    return s;
  }
  // Check Data blocks
  IndexBlockIter iiter_on_stack;
  BlockCacheLookupContext context{caller};
  InternalIteratorBase<IndexValue>* iiter = NewIndexIterator(
      read_options, /*disable_prefix_seek=*/false, &iiter_on_stack,
      /*get_context=*/nullptr, &context);
  std::unique_ptr<InternalIteratorBase<IndexValue>> iiter_unique_ptr;
  if (iiter != &iiter_on_stack) {
    iiter_unique_ptr = std::unique_ptr<InternalIteratorBase<IndexValue>>(iiter);
  }
  if (!iiter->status().ok()) {
    // error opening index iterator
    return iiter->status();
  }
  s = VerifyChecksumInBlocks(read_options, iiter);
  return s;
}

Status BlockBasedTable::VerifyChecksumInBlocks(
    const ReadOptions& read_options,
    InternalIteratorBase<IndexValue>* index_iter) {
  Status s;
  // We are scanning the whole file, so no need to do exponential
  // increasing of the buffer size.
  size_t readahead_size = (read_options.readahead_size != 0)
                              ? read_options.readahead_size
                              : kMaxAutoReadaheadSize;
  // FilePrefetchBuffer doesn't work in mmap mode and readahead is not
  // needed there.
  FilePrefetchBuffer prefetch_buffer(
      rep_->file.get(), readahead_size /* readadhead_size */,
      readahead_size /* max_readahead_size */,
      !rep_->ioptions.allow_mmap_reads /* enable */);

  for (index_iter->SeekToFirst(); index_iter->Valid(); index_iter->Next()) {
    s = index_iter->status();
    if (!s.ok()) {
      break;
    }
    BlockHandle handle = index_iter->value().handle;
    BlockContents contents;
    BlockFetcher block_fetcher(
        rep_->file.get(), &prefetch_buffer, rep_->footer, ReadOptions(), handle,
        &contents, rep_->ioptions, false /* decompress */,
        false /*maybe_compressed*/, BlockType::kData,
        UncompressionDict::GetEmptyDict(), rep_->persistent_cache_options);
    s = block_fetcher.ReadBlockContents();
    if (!s.ok()) {
      break;
    }
  }
  return s;
}

BlockType BlockBasedTable::GetBlockTypeForMetaBlockByName(
    const Slice& meta_block_name) {
  if (meta_block_name.starts_with(kFilterBlockPrefix) ||
      meta_block_name.starts_with(kFullFilterBlockPrefix) ||
      meta_block_name.starts_with(kPartitionedFilterBlockPrefix)) {
    return BlockType::kFilter;
  }

  if (meta_block_name == kPropertiesBlock) {
    return BlockType::kProperties;
  }

  if (meta_block_name == kCompressionDictBlock) {
    return BlockType::kCompressionDictionary;
  }

  if (meta_block_name == kRangeDelBlock) {
    return BlockType::kRangeDeletion;
  }

  if (meta_block_name == kHashIndexPrefixesBlock) {
    return BlockType::kHashIndexPrefixes;
  }

  if (meta_block_name == kHashIndexPrefixesMetadataBlock) {
    return BlockType::kHashIndexMetadata;
  }

  assert(false);
  return BlockType::kInvalid;
}

Status BlockBasedTable::VerifyChecksumInMetaBlocks(
    InternalIteratorBase<Slice>* index_iter) {
  Status s;
  for (index_iter->SeekToFirst(); index_iter->Valid(); index_iter->Next()) {
    s = index_iter->status();
    if (!s.ok()) {
      break;
    }
    BlockHandle handle;
    Slice input = index_iter->value();
    s = handle.DecodeFrom(&input);
    BlockContents contents;
    const Slice meta_block_name = index_iter->key();
    BlockFetcher block_fetcher(
        rep_->file.get(), nullptr /* prefetch buffer */, rep_->footer,
        ReadOptions(), handle, &contents, rep_->ioptions,
        false /* decompress */, false /*maybe_compressed*/,
        GetBlockTypeForMetaBlockByName(meta_block_name),
        UncompressionDict::GetEmptyDict(), rep_->persistent_cache_options);
    s = block_fetcher.ReadBlockContents();
    if (s.IsCorruption() && meta_block_name == kPropertiesBlock) {
      TableProperties* table_properties;
      s = TryReadPropertiesWithGlobalSeqno(nullptr /* prefetch_buffer */,
                                           index_iter->value(),
                                           &table_properties);
      delete table_properties;
    }
    if (!s.ok()) {
      break;
    }
  }
  return s;
}

bool BlockBasedTable::TEST_BlockInCache(const BlockHandle& handle) const {
  assert(rep_ != nullptr);

