rocksdb/db/dbformat.h

//  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.

#pragma once
#include <stdio.h>

#include <array>
#include <memory>
#include <optional>
#include <string>
#include <utility>

#include "rocksdb/comparator.h"
#include "rocksdb/slice.h"
#include "rocksdb/slice_transform.h"
#include "rocksdb/types.h"
#include "util/coding.h"
#include "util/user_comparator_wrapper.h"

namespace ROCKSDB_NAMESPACE {

// The file declares data structures and functions that deal with internal
// keys.
// Each internal key contains a user key, a sequence number (SequenceNumber)
// and a type (ValueType), and they are usually encoded together.
// There are some related helper classes here.

class InternalKey;

// Value types encoded as the last component of internal keys.
// DO NOT CHANGE THESE ENUM VALUES: they are embedded in the on-disk
// data structures.
// The highest bit of the value type needs to be reserved to SST tables
// for them to do more flexible encoding.
enum ValueType : unsigned char {
  kTypeDeletion = 0x0,
  kTypeValue = 0x1,
  kTypeMerge = 0x2,
  kTypeLogData = 0x3,               // WAL only.
  kTypeColumnFamilyDeletion = 0x4,  // WAL only.
  kTypeColumnFamilyValue = 0x5,     // WAL only.
  kTypeColumnFamilyMerge = 0x6,     // WAL only.
  kTypeSingleDeletion = 0x7,
  kTypeColumnFamilySingleDeletion = 0x8,  // WAL only.
  kTypeBeginPrepareXID = 0x9,             // WAL only.
  kTypeEndPrepareXID = 0xA,               // WAL only.
  kTypeCommitXID = 0xB,                   // WAL only.
  kTypeRollbackXID = 0xC,                 // WAL only.
  kTypeNoop = 0xD,                        // WAL only.
  kTypeColumnFamilyRangeDeletion = 0xE,   // WAL only.
  kTypeRangeDeletion = 0xF,               // meta block
  kTypeColumnFamilyBlobIndex = 0x10,      // Blob DB only
  kTypeBlobIndex = 0x11,                  // Blob DB only
  // When the prepared record is also persisted in db, we use a different
  // record. This is to ensure that the WAL that is generated by a WritePolicy
  // is not mistakenly read by another, which would result into data
  // inconsistency.
  kTypeBeginPersistedPrepareXID = 0x12,  // WAL only.
  // Similar to kTypeBeginPersistedPrepareXID, this is to ensure that WAL
  // generated by WriteUnprepared write policy is not mistakenly read by
  // another.
  kTypeBeginUnprepareXID = 0x13,  // WAL only.
  kTypeDeletionWithTimestamp = 0x14,
  kTypeCommitXIDAndTimestamp = 0x15,  // WAL only
  kTypeWideColumnEntity = 0x16,
  kTypeColumnFamilyWideColumnEntity = 0x17,     // WAL only
  kTypeValuePreferredSeqno = 0x18,              // Value with a unix write time
  kTypeColumnFamilyValuePreferredSeqno = 0x19,  // WAL only
  kTypeMaxValid,    // Should be after the last valid type, only used for
                    // validation
  kMaxValue = 0x7F  // Not used for storing records.
};

// Defined in dbformat.cc
extern const ValueType kValueTypeForSeek;
extern const ValueType kValueTypeForSeekForPrev;

// A range of user keys used internally by RocksDB. Also see `Range` used by
// public APIs.
// TODO: merge with Range in pubic API, but this is generally inclusive limit
// and it is maybe exclusive limit
struct UserKeyRange {
  // In case of user_defined timestamp, if enabled, `start` and `limit` should
  // include user_defined timestamps.
  Slice start;
  Slice limit;

  UserKeyRange() = default;
  UserKeyRange(const Slice& s, const Slice& l) : start(s), limit(l) {}
};

// A range of user keys used internally by RocksDB. Also see `RangeOpt` used by
// public APIs.
struct UserKeyRangeOpt {
  // In case of user_defined timestamp, if enabled, `start` and `limit` should
  // point to key with timestamp part.
  // An optional range start, if missing, indicating a start before all keys.
  OptSlice start;
  // An optional range end, if missing, indicating an end after all keys.
  OptSlice limit;

  UserKeyRangeOpt(const OptSlice& s, const OptSlice& l) : start(s), limit(l) {}
};

// Checks whether a type is an inline value type
// (i.e. a type used in memtable skiplist and sst file datablock).
inline bool IsValueType(ValueType t) {
  return t <= kTypeMerge || kTypeSingleDeletion == t || kTypeBlobIndex == t ||
         kTypeDeletionWithTimestamp == t || kTypeWideColumnEntity == t ||
         kTypeValuePreferredSeqno == t;
}

// Checks whether a type is from user operation
// kTypeRangeDeletion is in meta block so this API is separated from above
// kTypeMaxValid can be from keys generated by
// TruncatedRangeDelIterator::start_key()
inline bool IsExtendedValueType(ValueType t) {
  return IsValueType(t) || t == kTypeRangeDeletion || t == kTypeMaxValid;
}

// We leave eight bits empty at the bottom so a type and sequence#
// can be packed together into 64-bits.
static const SequenceNumber kMaxSequenceNumber = ((0x1ull << 56) - 1);

static const SequenceNumber kDisableGlobalSequenceNumber =
    std::numeric_limits<uint64_t>::max();

constexpr uint64_t kNumInternalBytes = 8;

// Defined in dbformat.cc
extern const std::string kDisableUserTimestamp;

// The data structure that represents an internal key in the way that user_key,
// sequence number and type are stored in separated forms.
struct ParsedInternalKey {
  Slice user_key;
  SequenceNumber sequence;
  ValueType type;

  ParsedInternalKey()
      : sequence(kMaxSequenceNumber),
        type(kTypeDeletion)  // Make code analyzer happy
  {}                         // Intentionally left uninitialized (for speed)
  // u contains timestamp if user timestamp feature is enabled.
  ParsedInternalKey(const Slice& u, const SequenceNumber& seq, ValueType t)
      : user_key(u), sequence(seq), type(t) {}
  std::string DebugString(bool log_err_key, bool hex,
                          const Comparator* ucmp = nullptr) const;

  void clear() {
    user_key.clear();
    sequence = 0;
    type = kTypeDeletion;
  }

  void SetTimestamp(const Slice& ts) {
    assert(ts.size() <= user_key.size());
    const char* addr = user_key.data() + user_key.size() - ts.size();
    memcpy(const_cast<char*>(addr), ts.data(), ts.size());
  }

