rocksdb/db/range_tombstone_fragmenter.h

//  Copyright (c) 2018-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).

#pragma once

#include <list>
#include <memory>
#include <set>
#include <string>
#include <vector>

#include "db/dbformat.h"
#include "db/pinned_iterators_manager.h"
#include "rocksdb/status.h"
#include "table/internal_iterator.h"

namespace rocksdb {

struct FragmentedRangeTombstoneList {
 public:
  // A compact representation of a "stack" of range tombstone fragments, which
  // start and end at the same user keys but have different sequence numbers.
  // The members seq_start_idx and seq_end_idx are intended to be parameters to
  // seq_iter().
  struct RangeTombstoneStack {
    RangeTombstoneStack(const Slice& start, const Slice& end, size_t start_idx,
                        size_t end_idx)
        : start_key(start),
          end_key(end),
          seq_start_idx(start_idx),
          seq_end_idx(end_idx) {}

    Slice start_key;
    Slice end_key;
    size_t seq_start_idx;
    size_t seq_end_idx;
  };
  FragmentedRangeTombstoneList(
      std::unique_ptr<InternalIterator> unfragmented_tombstones,
      const InternalKeyComparator& icmp, bool for_compaction = false,
      const std::vector<SequenceNumber>& snapshots = {});

  std::vector<RangeTombstoneStack>::const_iterator begin() const {
    return tombstones_.begin();
  }

  std::vector<RangeTombstoneStack>::const_iterator end() const {
    return tombstones_.end();
  }

  std::vector<SequenceNumber>::const_iterator seq_iter(size_t idx) const {
    return std::next(tombstone_seqs_.begin(), idx);
  }

  std::vector<SequenceNumber>::const_iterator seq_begin() const {
    return tombstone_seqs_.begin();
  }

  std::vector<SequenceNumber>::const_iterator seq_end() const {
    return tombstone_seqs_.end();
  }

  bool empty() const { return tombstones_.empty(); }

  // Returns true if the stored tombstones contain with one with a sequence
  // number in [lower, upper].
  bool ContainsRange(SequenceNumber lower, SequenceNumber upper) const;

 private:
  // Given an ordered range tombstone iterator unfragmented_tombstones,
  // "fragment" the tombstones into non-overlapping pieces, and store them in
  // tombstones_ and tombstone_seqs_.
  void FragmentTombstones(
      std::unique_ptr<InternalIterator> unfragmented_tombstones,
      const InternalKeyComparator& icmp, bool for_compaction,
      const std::vector<SequenceNumber>& snapshots);

  std::vector<RangeTombstoneStack> tombstones_;
  std::vector<SequenceNumber> tombstone_seqs_;
  std::set<SequenceNumber> seq_set_;
  std::list<std::string> pinned_slices_;
  PinnedIteratorsManager pinned_iters_mgr_;
};

// FragmentedRangeTombstoneIterator converts an InternalIterator of a range-del
// meta block into an iterator over non-overlapping tombstone fragments. The
// tombstone fragmentation process should be more efficient than the range
// tombstone collapsing algorithm in RangeDelAggregator because this leverages
// the internal key ordering already provided by the input iterator, if
// applicable (when the iterator is unsorted, a new sorted iterator is created
// before proceeding). If there are few overlaps, creating a
// FragmentedRangeTombstoneIterator should be O(n), while the RangeDelAggregator
// tombstone collapsing is always O(n log n).
class FragmentedRangeTombstoneIterator : public InternalIterator {
 public:
  FragmentedRangeTombstoneIterator(
      const FragmentedRangeTombstoneList* tombstones,
      const InternalKeyComparator& icmp, SequenceNumber upper_bound,
      SequenceNumber lower_bound = 0);
  FragmentedRangeTombstoneIterator(
      const std::shared_ptr<const FragmentedRangeTombstoneList>& tombstones,
      const InternalKeyComparator& icmp, SequenceNumber upper_bound,
      SequenceNumber lower_bound = 0);

  void SeekToFirst() override;
  void SeekToLast() override;

  void SeekToTopFirst();
  void SeekToTopLast();

  // NOTE: Seek and SeekForPrev do not behave in the way InternalIterator
  // seeking should behave. This is OK because they are not currently used, but
  // eventually FragmentedRangeTombstoneIterator should no longer implement
  // InternalIterator.
  //
  // Seeks to the range tombstone that covers target at a seqnum in the
  // snapshot. If no such tombstone exists, seek to the earliest tombstone in
  // the snapshot that ends after target.
  void Seek(const Slice& target) override;
  // Seeks to the range tombstone that covers target at a seqnum in the
  // snapshot. If no such tombstone exists, seek to the latest tombstone in the
  // snapshot that starts before target.
  void SeekForPrev(const Slice& target) override;

  void Next() override;
  void Prev() override;

  void TopNext();
  void TopPrev();

  bool Valid() const override;
  Slice key() const override {
    MaybePinKey();
    return current_start_key_.Encode();
  }
  Slice value() const override { return pos_->end_key; }
  bool IsKeyPinned() const override { return false; }
  bool IsValuePinned() const override { return true; }
  Status status() const override { return Status::OK(); }

  bool empty() const { return tombstones_->empty(); }
  void Invalidate() {
    pos_ = tombstones_->end();
    seq_pos_ = tombstones_->seq_end();
  }