  Cache* const cache = rep_->table_options.block_cache.get();
  if (cache == nullptr) {
    return false;
  }

  char cache_key_storage[kMaxCacheKeyPrefixSize + kMaxVarint64Length];
  Slice cache_key =
      GetCacheKey(rep_->cache_key_prefix, rep_->cache_key_prefix_size, handle,
                  cache_key_storage);

  Cache::Handle* const cache_handle = cache->Lookup(cache_key);
  if (cache_handle == nullptr) {
    return false;
  }

  cache->Release(cache_handle);

  return true;
}

bool BlockBasedTable::TEST_KeyInCache(const ReadOptions& options,
                                      const Slice& key) {
  std::unique_ptr<InternalIteratorBase<IndexValue>> iiter(NewIndexIterator(
      options, /*need_upper_bound_check=*/false, /*input_iter=*/nullptr,
      /*get_context=*/nullptr, /*lookup_context=*/nullptr));
  iiter->Seek(key);
  assert(iiter->Valid());

  return TEST_BlockInCache(iiter->value().handle);
}

// REQUIRES: The following fields of rep_ should have already been populated:
//  1. file
//  2. index_handle,
//  3. options
//  4. internal_comparator
//  5. index_type
Status BlockBasedTable::CreateIndexReader(
    FilePrefetchBuffer* prefetch_buffer,
    InternalIterator* preloaded_meta_index_iter, bool use_cache, bool prefetch,
    bool pin, BlockCacheLookupContext* lookup_context,
    std::unique_ptr<IndexReader>* index_reader) {
  // kHashSearch requires non-empty prefix_extractor but bypass checking
  // prefix_extractor here since we have no access to MutableCFOptions.
  // Add need_upper_bound_check flag in  BlockBasedTable::NewIndexIterator.
  // If prefix_extractor does not match prefix_extractor_name from table
  // properties, turn off Hash Index by setting total_order_seek to true

  switch (rep_->index_type) {
    case BlockBasedTableOptions::kTwoLevelIndexSearch: {
      return PartitionIndexReader::Create(this, prefetch_buffer, use_cache,
                                          prefetch, pin, lookup_context,
                                          index_reader);
    }
    case BlockBasedTableOptions::kBinarySearch:
      FALLTHROUGH_INTENDED;
    case BlockBasedTableOptions::kBinarySearchWithFirstKey: {
      return BinarySearchIndexReader::Create(this, prefetch_buffer, use_cache,
                                             prefetch, pin, lookup_context,
                                             index_reader);
    }
    case BlockBasedTableOptions::kHashSearch: {
      std::unique_ptr<Block> metaindex_guard;
      std::unique_ptr<InternalIterator> metaindex_iter_guard;
      auto meta_index_iter = preloaded_meta_index_iter;
      bool should_fallback = false;
      if (rep_->internal_prefix_transform.get() == nullptr) {
        ROCKS_LOG_WARN(rep_->ioptions.info_log,
                       "No prefix extractor passed in. Fall back to binary"
                       " search index.");
        should_fallback = true;
      } else if (meta_index_iter == nullptr) {
        auto s = ReadMetaIndexBlock(prefetch_buffer, &metaindex_guard,
                                    &metaindex_iter_guard);
        if (!s.ok()) {
          // we simply fall back to binary search in case there is any
          // problem with prefix hash index loading.
          ROCKS_LOG_WARN(rep_->ioptions.info_log,
                         "Unable to read the metaindex block."
                         " Fall back to binary search index.");
          should_fallback = true;
        }
        meta_index_iter = metaindex_iter_guard.get();
      }