  Slice GetTimestamp(size_t ts_sz) {
    assert(ts_sz <= user_key.size());
    const char* addr = user_key.data() + user_key.size() - ts_sz;
    return Slice(const_cast<char*>(addr), ts_sz);
  }
};

// Return the length of the encoding of "key".
inline size_t InternalKeyEncodingLength(const ParsedInternalKey& key) {
  return key.user_key.size() + kNumInternalBytes;
}

// Pack a sequence number and a ValueType into a uint64_t
inline uint64_t PackSequenceAndType(uint64_t seq, ValueType t) {
  assert(seq <= kMaxSequenceNumber);
  // kTypeMaxValid is used in TruncatedRangeDelIterator, see its constructor.
  assert(IsExtendedValueType(t) || t == kTypeMaxValid);
  return (seq << 8) | t;
}

// Given the result of PackSequenceAndType, store the sequence number in *seq
// and the ValueType in *t.
inline void UnPackSequenceAndType(uint64_t packed, uint64_t* seq,
                                  ValueType* t) {
  *seq = packed >> 8;
  *t = static_cast<ValueType>(packed & 0xff);

  // Commented the following two assertions in order to test key-value checksum
  // on corrupted keys without crashing ("DbKvChecksumTest").
  // assert(*seq <= kMaxSequenceNumber);
  // assert(IsExtendedValueType(*t));
}

const uint64_t kRangeTombstoneSentinel =
    PackSequenceAndType(kMaxSequenceNumber, kTypeRangeDeletion);

EntryType GetEntryType(ValueType value_type);

// Append the serialization of "key" to *result.
//
// input [internal key]: <user_key | seqno + type>
// output before: empty
// output:               <user_key | seqno + type>
void AppendInternalKey(std::string* result, const ParsedInternalKey& key);

// Append the serialization of "key" to *result, replacing the original
// timestamp with argument ts.
//
// input [internal key]:   <user_provided_key | original_ts | seqno + type>
// output before: empty
// output after:           <user_provided_key | ts          | seqno + type>
void AppendInternalKeyWithDifferentTimestamp(std::string* result,
                                             const ParsedInternalKey& key,
                                             const Slice& ts);

// Append the user key to *result, replacing the original timestamp with
// argument ts.
//
// input [user key]:       <user_provided_key | original_ts>
// output before: empty
// output after:           <user_provided_key | ts>
void AppendUserKeyWithDifferentTimestamp(std::string* result, const Slice& key,
                                         const Slice& ts);

// Serialized internal key consists of user key followed by footer.
// This function appends the footer to *result, assuming that *result already
// contains the user key at the end.
//
// output before: <user_key>
// output after:  <user_key | seqno + type>
void AppendInternalKeyFooter(std::string* result, SequenceNumber s,
                             ValueType t);

// Append the key and a minimal timestamp to *result
//
// input [user key without ts]:   <user_provided_key>
// output before: empty
// output after:                  <user_provided_key | min_ts>
void AppendKeyWithMinTimestamp(std::string* result, const Slice& key,
                               size_t ts_sz);

// Append the key and a maximal timestamp to *result
//
// input [user key without ts]:   <user_provided_key>
// output before: empty
// output after:                  <user_provided_key | max_ts>
void AppendKeyWithMaxTimestamp(std::string* result, const Slice& key,
                               size_t ts_sz);

// `key` is a user key with timestamp. Append the user key without timestamp
// and the minimum timestamp to *result.
//
// input [user key]:   <user_provided_key | original_ts>
// output before: empty
// output after:       <user_provided_key | min_ts>
void AppendUserKeyWithMinTimestamp(std::string* result, const Slice& key,
                                   size_t ts_sz);

// `key` is a user key with timestamp. Append the user key without timestamp
// and the maximal timestamp to *result.
//
// input [user key]:   <user_provided_key | original_ts>
// output before: empty
// output after:       <user_provided_key | max_ts>
void AppendUserKeyWithMaxTimestamp(std::string* result, const Slice& key,
                                   size_t ts_sz);

// `key` is an internal key containing a user key without timestamp. Create a
// new key in *result by padding a min timestamp of size `ts_sz` to the user key
// and copying the remaining internal key bytes.
//
// input [internal key]:   <user_provided_key | seqno + type>
// output before: empty
// output after:           <user_provided_key | min_ts | seqno + type>
void PadInternalKeyWithMinTimestamp(std::string* result, const Slice& key,
                                    size_t ts_sz);

// `key` is an internal key containing a user key without timestamp. Create a
// new key in *result by padding a max timestamp of size `ts_sz` to the user key
// and copying the remaining internal key bytes.
//
// input [internal key]:   <user_provided_key | seqno + type>
// output before: empty
// output after:           <user_provided_key | max_ts | seqno + type>
void PadInternalKeyWithMaxTimestamp(std::string* result, const Slice& key,
                                    size_t ts_sz);

// `key` is an internal key containing a user key with timestamp of size
// `ts_sz`. Create a new internal key in *result by stripping the timestamp from
// the user key and copying the remaining internal key bytes.
//
// input [internal key]:  <user_provided_key | original_ts | seqno + type>
// output before: empty
// output after:          <user_provided_key | seqno + type>
void StripTimestampFromInternalKey(std::string* result, const Slice& key,
                                   size_t ts_sz);

// `key` is an internal key containing a user key with timestamp of size
// `ts_sz`. Create a new internal key in *result while replace the original
// timestamp with min timestamp.
//
// input [internal key]:  <user_provided_key | original_ts | seqno + type>
// output before: empty
// output after:          <user_provided_key | min_ts      | seqno + type>
void ReplaceInternalKeyWithMinTimestamp(std::string* result, const Slice& key,
                                        size_t ts_sz);

// Attempt to parse an internal key from "internal_key".  On success,
// stores the parsed data in "*result", and returns true.
//
// On error, returns false, leaves "*result" in an undefined state.
Status ParseInternalKey(const Slice& internal_key, ParsedInternalKey* result,
                        bool log_err_key);

// Returns the user key portion of an internal key.
//
// input [internal key]: <user_key | seqno + type>
// output:               <user_key>
inline Slice ExtractUserKey(const Slice& internal_key) {
  assert(internal_key.size() >= kNumInternalBytes);
  return Slice(internal_key.data(), internal_key.size() - kNumInternalBytes);
}

