Inside Bitcoin

1// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file. See the AUTHORS file for names of contributors.
4
5#ifndef STORAGE_LEVELDB_DB_SNAPSHOT_H_
6#define STORAGE_LEVELDB_DB_SNAPSHOT_H_
7
8#include "db/dbformat.h"
9#include "leveldb/db.h"
10
11namespace leveldb {
12
13class SnapshotList;
14
15// Snapshots are kept in a doubly-linked list in the DB.
16// Each SnapshotImpl corresponds to a particular sequence number.
17class SnapshotImpl : public Snapshot {
18 public:
19  SnapshotImpl(SequenceNumber sequence_number)
20      : sequence_number_(sequence_number) {}
21
22  SequenceNumber sequence_number() const { return sequence_number_; }
23
24 private:
25  friend class SnapshotList;
26
27  // SnapshotImpl is kept in a doubly-linked circular list. The SnapshotList
28  // implementation operates on the next/previous fields directly.
29  SnapshotImpl* prev_;
30  SnapshotImpl* next_;
31
32  const SequenceNumber sequence_number_;
33
34#if !defined(NDEBUG)
35  SnapshotList* list_ = nullptr;
36#endif  // !defined(NDEBUG)
37};
38
39class SnapshotList {
40 public:
41  SnapshotList() : head_(0) {
42    head_.prev_ = &head_;
43    head_.next_ = &head_;
44  }
45
46  bool empty() const { return head_.next_ == &head_; }
47  SnapshotImpl* oldest() const {
48    assert(!empty());
49    return head_.next_;
50  }
51  SnapshotImpl* newest() const {
52    assert(!empty());
53    return head_.prev_;
54  }
55
56  // Creates a SnapshotImpl and appends it to the end of the list.
57  SnapshotImpl* New(SequenceNumber sequence_number) {
58    assert(empty() || newest()->sequence_number_ <= sequence_number);
59
60    SnapshotImpl* snapshot = new SnapshotImpl(sequence_number);
61
62#if !defined(NDEBUG)
63    snapshot->list_ = this;
64#endif  // !defined(NDEBUG)
65    snapshot->next_ = &head_;
66    snapshot->prev_ = head_.prev_;
67    snapshot->prev_->next_ = snapshot;
68    snapshot->next_->prev_ = snapshot;
69    return snapshot;
70  }
71
72  // Removes a SnapshotImpl from this list.
73  //
74  // The snapshot must have been created by calling New() on this list.
75  //
76  // The snapshot pointer should not be const, because its memory is
77  // deallocated. However, that would force us to change DB::ReleaseSnapshot(),
78  // which is in the API, and currently takes a const Snapshot.
79  void Delete(const SnapshotImpl* snapshot) {
80#if !defined(NDEBUG)
81    assert(snapshot->list_ == this);
82#endif  // !defined(NDEBUG)
83    snapshot->prev_->next_ = snapshot->next_;
84    snapshot->next_->prev_ = snapshot->prev_;
85    delete snapshot;
86  }
87
88 private:
89  // Dummy head of doubly-linked list of snapshots
90  SnapshotImpl head_;
91};
92
93}  // namespace leveldb
94
95#endif  // STORAGE_LEVELDB_DB_SNAPSHOT_H_
96