      if (should_fallback) {
        return BinarySearchIndexReader::Create(this, prefetch_buffer, use_cache,
                                               prefetch, pin, lookup_context,
                                               index_reader);
      } else {
        return HashIndexReader::Create(this, prefetch_buffer, meta_index_iter,
                                       use_cache, prefetch, pin, lookup_context,
                                       index_reader);
      }
    }
    default: {
      std::string error_message =
          "Unrecognized index type: " + ToString(rep_->index_type);
      return Status::InvalidArgument(error_message.c_str());
    }
  }
}

uint64_t BlockBasedTable::ApproximateOffsetOf(
    const InternalIteratorBase<IndexValue>& index_iter) const {
  uint64_t result = 0;
  if (index_iter.Valid()) {
    BlockHandle handle = index_iter.value().handle;
    result = handle.offset();
  } else {
    // The iterator is past the last key in the file. If table_properties is not
    // available, approximate the offset by returning the offset of the
    // metaindex block (which is right near the end of the file).
    if (rep_->table_properties) {
      result = rep_->table_properties->data_size;
    }
    // table_properties is not present in the table.
    if (result == 0) {
      result = rep_->footer.metaindex_handle().offset();
    }
  }

  return result;
}

uint64_t BlockBasedTable::ApproximateOffsetOf(const Slice& key,
                                              TableReaderCaller caller) {
  BlockCacheLookupContext context(caller);
  IndexBlockIter iiter_on_stack;
  ReadOptions ro;
  ro.total_order_seek = true;
  auto index_iter =
      NewIndexIterator(ro, /*disable_prefix_seek=*/true,
                       /*input_iter=*/&iiter_on_stack, /*get_context=*/nullptr,
                       /*lookup_context=*/&context);
  std::unique_ptr<InternalIteratorBase<IndexValue>> iiter_unique_ptr;
  if (index_iter != &iiter_on_stack) {
    iiter_unique_ptr.reset(index_iter);
  }

  index_iter->Seek(key);
  return ApproximateOffsetOf(*index_iter);
}

uint64_t BlockBasedTable::ApproximateSize(const Slice& start, const Slice& end,
                                          TableReaderCaller caller) {
  assert(rep_->internal_comparator.Compare(start, end) <= 0);

  BlockCacheLookupContext context(caller);
  IndexBlockIter iiter_on_stack;
  ReadOptions ro;
  ro.total_order_seek = true;
  auto index_iter =
      NewIndexIterator(ro, /*disable_prefix_seek=*/true,
                       /*input_iter=*/&iiter_on_stack, /*get_context=*/nullptr,
                       /*lookup_context=*/&context);
  std::unique_ptr<InternalIteratorBase<IndexValue>> iiter_unique_ptr;
  if (index_iter != &iiter_on_stack) {
    iiter_unique_ptr.reset(index_iter);
  }

  index_iter->Seek(start);
  uint64_t start_offset = ApproximateOffsetOf(*index_iter);
  index_iter->Seek(end);
  uint64_t end_offset = ApproximateOffsetOf(*index_iter);

  assert(end_offset >= start_offset);
  return end_offset - start_offset;
}

bool BlockBasedTable::TEST_FilterBlockInCache() const {
  assert(rep_ != nullptr);
  return TEST_BlockInCache(rep_->filter_handle);
}

bool BlockBasedTable::TEST_IndexBlockInCache() const {
  assert(rep_ != nullptr);

  return TEST_BlockInCache(rep_->footer.index_handle());
}

Status BlockBasedTable::GetKVPairsFromDataBlocks(
    std::vector<KVPairBlock>* kv_pair_blocks) {
  std::unique_ptr<InternalIteratorBase<IndexValue>> blockhandles_iter(
      NewIndexIterator(ReadOptions(), /*need_upper_bound_check=*/false,
                       /*input_iter=*/nullptr, /*get_context=*/nullptr,
                       /*lookup_contex=*/nullptr));

