rocksdb/table/block_based_table_reader.cc

//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same 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_table_reader.h"

#include <string>
#include <utility>

#include "db/dbformat.h"

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

#include "table/block.h"
#include "table/filter_block.h"
#include "table/format.h"
#include "table/meta_blocks.h"
#include "table/two_level_iterator.h"

#include "util/coding.h"
#include "util/perf_context_imp.h"
#include "util/stop_watch.h"

namespace rocksdb {

extern uint64_t kBlockBasedTableMagicNumber;
using std::unique_ptr;

typedef BlockBasedTable::IndexReader IndexReader;

namespace {

// The longest the prefix of the cache key used to identify blocks can be.
// We are using the fact that we know for Posix files the unique ID is three
// varints.
const size_t kMaxCacheKeyPrefixSize = kMaxVarint64Length*3+1;

// Read the block identified by "handle" from "file".
// The only relevant option is options.verify_checksums for now.
// Set *didIO to true if didIO is not null.
// On failure return non-OK.
// On success fill *result and return OK - caller owns *result
Status ReadBlockFromFile(RandomAccessFile* file, const ReadOptions& options,
                         const BlockHandle& handle, Block** result, Env* env,
                         bool* didIO = nullptr, bool do_uncompress = true) {
  BlockContents contents;
  Status s =
      ReadBlockContents(file, options, handle, &contents, env, do_uncompress);
  if (s.ok()) {
    *result = new Block(contents);
  }

  if (didIO != nullptr) {
    *didIO = true;
  }
  return s;
}

// Delete the resource that is held by the iterator.
template <class ResourceType>
void DeleteHeldResource(void* arg, void* ignored) {
  delete reinterpret_cast<ResourceType*>(arg);
}

// 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 ReleaseCachedEntry(void* arg, void* h) {
  Cache* cache = reinterpret_cast<Cache*>(arg);
  Cache::Handle* handle = reinterpret_cast<Cache::Handle*>(h);
  cache->Release(handle);
}

Slice 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));
}

Cache::Handle* GetEntryFromCache(Cache* block_cache, const Slice& key,
                                 Tickers block_cache_miss_ticker,
                                 Tickers block_cache_hit_ticker,
                                 Statistics* statistics) {
  auto cache_handle = block_cache->Lookup(key);
  if (cache_handle != nullptr) {
    BumpPerfCount(&perf_context.block_cache_hit_count);
    // overall cache hit
    RecordTick(statistics, BLOCK_CACHE_HIT);
    // block-type specific cache hit
    RecordTick(statistics, block_cache_hit_ticker);
  } else {
    // overall cache miss
    RecordTick(statistics, BLOCK_CACHE_MISS);
    // block-type specific cache miss
    RecordTick(statistics, block_cache_miss_ticker);
  }

  return cache_handle;
}

}  // namespace

// -- IndexReader and its subclasses
// IndexReader is the interface that provide the functionality for index access.
class BlockBasedTable::IndexReader {
 public:
  explicit IndexReader(const Comparator* comparator)
      : comparator_(comparator) {}

  virtual ~IndexReader() {}

  // Create an iterator for index access.
  virtual Iterator* NewIterator() = 0;

  // The size of the index.
  virtual size_t size() const = 0;

 protected:
  const Comparator* comparator_;
};

// 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 IndexReader {
 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(RandomAccessFile* file, const BlockHandle& index_handle,
                       Env* env, const Comparator* comparator,
                       IndexReader** index_reader) {
    Block* index_block = nullptr;
    auto s = ReadBlockFromFile(file, ReadOptions(), index_handle,
                               &index_block, env);

    if (s.ok()) {
      *index_reader = new BinarySearchIndexReader(comparator, index_block);
    }

    return s;
  }

  virtual Iterator* NewIterator() override {
    return index_block_->NewIterator(comparator_);
  }

  virtual size_t size() const override { return index_block_->size(); }

 private:
  BinarySearchIndexReader(const Comparator* comparator, Block* index_block)
      : IndexReader(comparator), index_block_(index_block) {
    assert(index_block_ != nullptr);
  }
  std::unique_ptr<Block> index_block_;
};