// input [internal key]: <user_provided_key | ts | seqno + type>
// output              : <user_provided_key>
inline Slice ExtractUserKeyAndStripTimestamp(const Slice& internal_key,
                                             size_t ts_sz) {
  assert(internal_key.size() >= kNumInternalBytes + ts_sz);
  return Slice(internal_key.data(),
               internal_key.size() - (kNumInternalBytes + ts_sz));
}

// input [user key]: <user_provided_key | ts>
// output:           <user_provided_key>
inline Slice StripTimestampFromUserKey(const Slice& user_key, size_t ts_sz) {
  assert(user_key.size() >= ts_sz);
  return Slice(user_key.data(), user_key.size() - ts_sz);
}

// input [user key]: <user_provided_key | ts>
// output:                               <ts>
inline Slice ExtractTimestampFromUserKey(const Slice& user_key, size_t ts_sz) {
  assert(user_key.size() >= ts_sz);
  return Slice(user_key.data() + user_key.size() - ts_sz, ts_sz);
}

// input [internal key]: <user_provided_key | ts | seqno + type>
// output:                                   <ts>
inline Slice ExtractTimestampFromKey(const Slice& internal_key, size_t ts_sz) {
  const size_t key_size = internal_key.size();
  assert(key_size >= kNumInternalBytes + ts_sz);
  return Slice(internal_key.data() + key_size - ts_sz - kNumInternalBytes,
               ts_sz);
}

// input [internal key]: <user_provided_key | ts | seqno + type>
// output:                                        <seqno + type>
inline uint64_t ExtractInternalKeyFooter(const Slice& internal_key) {
  assert(internal_key.size() >= kNumInternalBytes);
  const size_t n = internal_key.size();
  return DecodeFixed64(internal_key.data() + n - kNumInternalBytes);
}

// input [internal key]: <user_provided_key | ts | seqno + type>
// output:                                                <type>
inline ValueType ExtractValueType(const Slice& internal_key) {
  uint64_t num = ExtractInternalKeyFooter(internal_key);
  unsigned char c = num & 0xff;
  return static_cast<ValueType>(c);
}

// A comparator for internal keys that uses a specified comparator for
// the user key portion and breaks ties by decreasing sequence number.
class InternalKeyComparator
#ifdef NDEBUG
    final
#endif
    : public CompareInterface {
 private:
  UserComparatorWrapper user_comparator_;

 public:
  // `InternalKeyComparator`s constructed with the default constructor are not
  // usable and will segfault on any attempt to use them for comparisons.
  InternalKeyComparator() = default;

  // @param named If true, assign a name to this comparator based on the
  //    underlying comparator's name. This involves an allocation and copy in
  //    this constructor to precompute the result of `Name()`. To avoid this
  //    overhead, set `named` to false. In that case, `Name()` will return a
  //    generic name that is non-specific to the underlying comparator.
  explicit InternalKeyComparator(const Comparator* c) : user_comparator_(c) {}
  virtual ~InternalKeyComparator() {}

  int Compare(const Slice& a, const Slice& b) const override;

  bool Equal(const Slice& a, const Slice& b) const {
    // TODO Use user_comparator_.Equal(). Perhaps compare seqno before
    // comparing the user key too.
    return Compare(a, b) == 0;
  }

  // Same as Compare except that it excludes the value type from comparison
  int CompareKeySeq(const Slice& a, const Slice& b) const;
  int CompareKeySeq(const ParsedInternalKey& a, const Slice& b) const;

  const Comparator* user_comparator() const {
    return user_comparator_.user_comparator();
  }

  int Compare(const InternalKey& a, const InternalKey& b) const;
  int Compare(const ParsedInternalKey& a, const ParsedInternalKey& b) const;
  int Compare(const Slice& a, const ParsedInternalKey& b) const;
  int Compare(const ParsedInternalKey& a, const Slice& b) const;
  // In this `Compare()` overload, the sequence numbers provided in
  // `a_global_seqno` and `b_global_seqno` override the sequence numbers in `a`
  // and `b`, respectively. To disable sequence number override(s), provide the
  // value `kDisableGlobalSequenceNumber`.
  int Compare(const Slice& a, SequenceNumber a_global_seqno, const Slice& b,
              SequenceNumber b_global_seqno) const;
};

// The class represent the internal key in encoded form.
class InternalKey {
 private:
  std::string rep_;

 public:
  InternalKey() {}  // Leave rep_ as empty to indicate it is invalid
  InternalKey(const Slice& _user_key, SequenceNumber s, ValueType t) {
    AppendInternalKey(&rep_, ParsedInternalKey(_user_key, s, t));
  }
  InternalKey(const Slice& _user_key, SequenceNumber s, ValueType t, Slice ts) {
    AppendInternalKeyWithDifferentTimestamp(
        &rep_, ParsedInternalKey(_user_key, s, t), ts);
  }

  // sets the internal key to be bigger or equal to all internal keys with this
  // user key
  void SetMaxPossibleForUserKey(const Slice& _user_key) {
    AppendInternalKey(
        &rep_, ParsedInternalKey(_user_key, 0, static_cast<ValueType>(0)));
  }

  // sets the internal key to be smaller or equal to all internal keys with this
  // user key
  void SetMinPossibleForUserKey(const Slice& _user_key) {
    AppendInternalKey(&rep_, ParsedInternalKey(_user_key, kMaxSequenceNumber,
                                               kValueTypeForSeek));
  }

  bool Valid() const {
    ParsedInternalKey parsed;
    return (ParseInternalKey(Slice(rep_), &parsed, false /* log_err_key */)
                .ok());  // TODO
  }

  void DecodeFrom(const Slice& s) { rep_.assign(s.data(), s.size()); }
  Slice Encode() const {
    assert(!rep_.empty());
    return rep_;
  }