  Status s = blockhandles_iter->status();
  if (!s.ok()) {
    // Cannot read Index Block
    return s;
  }

  for (blockhandles_iter->SeekToFirst(); blockhandles_iter->Valid();
       blockhandles_iter->Next()) {
    s = blockhandles_iter->status();

    if (!s.ok()) {
      break;
    }

    std::unique_ptr<InternalIterator> datablock_iter;
    datablock_iter.reset(NewDataBlockIterator<DataBlockIter>(
        ReadOptions(), blockhandles_iter->value().handle,
        /*input_iter=*/nullptr, /*type=*/BlockType::kData,
        /*get_context=*/nullptr, /*lookup_context=*/nullptr, Status(),
        /*prefetch_buffer=*/nullptr));
    s = datablock_iter->status();

    if (!s.ok()) {
      // Error reading the block - Skipped
      continue;
    }

    KVPairBlock kv_pair_block;
    for (datablock_iter->SeekToFirst(); datablock_iter->Valid();
         datablock_iter->Next()) {
      s = datablock_iter->status();
      if (!s.ok()) {
        // Error reading the block - Skipped
        break;
      }
      const Slice& key = datablock_iter->key();
      const Slice& value = datablock_iter->value();
      std::string key_copy = std::string(key.data(), key.size());
      std::string value_copy = std::string(value.data(), value.size());

      kv_pair_block.push_back(
          std::make_pair(std::move(key_copy), std::move(value_copy)));
    }
    kv_pair_blocks->push_back(std::move(kv_pair_block));
  }
  return Status::OK();
}

Status BlockBasedTable::DumpTable(WritableFile* out_file) {
  // Output Footer
  out_file->Append(
      "Footer Details:\n"
      "--------------------------------------\n"
      "  ");
  out_file->Append(rep_->footer.ToString().c_str());
  out_file->Append("\n");

  // Output MetaIndex
  out_file->Append(
      "Metaindex Details:\n"
      "--------------------------------------\n");
  std::unique_ptr<Block> metaindex;
  std::unique_ptr<InternalIterator> metaindex_iter;
  Status s = ReadMetaIndexBlock(nullptr /* prefetch_buffer */, &metaindex,
                                &metaindex_iter);
  if (s.ok()) {
    for (metaindex_iter->SeekToFirst(); metaindex_iter->Valid();
         metaindex_iter->Next()) {
      s = metaindex_iter->status();
      if (!s.ok()) {
        return s;
      }
      if (metaindex_iter->key() == ROCKSDB_NAMESPACE::kPropertiesBlock) {
        out_file->Append("  Properties block handle: ");
        out_file->Append(metaindex_iter->value().ToString(true).c_str());
        out_file->Append("\n");
      } else if (metaindex_iter->key() ==
                 ROCKSDB_NAMESPACE::kCompressionDictBlock) {
        out_file->Append("  Compression dictionary block handle: ");
        out_file->Append(metaindex_iter->value().ToString(true).c_str());
        out_file->Append("\n");
      } else if (strstr(metaindex_iter->key().ToString().c_str(),
                        "filter.rocksdb.") != nullptr) {
        out_file->Append("  Filter block handle: ");
        out_file->Append(metaindex_iter->value().ToString(true).c_str());
        out_file->Append("\n");
      } else if (metaindex_iter->key() == ROCKSDB_NAMESPACE::kRangeDelBlock) {
        out_file->Append("  Range deletion block handle: ");
        out_file->Append(metaindex_iter->value().ToString(true).c_str());
        out_file->Append("\n");
      }
    }
    out_file->Append("\n");
  } else {
    return s;
  }

  // Output TableProperties
  const ROCKSDB_NAMESPACE::TableProperties* table_properties;
  table_properties = rep_->table_properties.get();

  if (table_properties != nullptr) {
    out_file->Append(
        "Table Properties:\n"
        "--------------------------------------\n"
        "  ");
    out_file->Append(table_properties->ToString("\n  ", ": ").c_str());
    out_file->Append("\n");
  }

  if (rep_->filter) {
    out_file->Append(
        "Filter Details:\n"
        "--------------------------------------\n"
        "  ");
    out_file->Append(rep_->filter->ToString().c_str());
    out_file->Append("\n");
  }

  // Output Index block
  s = DumpIndexBlock(out_file);
  if (!s.ok()) {
    return s;
  }