// TODO(kailiu) This class is only a stub for now. And the comment below is also
// not completed.
// Index that leverages an internal hash table to quicken the lookup for a given
// key.
class HashIndexReader : public IndexReader {
 public:
  static Status Create(RandomAccessFile* file, const BlockHandle& index_handle,
                       Env* env, const Comparator* comparator,
                       BlockBasedTable* table,
                       const SliceTransform* prefix_extractor,
                       IndexReader** index_reader) {
    return Status::NotSupported("not implemented yet!");
  }
};


struct BlockBasedTable::Rep {
  Rep(const EnvOptions& storage_options,
      const InternalKeyComparator& internal_comparator)
      : soptions(storage_options), internal_comparator(internal_comparator) {}

  Options options;
  const EnvOptions& soptions;
  const InternalKeyComparator& internal_comparator;
  Status status;
  unique_ptr<RandomAccessFile> file;
  char cache_key_prefix[kMaxCacheKeyPrefixSize];
  size_t cache_key_prefix_size = 0;
  char compressed_cache_key_prefix[kMaxCacheKeyPrefixSize];
  size_t compressed_cache_key_prefix_size = 0;

  // Handle to metaindex_block: saved from footer
  BlockHandle metaindex_handle;
  // Handle to index: saved from footer
  BlockHandle index_handle;
  // index_reader and filter will be populated and used only when
  // options.block_cache is nullptr; otherwise we will get the index block via
  // the block cache.
  unique_ptr<IndexReader> index_reader;
  unique_ptr<FilterBlockReader> filter;

  std::shared_ptr<const TableProperties> table_properties;
  BlockBasedTableOptions::IndexType index_type;
};

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

// CachableEntry represents the entries that *may* be fetched from block cache.
//  field `value` is the item we want to get.
//  field `cache_handle` is the cache handle to the block cache. If the value
//    was not read from cache, `cache_handle` will be nullptr.
template <class TValue>
struct BlockBasedTable::CachableEntry {
  CachableEntry(TValue* value, Cache::Handle* cache_handle)
    : value(value)
    , cache_handle(cache_handle) {
  }
  CachableEntry(): CachableEntry(nullptr, nullptr) { }
  void Release(Cache* cache) {
    if (cache_handle) {
      cache->Release(cache_handle);
      value = nullptr;
      cache_handle = nullptr;
    }
  }

  TValue* value = nullptr;
  // if the entry is from the cache, cache_handle will be populated.
  Cache::Handle* cache_handle = nullptr;
};

// 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->options.block_cache != nullptr) {
    GenerateCachePrefix(rep->options.block_cache.get(), rep->file.get(),
                        &rep->cache_key_prefix[0],
                        &rep->cache_key_prefix_size);
  }
  if (rep->options.block_cache_compressed != nullptr) {
    GenerateCachePrefix(rep->options.block_cache_compressed.get(),
                        rep->file.get(), &rep->compressed_cache_key_prefix[0],
                        &rep->compressed_cache_key_prefix_size);
  }
}

void BlockBasedTable::GenerateCachePrefix(Cache* cc,
    RandomAccessFile* 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 (*size == 0) {
    char* end = EncodeVarint64(buffer, cc->NewId());
    *size = static_cast<size_t>(end - buffer);
  }
}

void BlockBasedTable::GenerateCachePrefix(Cache* cc,
    WritableFile* 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 (*size == 0) {
    char* end = EncodeVarint64(buffer, cc->NewId());
    *size = static_cast<size_t>(end - buffer);
  }
}