  Slice user_key() const { return ExtractUserKey(rep_); }
  size_t size() const { return rep_.size(); }
  bool unset() const { return rep_.empty(); }

  void Set(const Slice& _user_key, SequenceNumber s, ValueType t) {
    SetFrom(ParsedInternalKey(_user_key, s, t));
  }

  void Set(const Slice& _user_key_with_ts, SequenceNumber s, ValueType t,
           const Slice& ts) {
    ParsedInternalKey pik(_user_key_with_ts, s, t);
    // Should not call pik.SetTimestamp() directly as it overwrites the buffer
    // containing _user_key.
    SetFrom(pik, ts);
  }

  void SetFrom(const ParsedInternalKey& p) {
    rep_.clear();
    AppendInternalKey(&rep_, p);
  }

  void SetFrom(const ParsedInternalKey& p, const Slice& ts) {
    rep_.clear();
    AppendInternalKeyWithDifferentTimestamp(&rep_, p, ts);
  }

  void Clear() { rep_.clear(); }

  // The underlying representation.
  // Intended only to be used together with ConvertFromUserKey().
  std::string* rep() { return &rep_; }

  const std::string* const_rep() const { return &rep_; }

  // Assuming that *rep() contains a user key, this method makes internal key
  // out of it in-place. This saves a memcpy compared to Set()/SetFrom().
  void ConvertFromUserKey(SequenceNumber s, ValueType t) {
    AppendInternalKeyFooter(&rep_, s, t);
  }

  std::string DebugString(bool hex, const Comparator* ucmp = nullptr) const;
};

inline int InternalKeyComparator::Compare(const InternalKey& a,
                                          const InternalKey& b) const {
  return Compare(a.Encode(), b.Encode());
}

inline Status ParseInternalKey(const Slice& internal_key,
                               ParsedInternalKey* result, bool log_err_key) {
  const size_t n = internal_key.size();

  if (n < kNumInternalBytes) {
    return Status::Corruption("Corrupted Key: Internal Key too small. Size=" +
                              std::to_string(n) + ". ");
  }

  uint64_t num = DecodeFixed64(internal_key.data() + n - kNumInternalBytes);
  unsigned char c = num & 0xff;
  result->sequence = num >> 8;
  result->type = static_cast<ValueType>(c);
  assert(result->type <= ValueType::kMaxValue);
  result->user_key = Slice(internal_key.data(), n - kNumInternalBytes);

  if (IsExtendedValueType(result->type)) {
    return Status::OK();
  } else {
    return Status::Corruption("Corrupted Key",
                              result->DebugString(log_err_key, true));
  }
}

// Update the sequence number in the internal key.
// Guarantees not to invalidate ikey.data().
inline void UpdateInternalKey(std::string* ikey, uint64_t seq, ValueType t) {
  size_t ikey_sz = ikey->size();
  assert(ikey_sz >= kNumInternalBytes);
  uint64_t newval = (seq << 8) | t;

  // Note: Since C++11, strings are guaranteed to be stored contiguously and
  // string::operator[]() is guaranteed not to change ikey.data().
  EncodeFixed64(&(*ikey)[ikey_sz - kNumInternalBytes], newval);
}

// Get the sequence number from the internal key
inline uint64_t GetInternalKeySeqno(const Slice& internal_key) {
  const size_t n = internal_key.size();
  assert(n >= kNumInternalBytes);
  uint64_t num = DecodeFixed64(internal_key.data() + n - kNumInternalBytes);
  return num >> 8;
}

// The class to store keys in an efficient way. It allows:
// 1. Users can either copy the key into it, or have it point to an unowned
//    address.
// 2. For copied key, a short inline buffer is kept to reduce memory
//    allocation for smaller keys.
// 3. It tracks user key or internal key, and allow conversion between them.
class IterKey {
  static constexpr size_t kInlineBufferSize = 39;
  // This is only used by user-defined timestamps in MemTable only feature,
  // which only supports uint64_t timestamps.
  static constexpr char kTsMin[] = "\x00\x00\x00\x00\x00\x00\x00\x00";

 public:
  IterKey()
      : buf_(space_),
        key_(buf_),
        key_size_(0),
        buf_size_(kInlineBufferSize),
        is_user_key_(true),
        secondary_buf_(space_for_secondary_buf_),
        secondary_buf_size_(kInlineBufferSize) {}
  // No copying allowed
  IterKey(const IterKey&) = delete;
  void operator=(const IterKey&) = delete;

  ~IterKey() {
    ResetBuffer();
    ResetSecondaryBuffer();
  }

  // The bool will be picked up by the next calls to SetKey
  void SetIsUserKey(bool is_user_key) { is_user_key_ = is_user_key; }

  // Returns the key in whichever format that was provided to KeyIter
  // If user-defined timestamp is enabled, then timestamp is included in the
  // return result.
  Slice GetKey() const { return Slice(key_, key_size_); }

  Slice GetInternalKey() const {
    assert(!IsUserKey());
    return Slice(key_, key_size_);
  }

  // If user-defined timestamp is enabled, then timestamp is included in the
  // return result of GetUserKey();
  Slice GetUserKey() const {
    if (IsUserKey()) {
      return Slice(key_, key_size_);
    } else {
      assert(key_size_ >= kNumInternalBytes);
      return Slice(key_, key_size_ - kNumInternalBytes);
    }
  }

  size_t Size() const { return key_size_; }

  void Clear() { key_size_ = 0; }

  // Append "non_shared_data" to its back, from "shared_len"
  // This function is used in Block::Iter::ParseNextKey
  // shared_len: bytes in [0, shard_len-1] would be remained
  // non_shared_data: data to be append, its length must be >= non_shared_len
  void TrimAppend(const size_t shared_len, const char* non_shared_data,
                  const size_t non_shared_len) {
    assert(shared_len <= key_size_);
    size_t total_size = shared_len + non_shared_len;

    if (IsKeyPinned() /* key is not in buf_ */) {
      // Copy the key from external memory to buf_ (copy shared_len bytes)
      EnlargeBufferIfNeeded(total_size);
      memcpy(buf_, key_, shared_len);
    } else if (total_size > buf_size_) {
      // Need to allocate space, delete previous space
      char* p = new char[total_size];
      memcpy(p, key_, shared_len);

      if (buf_ != space_) {
        delete[] buf_;
      }

      buf_ = p;
      buf_size_ = total_size;
    }

    memcpy(buf_ + shared_len, non_shared_data, non_shared_len);
    key_ = buf_;
    key_size_ = total_size;
  }

