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snapshot.h
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snapshot.h
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// 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.
#ifndef STORAGE_LEVELDB_DB_SNAPSHOT_H_
#define STORAGE_LEVELDB_DB_SNAPSHOT_H_
#include "db/dbformat.h"
#include "leveldb/db.h"
namespace leveldb {
class SnapshotList;
// Snapshots are kept in a doubly-linked list in the DB.
// Each SnapshotImpl corresponds to a particular sequence number.
class SnapshotImpl : public Snapshot {
public:
SnapshotImpl(SequenceNumber sequence_number)
: sequence_number_(sequence_number) {}
SequenceNumber sequence_number() const { return sequence_number_; }
private:
friend class SnapshotList;
// SnapshotImpl is kept in a doubly-linked circular list. The SnapshotList
// implementation operates on the next/previous fields direcly.
SnapshotImpl* prev_;
SnapshotImpl* next_;
const SequenceNumber sequence_number_;
#if !defined(NDEBUG)
SnapshotList* list_ = nullptr;
#endif // !defined(NDEBUG)
};
class SnapshotList {
public:
SnapshotList() : head_(0) {
head_.prev_ = &head_;
head_.next_ = &head_;
}
bool empty() const { return head_.next_ == &head_; }
SnapshotImpl* oldest() const {
assert(!empty());
return head_.next_;
}
SnapshotImpl* newest() const {
assert(!empty());
return head_.prev_;
}
// Creates a SnapshotImpl and appends it to the end of the list.
SnapshotImpl* New(SequenceNumber sequence_number) {
assert(empty() || newest()->sequence_number_ <= sequence_number);
SnapshotImpl* snapshot = new SnapshotImpl(sequence_number);
#if !defined(NDEBUG)
snapshot->list_ = this;
#endif // !defined(NDEBUG)
snapshot->next_ = &head_;
snapshot->prev_ = head_.prev_;
snapshot->prev_->next_ = snapshot;
snapshot->next_->prev_ = snapshot;
return snapshot;
}
// Removes a SnapshotImpl from this list.
//
// The snapshot must have been created by calling New() on this list.
//
// The snapshot pointer should not be const, because its memory is
// deallocated. However, that would force us to change DB::ReleaseSnapshot(),
// which is in the API, and currently takes a const Snapshot.
void Delete(const SnapshotImpl* snapshot) {
#if !defined(NDEBUG)
assert(snapshot->list_ == this);
#endif // !defined(NDEBUG)
snapshot->prev_->next_ = snapshot->next_;
snapshot->next_->prev_ = snapshot->prev_;
delete snapshot;
}
private:
// Dummy head of doubly-linked list of snapshots
SnapshotImpl head_;
};
} // namespace leveldb
#endif // STORAGE_LEVELDB_DB_SNAPSHOT_H_