Status BlockBasedTable::Open(const Options& options, const EnvOptions& soptions,
                             const BlockBasedTableOptions& table_options,
                             const InternalKeyComparator& internal_comparator,
                             unique_ptr<RandomAccessFile>&& file,
                             uint64_t file_size,
                             unique_ptr<TableReader>* table_reader) {
  table_reader->reset();

  Footer footer(kBlockBasedTableMagicNumber);
  auto s = ReadFooterFromFile(file.get(), file_size, &footer);
  if (!s.ok()) return s;

  // We've successfully read the footer and the index block: we're
  // ready to serve requests.
  Rep* rep = new BlockBasedTable::Rep(soptions, internal_comparator);
  rep->options = options;
  rep->file = std::move(file);
  rep->metaindex_handle = footer.metaindex_handle();
  rep->index_handle = footer.index_handle();
  rep->index_type = table_options.index_type;
  SetupCacheKeyPrefix(rep);
  unique_ptr<BlockBasedTable> new_table(new BlockBasedTable(rep));

  // Read meta index
  std::unique_ptr<Block> meta;
  std::unique_ptr<Iterator> meta_iter;
  s = ReadMetaBlock(rep, &meta, &meta_iter);

  // Read the properties
  meta_iter->Seek(kPropertiesBlock);
  if (meta_iter->Valid() && meta_iter->key() == kPropertiesBlock) {
    s = meta_iter->status();
    TableProperties* table_properties = nullptr;
    if (s.ok()) {
      s = ReadProperties(meta_iter->value(), rep->file.get(), rep->options.env,
                         rep->options.info_log.get(), &table_properties);
    }

    if (!s.ok()) {
      auto err_msg =
        "[Warning] Encountered error while reading data from properties "
        "block " + s.ToString();
      Log(rep->options.info_log, "%s", err_msg.c_str());
    } else {
      rep->table_properties.reset(table_properties);
    }
  }

  // Will use block cache for index/filter blocks access?
  if (options.block_cache && table_options.cache_index_and_filter_blocks) {
    // Hack: Call NewIndexIterator() to implicitly add index to the block_cache
    unique_ptr<Iterator> iter(new_table->NewIndexIterator(ReadOptions()));
    s = iter->status();

    if (s.ok()) {
      // Hack: Call GetFilter() to implicitly add filter to the block_cache
      auto filter_entry = new_table->GetFilter();
      filter_entry.Release(options.block_cache.get());
    }
  } else {
    // If we don't use block cache for index/filter blocks access, we'll
    // pre-load these blocks, which will kept in member variables in Rep
    // and with a same life-time as this table object.
    IndexReader* index_reader = nullptr;
    // TODO: we never really verify check sum for index block
    s = new_table->CreateIndexReader(&index_reader);

    if (s.ok()) {
      rep->index_reader.reset(index_reader);

      // Set filter block
      if (rep->options.filter_policy) {
        std::string key = kFilterBlockPrefix;
        key.append(rep->options.filter_policy->Name());
        meta_iter->Seek(key);

        if (meta_iter->Valid() && meta_iter->key() == Slice(key)) {
          rep->filter.reset(ReadFilter(meta_iter->value(), rep));
        }
      }
    } else {
      delete index_reader;
    }
  }

  if (s.ok()) {
    *table_reader = std::move(new_table);
  }

  return s;
}

void BlockBasedTable::SetupForCompaction() {
  switch (rep_->options.access_hint_on_compaction_start) {
    case Options::NONE:
      break;
    case Options::NORMAL:
      rep_->file->Hint(RandomAccessFile::NORMAL);
      break;
    case Options::SEQUENTIAL:
      rep_->file->Hint(RandomAccessFile::SEQUENTIAL);
      break;
    case Options::WILLNEED:
      rep_->file->Hint(RandomAccessFile::WILLNEED);
      break;
    default:
      assert(false);
  }
  compaction_optimized_ = true;
}