  // A version of `TrimAppend` assuming the last bytes of length `ts_sz` in the
  // user key part of `key_` is not counted towards shared bytes. And the
  // decoded key needed a min timestamp of length `ts_sz` pad to the user key.
  void TrimAppendWithTimestamp(const size_t shared_len,
                               const char* non_shared_data,
                               const size_t non_shared_len,
                               const size_t ts_sz) {
    // This function is only used by the UDT in memtable feature, which only
    // support built in comparators with uint64 timestamps.
    assert(ts_sz == sizeof(uint64_t));
    size_t next_key_slice_index = 0;
    if (IsUserKey()) {
      key_slices_[next_key_slice_index++] = Slice(key_, shared_len);
      key_slices_[next_key_slice_index++] =
          Slice(non_shared_data, non_shared_len);
      key_slices_[next_key_slice_index++] = Slice(kTsMin, ts_sz);
    } else {
      assert(shared_len + non_shared_len >= kNumInternalBytes);
      // Invaraint: shared_user_key_len + shared_internal_bytes_len = shared_len
      // In naming below `*_len` variables, keyword `user_key` refers to the
      // user key part of the existing key in `key_` as apposed to the new key.
      // Similary, `internal_bytes` refers to the footer part of the existing
      // key. These bytes potentially will move between user key part and the
      // footer part in the new key.
      const size_t user_key_len = key_size_ - kNumInternalBytes;
      const size_t sharable_user_key_len = user_key_len - ts_sz;
      const size_t shared_user_key_len =
          std::min(shared_len, sharable_user_key_len);
      const size_t shared_internal_bytes_len = shared_len - shared_user_key_len;

      // One Slice among the three Slices will get split into two Slices, plus
      // a timestamp slice.
      bool ts_added = false;
      // Add slice parts and find the right location to add the min timestamp.
      MaybeAddKeyPartsWithTimestamp(
          key_, shared_user_key_len,
          shared_internal_bytes_len + non_shared_len < kNumInternalBytes,
          shared_len + non_shared_len - kNumInternalBytes, ts_sz,
          &next_key_slice_index, &ts_added);
      MaybeAddKeyPartsWithTimestamp(
          key_ + user_key_len, shared_internal_bytes_len,
          non_shared_len < kNumInternalBytes,
          shared_internal_bytes_len + non_shared_len - kNumInternalBytes, ts_sz,
          &next_key_slice_index, &ts_added);
      MaybeAddKeyPartsWithTimestamp(non_shared_data, non_shared_len,
                                    non_shared_len >= kNumInternalBytes,
                                    non_shared_len - kNumInternalBytes, ts_sz,
                                    &next_key_slice_index, &ts_added);
      assert(ts_added);
    }
    SetKeyImpl(next_key_slice_index,
               /* total_bytes= */ shared_len + non_shared_len + ts_sz);
  }

  Slice SetKeyWithPaddedMinTimestamp(const Slice& key, size_t ts_sz) {
    // This function is only used by the UDT in memtable feature, which only
    // support built in comparators with uint64 timestamps.
    assert(ts_sz == sizeof(uint64_t));
    size_t num_key_slices = 0;
    if (is_user_key_) {
      key_slices_[0] = key;
      key_slices_[1] = Slice(kTsMin, ts_sz);
      num_key_slices = 2;
    } else {
      assert(key.size() >= kNumInternalBytes);
      size_t user_key_size = key.size() - kNumInternalBytes;
      key_slices_[0] = Slice(key.data(), user_key_size);
      key_slices_[1] = Slice(kTsMin, ts_sz);
      key_slices_[2] = Slice(key.data() + user_key_size, kNumInternalBytes);
      num_key_slices = 3;
    }
    return SetKeyImpl(num_key_slices, key.size() + ts_sz);
  }

  Slice SetKey(const Slice& key, bool copy = true) {
    // is_user_key_ expected to be set already via SetIsUserKey
    return SetKeyImpl(key, copy);
  }

  // If user-defined timestamp is enabled, then `key` includes timestamp.
  // TODO(yanqin) this is also used to set prefix, which do not include
  // timestamp. Should be handled.
  Slice SetUserKey(const Slice& key, bool copy = true) {
    is_user_key_ = true;
    return SetKeyImpl(key, copy);
  }

  Slice SetInternalKey(const Slice& key, bool copy = true) {
    is_user_key_ = false;
    return SetKeyImpl(key, copy);
  }

  // Copies the content of key, updates the reference to the user key in ikey
  // and returns a Slice referencing the new copy.
  Slice SetInternalKey(const Slice& key, ParsedInternalKey* ikey) {
    size_t key_n = key.size();
    assert(key_n >= kNumInternalBytes);
    SetInternalKey(key);
    ikey->user_key = Slice(key_, key_n - kNumInternalBytes);
    return Slice(key_, key_n);
  }

  // Update the sequence number in the internal key.  Guarantees not to
  // invalidate slices to the key (and the user key).
  void UpdateInternalKey(uint64_t seq, ValueType t, const Slice* ts = nullptr) {
    assert(!IsKeyPinned());
    assert(key_size_ >= kNumInternalBytes);
    if (ts) {
      assert(key_size_ >= kNumInternalBytes + ts->size());
      memcpy(&buf_[key_size_ - kNumInternalBytes - ts->size()], ts->data(),
             ts->size());
    }
    uint64_t newval = (seq << 8) | t;
    if (key_ == buf_) {
      EncodeFixed64(&buf_[key_size_ - kNumInternalBytes], newval);
    } else {
      assert(key_ == secondary_buf_);
      EncodeFixed64(&secondary_buf_[key_size_ - kNumInternalBytes], newval);
    }
  }

  bool IsKeyPinned() const { return key_ != buf_ && key_ != secondary_buf_; }

  // If `ts` is provided, user_key should not contain timestamp,
  // and `ts` is appended after user_key.
  // TODO: more efficient storage for timestamp.
  void SetInternalKey(const Slice& key_prefix, const Slice& user_key,
                      SequenceNumber s,
                      ValueType value_type = kValueTypeForSeek,
                      const Slice* ts = nullptr) {
    size_t psize = key_prefix.size();
    size_t usize = user_key.size();
    size_t ts_sz = (ts != nullptr ? ts->size() : 0);
    EnlargeBufferIfNeeded(psize + usize + sizeof(uint64_t) + ts_sz);
    if (psize > 0) {
      memcpy(buf_, key_prefix.data(), psize);
    }
    memcpy(buf_ + psize, user_key.data(), usize);
    if (ts) {
      memcpy(buf_ + psize + usize, ts->data(), ts_sz);
    }
    EncodeFixed64(buf_ + usize + psize + ts_sz,
                  PackSequenceAndType(s, value_type));

    key_ = buf_;
    key_size_ = psize + usize + sizeof(uint64_t) + ts_sz;
    is_user_key_ = false;
  }

  void SetInternalKey(const Slice& user_key, SequenceNumber s,
                      ValueType value_type = kValueTypeForSeek,
                      const Slice* ts = nullptr) {
    SetInternalKey(Slice(), user_key, s, value_type, ts);
  }