  // Output compression dictionary
  if (rep_->uncompression_dict_reader) {
    CachableEntry<UncompressionDict> uncompression_dict;
    s = rep_->uncompression_dict_reader->GetOrReadUncompressionDictionary(
        nullptr /* prefetch_buffer */, false /* no_io */,
        nullptr /* get_context */, nullptr /* lookup_context */,
        &uncompression_dict);
    if (!s.ok()) {
      return s;
    }

    assert(uncompression_dict.GetValue());

    const Slice& raw_dict = uncompression_dict.GetValue()->GetRawDict();
    out_file->Append(
        "Compression Dictionary:\n"
        "--------------------------------------\n");
    out_file->Append("  size (bytes): ");
    out_file->Append(ROCKSDB_NAMESPACE::ToString(raw_dict.size()));
    out_file->Append("\n\n");
    out_file->Append("  HEX    ");
    out_file->Append(raw_dict.ToString(true).c_str());
    out_file->Append("\n\n");
  }

  // Output range deletions block
  auto* range_del_iter = NewRangeTombstoneIterator(ReadOptions());
  if (range_del_iter != nullptr) {
    range_del_iter->SeekToFirst();
    if (range_del_iter->Valid()) {
      out_file->Append(
          "Range deletions:\n"
          "--------------------------------------\n"
          "  ");
      for (; range_del_iter->Valid(); range_del_iter->Next()) {
        DumpKeyValue(range_del_iter->key(), range_del_iter->value(), out_file);
      }
      out_file->Append("\n");
    }
    delete range_del_iter;
  }
  // Output Data blocks
  s = DumpDataBlocks(out_file);

  return s;
}

Status BlockBasedTable::DumpIndexBlock(WritableFile* out_file) {
  out_file->Append(
      "Index Details:\n"
      "--------------------------------------\n");
  std::unique_ptr<InternalIteratorBase<IndexValue>> blockhandles_iter(
      NewIndexIterator(ReadOptions(), /*need_upper_bound_check=*/false,
                       /*input_iter=*/nullptr, /*get_context=*/nullptr,
                       /*lookup_contex=*/nullptr));
  Status s = blockhandles_iter->status();
  if (!s.ok()) {
    out_file->Append("Can not read Index Block \n\n");
    return s;
  }

  out_file->Append("  Block key hex dump: Data block handle\n");
  out_file->Append("  Block key ascii\n\n");
  for (blockhandles_iter->SeekToFirst(); blockhandles_iter->Valid();
       blockhandles_iter->Next()) {
    s = blockhandles_iter->status();
    if (!s.ok()) {
      break;
    }
    Slice key = blockhandles_iter->key();
    Slice user_key;
    InternalKey ikey;
    if (!rep_->index_key_includes_seq) {
      user_key = key;
    } else {
      ikey.DecodeFrom(key);
      user_key = ikey.user_key();
    }

    out_file->Append("  HEX    ");
    out_file->Append(user_key.ToString(true).c_str());
    out_file->Append(": ");
    out_file->Append(blockhandles_iter->value()
                         .ToString(true, rep_->index_has_first_key)
                         .c_str());
    out_file->Append("\n");

    std::string str_key = user_key.ToString();
    std::string res_key("");
    char cspace = ' ';
    for (size_t i = 0; i < str_key.size(); i++) {
      res_key.append(&str_key[i], 1);
      res_key.append(1, cspace);
    }
    out_file->Append("  ASCII  ");
    out_file->Append(res_key.c_str());
    out_file->Append("\n  ------\n");
  }
  out_file->Append("\n");
  return Status::OK();
}

Status BlockBasedTable::DumpDataBlocks(WritableFile* out_file) {
  std::unique_ptr<InternalIteratorBase<IndexValue>> blockhandles_iter(
      NewIndexIterator(ReadOptions(), /*need_upper_bound_check=*/false,
                       /*input_iter=*/nullptr, /*get_context=*/nullptr,
                       /*lookup_contex=*/nullptr));
  Status s = blockhandles_iter->status();
  if (!s.ok()) {
    out_file->Append("Can not read Index Block \n\n");
    return s;
  }

  uint64_t datablock_size_min = std::numeric_limits<uint64_t>::max();
  uint64_t datablock_size_max = 0;
  uint64_t datablock_size_sum = 0;

  size_t block_id = 1;
  for (blockhandles_iter->SeekToFirst(); blockhandles_iter->Valid();
       block_id++, blockhandles_iter->Next()) {
    s = blockhandles_iter->status();
    if (!s.ok()) {
      break;
    }