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

// Load the meta-block from the file. On success, return the loaded meta block
// and its iterator.
Status BlockBasedTable::ReadMetaBlock(
    Rep* rep,
    std::unique_ptr<Block>* meta_block,
    std::unique_ptr<Iterator>* iter) {
  // TODO(sanjay): Skip this if footer.metaindex_handle() size indicates
  // it is an empty block.
  //  TODO: we never really verify check sum for meta index block
  Block* meta = nullptr;
  Status s = ReadBlockFromFile(
      rep->file.get(),
      ReadOptions(),
      rep->metaindex_handle,
      &meta,
      rep->options.env);

    if (!s.ok()) {
      auto err_msg =
        "[Warning] Encountered error while reading data from properties"
        "block " + s.ToString();
      Log(rep->options.info_log, "%s", err_msg.c_str());
    }
  if (!s.ok()) {
    delete meta;
    return s;
  }

  meta_block->reset(meta);
  // meta block uses bytewise comparator.
  iter->reset(meta->NewIterator(BytewiseComparator()));
  return Status::OK();
}

Status BlockBasedTable::GetDataBlockFromCache(
    const Slice& block_cache_key, const Slice& compressed_block_cache_key,
    Cache* block_cache, Cache* block_cache_compressed, Statistics* statistics,
    const ReadOptions& read_options,
    BlockBasedTable::CachableEntry<Block>* block) {
  Status s;
  Block* compressed_block = nullptr;
  Cache::Handle* block_cache_compressed_handle = nullptr;

  // Lookup uncompressed cache first
  if (block_cache != nullptr) {
    block->cache_handle =
        GetEntryFromCache(block_cache, block_cache_key, BLOCK_CACHE_DATA_MISS,
                          BLOCK_CACHE_DATA_HIT, statistics);
    if (block->cache_handle != nullptr) {
      block->value =
          reinterpret_cast<Block*>(block_cache->Value(block->cache_handle));
      return s;
    }
  }

  // If not found, search from the compressed block cache.
  assert(block->cache_handle == nullptr && block->value == nullptr);

  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);
  // 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<Block*>(
      block_cache_compressed->Value(block_cache_compressed_handle));
  assert(compressed_block->compression_type() != kNoCompression);

  // Retrieve the uncompressed contents into a new buffer
  BlockContents contents;
  s = UncompressBlockContents(compressed_block->data(),
                              compressed_block->size(), &contents);

  // Insert uncompressed block into block cache
  if (s.ok()) {
    block->value = new Block(contents);  // uncompressed block
    assert(block->value->compression_type() == kNoCompression);
    if (block_cache != nullptr && block->value->cachable() &&
        read_options.fill_cache) {
      block->cache_handle =
          block_cache->Insert(block_cache_key, block->value,
                              block->value->size(), &DeleteCachedEntry<Block>);
      assert(reinterpret_cast<Block*>(
                 block_cache->Value(block->cache_handle)) == block->value);
    }
  }

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

Status BlockBasedTable::PutDataBlockToCache(
    const Slice& block_cache_key, const Slice& compressed_block_cache_key,
    Cache* block_cache, Cache* block_cache_compressed,
    const ReadOptions& read_options, Statistics* statistics,
    CachableEntry<Block>* block, Block* raw_block) {
  assert(raw_block->compression_type() == kNoCompression ||
         block_cache_compressed != nullptr);

  Status s;
  // Retrieve the uncompressed contents into a new buffer
  BlockContents contents;
  if (raw_block->compression_type() != kNoCompression) {
    s = UncompressBlockContents(raw_block->data(), raw_block->size(),
                                &contents);
  }
  if (!s.ok()) {
    delete raw_block;
    return s;
  }

  if (raw_block->compression_type() != kNoCompression) {
    block->value = new Block(contents);  // uncompressed block
  } else {
    block->value = raw_block;
    raw_block = nullptr;
  }