  void Reserve(size_t size) {
    EnlargeBufferIfNeeded(size);
    key_size_ = size;
  }

  void SetInternalKey(const ParsedInternalKey& parsed_key) {
    SetInternalKey(Slice(), parsed_key);
  }

  void SetInternalKey(const Slice& key_prefix,
                      const ParsedInternalKey& parsed_key_suffix) {
    SetInternalKey(key_prefix, parsed_key_suffix.user_key,
                   parsed_key_suffix.sequence, parsed_key_suffix.type);
  }

  void EncodeLengthPrefixedKey(const Slice& key) {
    auto size = key.size();
    EnlargeBufferIfNeeded(size + static_cast<size_t>(VarintLength(size)));
    char* ptr = EncodeVarint32(buf_, static_cast<uint32_t>(size));
    memcpy(ptr, key.data(), size);
    key_ = buf_;
    is_user_key_ = true;
  }

  bool IsUserKey() const { return is_user_key_; }

  void Swap(IterKey& other) {
    // Record pointer relationships before any mutation.
    bool this_key_in_buf = (key_ == buf_);
    bool this_key_in_sec = (key_ == secondary_buf_);
    bool other_key_in_buf = (other.key_ == other.buf_);
    bool other_key_in_sec = (other.key_ == other.secondary_buf_);
    const char* orig_this_key = key_;
    const char* orig_other_key = other.key_;

    // --- Swap primary buffer ---
    bool this_buf_inline = (buf_ == space_);
    bool other_buf_inline = (other.buf_ == other.space_);
    if (this_buf_inline && other_buf_inline) {
      char temp[kInlineBufferSize];
      size_t max_sz =
          std::min(std::max(key_size_, other.key_size_), kInlineBufferSize);
      memcpy(temp, space_, max_sz);
      memcpy(space_, other.space_, max_sz);
      memcpy(other.space_, temp, max_sz);
    } else if (this_buf_inline) {
      memcpy(other.space_, space_, key_size_);
      buf_ = other.buf_;
      other.buf_ = other.space_;
    } else if (other_buf_inline) {
      memcpy(space_, other.space_, other.key_size_);
      other.buf_ = buf_;
      buf_ = space_;
    } else {
      std::swap(buf_, other.buf_);
    }

    // --- Swap secondary buffer ---
    bool this_sec_inline = (secondary_buf_ == space_for_secondary_buf_);
    bool other_sec_inline =
        (other.secondary_buf_ == other.space_for_secondary_buf_);
    if (this_sec_inline && other_sec_inline) {
      char temp[kInlineBufferSize];
      memcpy(temp, space_for_secondary_buf_, kInlineBufferSize);
      memcpy(space_for_secondary_buf_, other.space_for_secondary_buf_,
             kInlineBufferSize);
      memcpy(other.space_for_secondary_buf_, temp, kInlineBufferSize);
    } else if (this_sec_inline) {
      memcpy(other.space_for_secondary_buf_, space_for_secondary_buf_,
             secondary_buf_size_);
      secondary_buf_ = other.secondary_buf_;
      other.secondary_buf_ = other.space_for_secondary_buf_;
    } else if (other_sec_inline) {
      memcpy(space_for_secondary_buf_, other.space_for_secondary_buf_,
             other.secondary_buf_size_);
      other.secondary_buf_ = secondary_buf_;
      secondary_buf_ = space_for_secondary_buf_;
    } else {
      std::swap(secondary_buf_, other.secondary_buf_);
    }

    // --- Redirect key_ pointers ---
    if (other_key_in_buf) {
      key_ = buf_;
    } else if (other_key_in_sec) {
      key_ = secondary_buf_;
    } else {
      key_ = orig_other_key;
    }

    if (this_key_in_buf) {
      other.key_ = other.buf_;
    } else if (this_key_in_sec) {
      other.key_ = other.secondary_buf_;
    } else {
      other.key_ = orig_this_key;
    }

    std::swap(key_size_, other.key_size_);
    std::swap(buf_size_, other.buf_size_);
    std::swap(secondary_buf_size_, other.secondary_buf_size_);
    std::swap(is_user_key_, other.is_user_key_);
    std::swap(key_slices_, other.key_slices_);
  }

 private:
  char* buf_;
  const char* key_;
  size_t key_size_;
  size_t buf_size_;
  char space_[kInlineBufferSize];  // Avoid allocation for short keys
  bool is_user_key_;
  // Below variables are only used by user-defined timestamps in MemTable only
  // feature for iterating keys in an index block or a data block.
  //
  // We will alternate between buf_ and secondary_buf_ to hold the key. key_
  // will be modified in accordance to point to the right one. This is to avoid
  // an extra copy when we need to copy some shared bytes from previous key
  // (delta encoding), and we need to pad a min timestamp at the right location.
  char space_for_secondary_buf_[kInlineBufferSize];  // Avoid allocation for
                                                     // short keys
  char* secondary_buf_;
  size_t secondary_buf_size_;
  // Use to track the pieces that together make the whole key. We then copy
  // these pieces in order either into buf_ or secondary_buf_ depending on where
  // the previous key is held.
  std::array<Slice, 5> key_slices_;
  // End of variables used by user-defined timestamps in MemTable only feature.

  Slice SetKeyImpl(const Slice& key, bool copy) {
    size_t size = key.size();
    if (copy) {
      // Copy key to buf_
      EnlargeBufferIfNeeded(size);
      memcpy(buf_, key.data(), size);
      key_ = buf_;
    } else {
      // Update key_ to point to external memory
      key_ = key.data();
    }
    key_size_ = size;
    return Slice(key_, key_size_);
  }