    BlockHandle bh = blockhandles_iter->value().handle;
    uint64_t datablock_size = bh.size();
    datablock_size_min = std::min(datablock_size_min, datablock_size);
    datablock_size_max = std::max(datablock_size_max, datablock_size);
    datablock_size_sum += datablock_size;

    out_file->Append("Data Block # ");
    out_file->Append(ROCKSDB_NAMESPACE::ToString(block_id));
    out_file->Append(" @ ");
    out_file->Append(blockhandles_iter->value().handle.ToString(true).c_str());
    out_file->Append("\n");
    out_file->Append("--------------------------------------\n");

    std::unique_ptr<InternalIterator> datablock_iter;
    datablock_iter.reset(NewDataBlockIterator<DataBlockIter>(
        ReadOptions(), blockhandles_iter->value().handle,
        /*input_iter=*/nullptr, /*type=*/BlockType::kData,
        /*get_context=*/nullptr, /*lookup_context=*/nullptr, Status(),
        /*prefetch_buffer=*/nullptr));
    s = datablock_iter->status();

    if (!s.ok()) {
      out_file->Append("Error reading the block - Skipped \n\n");
      continue;
    }

    for (datablock_iter->SeekToFirst(); datablock_iter->Valid();
         datablock_iter->Next()) {
      s = datablock_iter->status();
      if (!s.ok()) {
        out_file->Append("Error reading the block - Skipped \n");
        break;
      }
      DumpKeyValue(datablock_iter->key(), datablock_iter->value(), out_file);
    }
    out_file->Append("\n");
  }

  uint64_t num_datablocks = block_id - 1;
  if (num_datablocks) {
    double datablock_size_avg =
        static_cast<double>(datablock_size_sum) / num_datablocks;
    out_file->Append("Data Block Summary:\n");
    out_file->Append("--------------------------------------");
    out_file->Append("\n  # data blocks: ");
    out_file->Append(ROCKSDB_NAMESPACE::ToString(num_datablocks));
    out_file->Append("\n  min data block size: ");
    out_file->Append(ROCKSDB_NAMESPACE::ToString(datablock_size_min));
    out_file->Append("\n  max data block size: ");
    out_file->Append(ROCKSDB_NAMESPACE::ToString(datablock_size_max));
    out_file->Append("\n  avg data block size: ");
    out_file->Append(ROCKSDB_NAMESPACE::ToString(datablock_size_avg));
    out_file->Append("\n");
  }

  return Status::OK();
}

void BlockBasedTable::DumpKeyValue(const Slice& key, const Slice& value,
                                   WritableFile* out_file) {
  InternalKey ikey;
  ikey.DecodeFrom(key);

  out_file->Append("  HEX    ");
  out_file->Append(ikey.user_key().ToString(true).c_str());
  out_file->Append(": ");
  out_file->Append(value.ToString(true).c_str());
  out_file->Append("\n");

  std::string str_key = ikey.user_key().ToString();
  std::string str_value = value.ToString();
  std::string res_key(""), res_value("");
  char cspace = ' ';
  for (size_t i = 0; i < str_key.size(); i++) {
    if (str_key[i] == '\0') {
      res_key.append("\\0", 2);
    } else {
      res_key.append(&str_key[i], 1);
    }
    res_key.append(1, cspace);
  }
  for (size_t i = 0; i < str_value.size(); i++) {
    if (str_value[i] == '\0') {
      res_value.append("\\0", 2);
    } else {
      res_value.append(&str_value[i], 1);
    }
    res_value.append(1, cspace);
  }

  out_file->Append("  ASCII  ");
  out_file->Append(res_key.c_str());
  out_file->Append(": ");
  out_file->Append(res_value.c_str());
  out_file->Append("\n  ------\n");
}

}  // namespace ROCKSDB_NAMESPACE