  // Insert compressed block into compressed block cache.
  // Release the hold on the compressed cache entry immediately.
  if (block_cache_compressed != nullptr && raw_block != nullptr &&
      raw_block->cachable()) {
    auto cache_handle = block_cache_compressed->Insert(
        compressed_block_cache_key, raw_block, raw_block->size(),
        &DeleteCachedEntry<Block>);
    block_cache_compressed->Release(cache_handle);
    RecordTick(statistics, BLOCK_CACHE_COMPRESSED_MISS);
    // Avoid the following code to delete this cached block.
    raw_block = nullptr;
  }
  delete raw_block;

  // insert into uncompressed block cache
  assert((block->value->compression_type() == kNoCompression));
  if (block_cache != nullptr && block->value->cachable()) {
    block->cache_handle =
        block_cache->Insert(block_cache_key, block->value, block->value->size(),
                            &DeleteCachedEntry<Block>);
    RecordTick(statistics, BLOCK_CACHE_ADD);
    assert(reinterpret_cast<Block*>(block_cache->Value(block->cache_handle)) ==
           block->value);
  }

  return s;
}

FilterBlockReader* BlockBasedTable::ReadFilter (
    const Slice& filter_handle_value,
    BlockBasedTable::Rep* rep,
    size_t* filter_size) {
  Slice v = filter_handle_value;
  BlockHandle filter_handle;
  if (!filter_handle.DecodeFrom(&v).ok()) {
    return nullptr;
  }

  // TODO: We might want to unify with ReadBlockFromFile() if we start
  // requiring checksum verification in Table::Open.
  ReadOptions opt;
  BlockContents block;
  if (!ReadBlockContents(rep->file.get(), opt, filter_handle, &block,
                        rep->options.env, false).ok()) {
    return nullptr;
  }

  if (filter_size) {
    *filter_size = block.data.size();
  }

  return new FilterBlockReader(
       rep->options, block.data, block.heap_allocated);
}

// Convert an index iterator value (i.e., an encoded BlockHandle)
// into an iterator over the contents of the corresponding block.
Iterator* BlockBasedTable::DataBlockReader(void* arg,
                                           const ReadOptions& options,
                                           const Slice& index_value,
                                           bool* didIO, bool for_compaction) {
  const bool no_io = (options.read_tier == kBlockCacheTier);
  BlockBasedTable* table = reinterpret_cast<BlockBasedTable*>(arg);
  Cache* block_cache = table->rep_->options.block_cache.get();
  Cache* block_cache_compressed = table->rep_->options.
                                    block_cache_compressed.get();
  CachableEntry<Block> block;

  BlockHandle handle;
  Slice input = index_value;
  // We intentionally allow extra stuff in index_value so that we
  // can add more features in the future.
  Status s = handle.DecodeFrom(&input);

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

  // If either block cache is enabled, we'll try to read from it.
  if (block_cache != nullptr || block_cache_compressed != nullptr) {
    Statistics* statistics = table->rep_->options.statistics.get();
    char cache_key[kMaxCacheKeyPrefixSize + kMaxVarint64Length];
    char compressed_cache_key[kMaxCacheKeyPrefixSize + kMaxVarint64Length];
    Slice key, /* key to the block cache */
        ckey /* key to the compressed block cache */;

    // create key for block cache
    if (block_cache != nullptr) {
      key = GetCacheKey(table->rep_->cache_key_prefix,
                        table->rep_->cache_key_prefix_size, handle, cache_key);
    }

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

    s = GetDataBlockFromCache(key, ckey, block_cache, block_cache_compressed,
                              statistics, options, &block);

    if (block.value == nullptr && !no_io && options.fill_cache) {
      Histograms histogram = for_compaction ?
        READ_BLOCK_COMPACTION_MICROS : READ_BLOCK_GET_MICROS;
      Block* raw_block = nullptr;
      {
        StopWatch sw(table->rep_->options.env, statistics, histogram);
        s = ReadBlockFromFile(table->rep_->file.get(), options, handle,
                              &raw_block, table->rep_->options.env, didIO,
                              block_cache_compressed == nullptr);
      }

      if (s.ok()) {
        s = PutDataBlockToCache(key, ckey, block_cache, block_cache_compressed,
                                options, statistics, &block, raw_block);
      }
    }
  }