  Slice SetKeyImpl(size_t num_key_slices, size_t total_bytes) {
    assert(num_key_slices <= 5);
    char* buf_start = nullptr;
    if (key_ == buf_) {
      // If the previous key is in buf_, we copy key_slices_ in order into
      // secondary_buf_.
      EnlargeSecondaryBufferIfNeeded(total_bytes);
      buf_start = secondary_buf_;
      key_ = secondary_buf_;
    } else {
      // Copy key_slices_ in order into buf_.
      EnlargeBufferIfNeeded(total_bytes);
      buf_start = buf_;
      key_ = buf_;
    }
#ifndef NDEBUG
    size_t actual_total_bytes = 0;
#endif  // NDEBUG
    for (size_t i = 0; i < num_key_slices; i++) {
      size_t key_slice_size = key_slices_[i].size();
      memcpy(buf_start, key_slices_[i].data(), key_slice_size);
      buf_start += key_slice_size;
#ifndef NDEBUG
      actual_total_bytes += key_slice_size;
#endif  // NDEBUG
    }
#ifndef NDEBUG
    assert(actual_total_bytes == total_bytes);
#endif  // NDEBUG
    key_size_ = total_bytes;
    return Slice(key_, key_size_);
  }

  void ResetBuffer() {
    if (key_ == buf_) {
      key_size_ = 0;
    }
    if (buf_ != space_) {
      delete[] buf_;
      buf_ = space_;
    }
    buf_size_ = kInlineBufferSize;
  }

  void ResetSecondaryBuffer() {
    if (key_ == secondary_buf_) {
      key_size_ = 0;
    }
    if (secondary_buf_ != space_for_secondary_buf_) {
      delete[] secondary_buf_;
      secondary_buf_ = space_for_secondary_buf_;
    }
    secondary_buf_size_ = kInlineBufferSize;
  }

  // Enlarge the buffer size if needed based on key_size.
  // By default, inline buffer is used. Once there is a key
  // larger than the inline buffer, another buffer is dynamically
  // allocated, until a larger key buffer is requested. In that case, we
  // reallocate buffer and delete the old one.
  void EnlargeBufferIfNeeded(size_t key_size) {
    // If size is smaller than buffer size, continue using current buffer,
    // or the static allocated one, as default
    if (key_size > buf_size_) {
      EnlargeBuffer(key_size);
    }
  }

  void EnlargeSecondaryBufferIfNeeded(size_t key_size);

  void EnlargeBuffer(size_t key_size);

  void MaybeAddKeyPartsWithTimestamp(const char* slice_data,
                                     const size_t slice_sz, bool add_timestamp,
                                     const size_t left_sz, const size_t ts_sz,
                                     size_t* next_key_slice_idx,
                                     bool* ts_added) {
    assert(next_key_slice_idx);
    if (add_timestamp && !*ts_added) {
      assert(slice_sz >= left_sz);
      key_slices_[(*next_key_slice_idx)++] = Slice(slice_data, left_sz);
      key_slices_[(*next_key_slice_idx)++] = Slice(kTsMin, ts_sz);
      key_slices_[(*next_key_slice_idx)++] =
          Slice(slice_data + left_sz, slice_sz - left_sz);
      *ts_added = true;
    } else {
      key_slices_[(*next_key_slice_idx)++] = Slice(slice_data, slice_sz);
    }
    assert(*next_key_slice_idx <= 5);
  }
};

// Convert from a SliceTransform of user keys, to a SliceTransform of
// internal keys.
class InternalKeySliceTransform : public SliceTransform {
 public:
  explicit InternalKeySliceTransform(const SliceTransform* transform)
      : transform_(transform) {}

  const char* Name() const override { return transform_->Name(); }

  Slice Transform(const Slice& src) const override {
    auto user_key = ExtractUserKey(src);
    return transform_->Transform(user_key);
  }

  bool InDomain(const Slice& src) const override {
    auto user_key = ExtractUserKey(src);
    return transform_->InDomain(user_key);
  }

  const SliceTransform* user_prefix_extractor() const { return transform_; }

 private:
  // Like comparator, InternalKeySliceTransform will not take care of the
  // deletion of transform_
  const SliceTransform* const transform_;
};

// Read the key of a record from a write batch.
// if this record represent the default column family then cf_record
// must be passed as false, otherwise it must be passed as true.
bool ReadKeyFromWriteBatchEntry(Slice* input, Slice* key, bool cf_record);

// Read record from a write batch piece from input.
// tag, column_family, key, value and blob are return values. Callers own the
// slice they point to.
// Tag is defined as ValueType.
// input will be advanced to after the record.
// If user-defined timestamp is enabled for a column family, then the `key`
// resulting from this call will include timestamp.
Status ReadRecordFromWriteBatch(Slice* input, char* tag,
                                uint32_t* column_family, Slice* key,
                                Slice* value, Slice* blob, Slice* xid,
                                uint64_t* write_unix_time);

// When user call DeleteRange() to delete a range of keys,
// we will store a serialized RangeTombstone in MemTable and SST.
// the struct here is an easy-understood form
// start/end_key_ is the start/end user key of the range to be deleted
struct RangeTombstone {
  Slice start_key_;
  Slice end_key_;
  SequenceNumber seq_;
  // TODO: we should optimize the storage here when user-defined timestamp
  //  is NOT enabled: they currently take up (16 + 32 + 32) bytes per tombstone.
  Slice ts_;
  std::string pinned_start_key_;
  std::string pinned_end_key_;

  RangeTombstone() = default;
  RangeTombstone(Slice sk, Slice ek, SequenceNumber sn)
      : start_key_(sk), end_key_(ek), seq_(sn) {}

  // User-defined timestamp is enabled, `sk` and `ek` should be user key
  // with timestamp, `ts` will replace the timestamps in `sk` and
  // `ek`.
  RangeTombstone(Slice sk, Slice ek, SequenceNumber sn, Slice ts) : seq_(sn) {
    const size_t ts_sz = ts.size();
    assert(ts_sz > 0);
    pinned_start_key_.reserve(sk.size());
    pinned_end_key_.reserve(ek.size());
    AppendUserKeyWithDifferentTimestamp(&pinned_start_key_, sk, ts);
    AppendUserKeyWithDifferentTimestamp(&pinned_end_key_, ek, ts);
    start_key_ = pinned_start_key_;
    end_key_ = pinned_end_key_;
    ts_ = Slice(pinned_start_key_.data() + sk.size() - ts_sz, ts_sz);
  }

  RangeTombstone(ParsedInternalKey parsed_key, Slice value) {
    start_key_ = parsed_key.user_key;
    seq_ = parsed_key.sequence;
    end_key_ = value;
  }

  // be careful to use Serialize(), allocates new memory
  std::pair<InternalKey, Slice> Serialize() const {
    auto key = InternalKey(start_key_, seq_, kTypeRangeDeletion);
    return std::make_pair(std::move(key), end_key_);
  }

  // be careful to use SerializeKey(), allocates new memory
  InternalKey SerializeKey() const {
    return InternalKey(start_key_, seq_, kTypeRangeDeletion);
  }