  Slice start_key() const { return pos_->start_key; }
  Slice end_key() const { return pos_->end_key; }
  SequenceNumber seq() const { return *seq_pos_; }
  ParsedInternalKey parsed_start_key() const {
    return ParsedInternalKey(pos_->start_key, kMaxSequenceNumber,
                             kTypeRangeDeletion);
  }
  ParsedInternalKey parsed_end_key() const {
    return ParsedInternalKey(pos_->end_key, kMaxSequenceNumber,
                             kTypeRangeDeletion);
  }

  SequenceNumber MaxCoveringTombstoneSeqnum(const Slice& user_key);

  // Splits the iterator into n+1 iterators (where n is the number of
  // snapshots), each providing a view over a "stripe" of sequence numbers. The
  // iterators are keyed by the upper bound of their ranges (the provided
  // snapshots + kMaxSequenceNumber).
  //
  // NOTE: the iterators in the returned map are no longer valid if their
  // parent iterator is deleted, since they do not modify the refcount of the
  // underlying tombstone list. Therefore, this map should be deleted before
  // the parent iterator.
  std::map<SequenceNumber, std::unique_ptr<FragmentedRangeTombstoneIterator>>
  SplitBySnapshot(const std::vector<SequenceNumber>& snapshots);

  SequenceNumber upper_bound() const { return upper_bound_; }
  SequenceNumber lower_bound() const { return lower_bound_; }

 private:
  using RangeTombstoneStack = FragmentedRangeTombstoneList::RangeTombstoneStack;

  struct RangeTombstoneStackStartComparator {
    explicit RangeTombstoneStackStartComparator(const Comparator* c) : cmp(c) {}

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

    bool operator()(const RangeTombstoneStack& a, const Slice& b) const {
      return cmp->Compare(a.start_key, b) < 0;
    }

    bool operator()(const Slice& a, const RangeTombstoneStack& b) const {
      return cmp->Compare(a, b.start_key) < 0;
    }

    const Comparator* cmp;
  };

  struct RangeTombstoneStackEndComparator {
    explicit RangeTombstoneStackEndComparator(const Comparator* c) : cmp(c) {}

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

    bool operator()(const RangeTombstoneStack& a, const Slice& b) const {
      return cmp->Compare(a.end_key, b) < 0;
    }

    bool operator()(const Slice& a, const RangeTombstoneStack& b) const {
      return cmp->Compare(a, b.end_key) < 0;
    }

    const Comparator* cmp;
  };

  void MaybePinKey() const {
    if (pos_ != tombstones_->end() && seq_pos_ != tombstones_->seq_end() &&
        (pinned_pos_ != pos_ || pinned_seq_pos_ != seq_pos_)) {
      current_start_key_.Set(pos_->start_key, *seq_pos_, kTypeRangeDeletion);
      pinned_pos_ = pos_;
      pinned_seq_pos_ = seq_pos_;
    }
  }

  void SeekToCoveringTombstone(const Slice& key);
  void SeekForPrevToCoveringTombstone(const Slice& key);
  void ScanForwardToVisibleTombstone();
  void ScanBackwardToVisibleTombstone();
  bool ValidPos() const {
    return Valid() && seq_pos_ != tombstones_->seq_iter(pos_->seq_end_idx);
  }

  const RangeTombstoneStackStartComparator tombstone_start_cmp_;
  const RangeTombstoneStackEndComparator tombstone_end_cmp_;
  const InternalKeyComparator* icmp_;
  const Comparator* ucmp_;
  std::shared_ptr<const FragmentedRangeTombstoneList> tombstones_ref_;
  const FragmentedRangeTombstoneList* tombstones_;
  SequenceNumber upper_bound_;
  SequenceNumber lower_bound_;
  std::vector<RangeTombstoneStack>::const_iterator pos_;
  std::vector<SequenceNumber>::const_iterator seq_pos_;
  mutable std::vector<RangeTombstoneStack>::const_iterator pinned_pos_;
  mutable std::vector<SequenceNumber>::const_iterator pinned_seq_pos_;
  mutable InternalKey current_start_key_;
};

}  // namespace rocksdb