  // Didn't get any data from block caches.
  if (block.value == nullptr) {
    if (no_io) {
      // Could not read from block_cache and can't do IO
      return NewErrorIterator(Status::Incomplete("no blocking io"));
    }
    s = ReadBlockFromFile(table->rep_->file.get(), options, handle,
                          &block.value, table->rep_->options.env, didIO);
  }

  Iterator* iter;
  if (block.value != nullptr) {
    iter = block.value->NewIterator(&table->rep_->internal_comparator);
    if (block.cache_handle != nullptr) {
      iter->RegisterCleanup(&ReleaseCachedEntry, block_cache,
                            block.cache_handle);
    } else {
      iter->RegisterCleanup(&DeleteHeldResource<Block>, block.value, nullptr);
    }
  } else {
    iter = NewErrorIterator(s);
  }
  return iter;
}

BlockBasedTable::CachableEntry<FilterBlockReader> BlockBasedTable::GetFilter(
    bool no_io) const {
  // filter pre-populated
  if (rep_->filter != nullptr) {
    return {rep_->filter.get(), nullptr /* cache handle */};
  }

  if (rep_->options.filter_policy == nullptr /* do not use filter at all */ ||
      rep_->options.block_cache == nullptr /* no block cache at all */) {
    return {nullptr /* filter */, nullptr /* cache handle */};
  }

  // Fetching from the cache
  Cache* block_cache = rep_->options.block_cache.get();
  char cache_key[kMaxCacheKeyPrefixSize + kMaxVarint64Length];
  auto key = GetCacheKey(
      rep_->cache_key_prefix,
      rep_->cache_key_prefix_size,
      rep_->metaindex_handle,
      cache_key
  );

  Statistics* statistics = rep_->options.statistics.get();
  auto cache_handle =
      GetEntryFromCache(block_cache, key, BLOCK_CACHE_FILTER_MISS,
                        BLOCK_CACHE_FILTER_HIT, statistics);

  FilterBlockReader* filter = nullptr;
  if (cache_handle != nullptr) {
     filter = reinterpret_cast<FilterBlockReader*>(
         block_cache->Value(cache_handle));
  } else if (no_io) {
    // Do not invoke any io.
    return CachableEntry<FilterBlockReader>();
  } else {
    size_t filter_size = 0;
    std::unique_ptr<Block> meta;
    std::unique_ptr<Iterator> iter;
    auto s = ReadMetaBlock(rep_, &meta, &iter);

    if (s.ok()) {
      std::string filter_block_key = kFilterBlockPrefix;
      filter_block_key.append(rep_->options.filter_policy->Name());
      iter->Seek(filter_block_key);

      if (iter->Valid() && iter->key() == Slice(filter_block_key)) {
        filter = ReadFilter(iter->value(), rep_, &filter_size);
        assert(filter);
        assert(filter_size > 0);

        cache_handle = block_cache->Insert(
            key, filter, filter_size, &DeleteCachedEntry<FilterBlockReader>);
        RecordTick(statistics, BLOCK_CACHE_ADD);
      }
    }
  }

  return { filter, cache_handle };
}

Iterator* BlockBasedTable::NewIndexIterator(const ReadOptions& read_options)
    const {
  // index reader has already been pre-populated.
  if (rep_->index_reader) {
    return rep_->index_reader->NewIterator();
  }

  bool no_io = read_options.read_tier == kBlockCacheTier;
  Cache* block_cache = rep_->options.block_cache.get();
  char cache_key[kMaxCacheKeyPrefixSize + kMaxVarint64Length];
  auto key = GetCacheKey(rep_->cache_key_prefix, rep_->cache_key_prefix_size,
                         rep_->index_handle, cache_key);
  Statistics* statistics = rep_->options.statistics.get();
  auto cache_handle =
      GetEntryFromCache(block_cache, key, BLOCK_CACHE_INDEX_MISS,
                        BLOCK_CACHE_INDEX_HIT, statistics);

  if (cache_handle == nullptr && no_io) {
    return NewErrorIterator(Status::Incomplete("no blocking io"));
  }