  // The tombstone end-key is exclusive, so we generate an internal-key here
  // which has a similar property. Using kMaxSequenceNumber guarantees that
  // the returned internal-key will compare less than any other internal-key
  // with the same user-key. This in turn guarantees that the serialized
  // end-key for a tombstone such as [a-b] will compare less than the key "b".
  //
  // be careful to use SerializeEndKey(), allocates new memory
  InternalKey SerializeEndKey() const {
    if (!ts_.empty()) {
      static constexpr char kTsMax[] = "\xff\xff\xff\xff\xff\xff\xff\xff\xff";
      if (ts_.size() <= strlen(kTsMax)) {
        return InternalKey(end_key_, kMaxSequenceNumber, kTypeRangeDeletion,
                           Slice(kTsMax, ts_.size()));
      } else {
        return InternalKey(end_key_, kMaxSequenceNumber, kTypeRangeDeletion,
                           std::string(ts_.size(), '\xff'));
      }
    }
    return InternalKey(end_key_, kMaxSequenceNumber, kTypeRangeDeletion);
  }
};

inline int InternalKeyComparator::Compare(const Slice& akey,
                                          const Slice& bkey) const {
  // Order by:
  //    increasing user key (according to user-supplied comparator)
  //    decreasing sequence number
  //    decreasing type (though sequence# should be enough to disambiguate)
  int r = user_comparator_.Compare(ExtractUserKey(akey), ExtractUserKey(bkey));
  if (r == 0) {
    const uint64_t anum =
        DecodeFixed64(akey.data() + akey.size() - kNumInternalBytes);
    const uint64_t bnum =
        DecodeFixed64(bkey.data() + bkey.size() - kNumInternalBytes);
    if (anum > bnum) {
      r = -1;
    } else if (anum < bnum) {
      r = +1;
    }
  }
  return r;
}

inline int InternalKeyComparator::CompareKeySeq(const Slice& akey,
                                                const Slice& bkey) const {
  // Order by:
  //    increasing user key (according to user-supplied comparator)
  //    decreasing sequence number
  int r = user_comparator_.Compare(ExtractUserKey(akey), ExtractUserKey(bkey));
  if (r == 0) {
    // Shift the number to exclude the last byte which contains the value type
    const uint64_t anum =
        DecodeFixed64(akey.data() + akey.size() - kNumInternalBytes) >> 8;
    const uint64_t bnum =
        DecodeFixed64(bkey.data() + bkey.size() - kNumInternalBytes) >> 8;
    if (anum > bnum) {
      r = -1;
    } else if (anum < bnum) {
      r = +1;
    }
  }
  return r;
}

inline int InternalKeyComparator::CompareKeySeq(const ParsedInternalKey& a,
                                                const Slice& b) const {
  // Order by:
  //    increasing user key (according to user-supplied comparator)
  //    decreasing sequence number
  int r = user_comparator_.Compare(a.user_key, ExtractUserKey(b));
  if (r == 0) {
    // Shift the number to exclude the last byte which contains the value type
    const uint64_t anum = a.sequence;
    const uint64_t bnum =
        DecodeFixed64(b.data() + b.size() - kNumInternalBytes) >> 8;
    if (anum > bnum) {
      r = -1;
    } else if (anum < bnum) {
      r = +1;
    }
  }
  return r;
}

inline int InternalKeyComparator::Compare(const Slice& a,
                                          SequenceNumber a_global_seqno,
                                          const Slice& b,
                                          SequenceNumber b_global_seqno) const {
  int r = user_comparator_.Compare(ExtractUserKey(a), ExtractUserKey(b));
  if (r == 0) {
    uint64_t a_footer, b_footer;
    if (a_global_seqno == kDisableGlobalSequenceNumber) {
      a_footer = ExtractInternalKeyFooter(a);
    } else {
      a_footer = PackSequenceAndType(a_global_seqno, ExtractValueType(a));
    }
    if (b_global_seqno == kDisableGlobalSequenceNumber) {
      b_footer = ExtractInternalKeyFooter(b);
    } else {
      b_footer = PackSequenceAndType(b_global_seqno, ExtractValueType(b));
    }
    if (a_footer > b_footer) {
      r = -1;
    } else if (a_footer < b_footer) {
      r = +1;
    }
  }
  return r;
}

// Wrap InternalKeyComparator as a comparator class for ParsedInternalKey.
struct ParsedInternalKeyComparator {
  explicit ParsedInternalKeyComparator(const InternalKeyComparator* c)
      : cmp(c) {}

  bool operator()(const ParsedInternalKey& a,
                  const ParsedInternalKey& b) const {
    return cmp->Compare(a, b) < 0;
  }

  const InternalKeyComparator* cmp;
};

class PredecessorWALInfo {
 public:
  PredecessorWALInfo()
      : log_number_(0),
        size_bytes_(0),
        last_seqno_recorded_(0),
        initialized_(false) {}

  explicit PredecessorWALInfo(uint64_t log_number, uint64_t size_bytes,
                              SequenceNumber last_seqno_recorded)
      : log_number_(log_number),
        size_bytes_(size_bytes),
        last_seqno_recorded_(last_seqno_recorded),
        initialized_(true) {}

  uint64_t GetLogNumber() const {
    assert(initialized_);
    return log_number_;
  }

  uint64_t GetSizeBytes() const {
    assert(initialized_);
    return size_bytes_;
  }

  SequenceNumber GetLastSeqnoRecorded() const {
    assert(initialized_);
    return last_seqno_recorded_;
  }

  bool IsInitialized() const { return initialized_; }

  inline void EncodeTo(std::string* dst) const {
    assert(dst != nullptr);
    assert(initialized_);
    PutFixed64(dst, log_number_);
    PutFixed64(dst, size_bytes_);
    PutFixed64(dst, last_seqno_recorded_);
  }

  inline Status DecodeFrom(Slice* src) {
    if (!GetFixed64(src, &log_number_)) {
      return Status::Corruption("Error decoding log number");
    }
    if (!GetFixed64(src, &size_bytes_)) {
      return Status::Corruption("Error decoding size bytes");
    }
    if (!GetFixed64(src, &last_seqno_recorded_)) {
      return Status::Corruption("Error decoding last seqno recorded");
    }
    initialized_ = true;
    return Status::OK();
  }

 private:
  uint64_t log_number_;
  uint64_t size_bytes_;
  SequenceNumber last_seqno_recorded_;
  bool initialized_;
};
}  // namespace ROCKSDB_NAMESPACE