  IndexReader* index_reader = nullptr;
  if (cache_handle != nullptr) {
    index_reader =
        reinterpret_cast<IndexReader*>(block_cache->Value(cache_handle));
  } else {
    // Create index reader and put it in the cache.
    Status s;
    s = CreateIndexReader(&index_reader);

    if (!s.ok()) {
      // make sure if something goes wrong, index_reader shall remain intact.
      assert(index_reader == nullptr);
      return NewErrorIterator(s);
    }

    cache_handle = block_cache->Insert(key, index_reader, index_reader->size(),
                                       &DeleteCachedEntry<IndexReader>);
    RecordTick(statistics, BLOCK_CACHE_ADD);
  }

  assert(cache_handle);
  auto iter = index_reader->NewIterator();
  iter->RegisterCleanup(&ReleaseCachedEntry, block_cache, cache_handle);

  return iter;
}

Iterator* BlockBasedTable::DataBlockReader(
    void* arg, const ReadOptions& options, const EnvOptions& soptions,
    const InternalKeyComparator& icomparator, const Slice& index_value,
    bool for_compaction) {
  return DataBlockReader(arg, options, index_value, nullptr, for_compaction);
}

// 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 Options.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_prefix) {
  bool may_match = true;
  Status s;

  if (!rep_->options.filter_policy) {
    return true;
  }

  // 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;
  unique_ptr<Iterator> iiter(NewIndexIterator(no_io_read_options));
  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 (ExtractUserKey(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 {
    // 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 one which could potentially contain the prefix.
    Slice handle_value = iiter->value();
    BlockHandle handle;
    s = handle.DecodeFrom(&handle_value);
    assert(s.ok());
    auto filter_entry = GetFilter(true /* no io */);
    may_match =
      filter_entry.value == nullptr ||
      filter_entry.value->PrefixMayMatch(handle.offset(), internal_prefix);
    filter_entry.Release(rep_->options.block_cache.get());
  }

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

  return may_match;
}

Iterator* BlockBasedTable::NewIterator(const ReadOptions& options) {
  if (options.prefix) {
    InternalKey internal_prefix(*options.prefix, 0, kTypeValue);
    if (!PrefixMayMatch(internal_prefix.Encode())) {
      // nothing in this file can match the prefix, so we should not
      // bother doing I/O to this file when iterating.
      return NewEmptyIterator();
    }
  }

  return NewTwoLevelIterator(NewIndexIterator(options),
                             &BlockBasedTable::DataBlockReader,
                             const_cast<BlockBasedTable*>(this), options,
                             rep_->soptions, rep_->internal_comparator);
}

Status BlockBasedTable::Get(
    const ReadOptions& read_options, const Slice& key, void* handle_context,
    bool (*result_handler)(void* handle_context, const ParsedInternalKey& k,
                           const Slice& v, bool didIO),
    void (*mark_key_may_exist_handler)(void* handle_context)) {
  Status s;
  Iterator* iiter = NewIndexIterator(read_options);
  auto filter_entry = GetFilter(read_options.read_tier == kBlockCacheTier);
  FilterBlockReader* filter = filter_entry.value;
  bool done = false;
  for (iiter->Seek(key); iiter->Valid() && !done; iiter->Next()) {
    Slice handle_value = iiter->value();

    BlockHandle handle;
    bool may_not_exist_in_filter =
      filter != nullptr &&
      handle.DecodeFrom(&handle_value).ok() &&
      !filter->KeyMayMatch(handle.offset(), key);

    if (may_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_->options.statistics.get(), BLOOM_FILTER_USEFUL);
      break;
    } else {
      bool didIO = false;
      unique_ptr<Iterator> block_iter(
          DataBlockReader(this, read_options, iiter->value(), &didIO));

      if (read_options.read_tier && block_iter->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
        (*mark_key_may_exist_handler)(handle_context);
        break;
      }

      // Call the *saver function on each entry/block until it returns false
      for (block_iter->Seek(key); block_iter->Valid(); block_iter->Next()) {
        ParsedInternalKey parsed_key;
        if (!ParseInternalKey(block_iter->key(), &parsed_key)) {
          s = Status::Corruption(Slice());
        }

        if (!(*result_handler)(handle_context, parsed_key, block_iter->value(),
                               didIO)) {
          done = true;
          break;
        }
      }
      s = block_iter->status();
    }
  }

  filter_entry.Release(rep_->options.block_cache.get());
  if (s.ok()) {
    s = iiter->status();
  }
  delete iiter;
  return s;
}

bool SaveDidIO(void* arg, const ParsedInternalKey& key, const Slice& value,
               bool didIO) {
  *reinterpret_cast<bool*>(arg) = didIO;
  return false;
}
bool BlockBasedTable::TEST_KeyInCache(const ReadOptions& options,
                                      const Slice& key) {
  // We use Get() as it has logic that checks whether we read the
  // block from the disk or not.
  bool didIO = false;
  Status s = Get(options, key, &didIO, SaveDidIO);
  assert(s.ok());
  return !didIO;
}

// 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(IndexReader** index_reader) const {
  // Some old version of block-based tables don't have index type present in
  // table properties. If that's the case we can safely use the kBinarySearch.
  auto index_type = BlockBasedTableOptions::kBinarySearch;
  auto& props = rep_->table_properties->user_collected_properties;
  auto pos = props.find(BlockBasedTablePropertyNames::kIndexType);
  if (pos != props.end()) {
    index_type = static_cast<BlockBasedTableOptions::IndexType>(
        DecodeFixed32(pos->second.c_str()));
  }

  switch (index_type) {
    case BlockBasedTableOptions::kBinarySearch: {
      return BinarySearchIndexReader::Create(
          rep_->file.get(), rep_->index_handle, rep_->options.env,
          &rep_->internal_comparator, index_reader);
    }
    default: {
      std::string error_message =
          "Unrecognized index type: " + std::to_string(rep_->index_type);
      // equivalent to assert(false), but more informative.
      assert(!error_message.c_str());
      return Status::InvalidArgument(error_message.c_str());
    }
  }
}

uint64_t BlockBasedTable::ApproximateOffsetOf(const Slice& key) {
  unique_ptr<Iterator> index_iter(NewIndexIterator(ReadOptions()));

  index_iter->Seek(key);
  uint64_t result;
  if (index_iter->Valid()) {
    BlockHandle handle;
    Slice input = index_iter->value();
    Status s = handle.DecodeFrom(&input);
    if (s.ok()) {
      result = handle.offset();
    } else {
      // Strange: we can't decode the block handle in the index block.
      // We'll just return the offset of the metaindex block, which is
      // close to the whole file size for this case.
      result = rep_->metaindex_handle.offset();
    }
  } else {
    // key 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).
    result = rep_->table_properties->data_size;
    // table_properties is not present in the table.
    if (result == 0) {
      result = rep_->metaindex_handle.offset();
    }
  }
  return result;
}

bool BlockBasedTable::TEST_filter_block_preloaded() const {
  return rep_->filter != nullptr;
}

bool BlockBasedTable::TEST_index_reader_preloaded() const {
  return rep_->index_reader != nullptr;
}

}  // namespace rocksdb