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89 行
2.6 KiB
89 行
2.6 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef STORAGE_LEVELDB_DB_SNAPSHOT_H_
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#define STORAGE_LEVELDB_DB_SNAPSHOT_H_
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#include "db/dbformat.h"
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#include "leveldb/db.h"
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namespace leveldb {
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class SnapshotList;
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// Snapshots are kept in a doubly-linked list in the DB.
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// Each SnapshotImpl corresponds to a particular sequence number.
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class SnapshotImpl : public Snapshot {
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public:
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SnapshotImpl(SequenceNumber sequence_number)
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: sequence_number_(sequence_number) {}
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SequenceNumber sequence_number() const { return sequence_number_; }
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private:
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friend class SnapshotList;
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// SnapshotImpl is kept in a doubly-linked circular list. The SnapshotList
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// implementation operates on the next/previous fields direcly.
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SnapshotImpl* prev_;
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SnapshotImpl* next_;
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const SequenceNumber sequence_number_;
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#if !defined(NDEBUG)
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SnapshotList* list_ = nullptr;
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#endif // !defined(NDEBUG)
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};
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class SnapshotList {
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public:
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SnapshotList() : head_(0) {
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head_.prev_ = &head_;
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head_.next_ = &head_;
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}
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bool empty() const { return head_.next_ == &head_; }
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SnapshotImpl* oldest() const { assert(!empty()); return head_.next_; }
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SnapshotImpl* newest() const { assert(!empty()); return head_.prev_; }
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// Creates a SnapshotImpl and appends it to the end of the list.
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SnapshotImpl* New(SequenceNumber sequence_number) {
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assert(empty() || newest()->sequence_number_ <= sequence_number);
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SnapshotImpl* snapshot = new SnapshotImpl(sequence_number);
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#if !defined(NDEBUG)
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snapshot->list_ = this;
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#endif // !defined(NDEBUG)
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snapshot->next_ = &head_;
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snapshot->prev_ = head_.prev_;
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snapshot->prev_->next_ = snapshot;
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snapshot->next_->prev_ = snapshot;
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return snapshot;
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}
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// Removes a SnapshotImpl from this list.
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//
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// The snapshot must have been created by calling New() on this list.
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//
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// The snapshot pointer should not be const, because its memory is
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// deallocated. However, that would force us to change DB::ReleaseSnapshot(),
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// which is in the API, and currently takes a const Snapshot.
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void Delete(const SnapshotImpl* snapshot) {
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#if !defined(NDEBUG)
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assert(snapshot->list_ == this);
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#endif // !defined(NDEBUG)
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snapshot->prev_->next_ = snapshot->next_;
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snapshot->next_->prev_ = snapshot->prev_;
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delete snapshot;
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}
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private:
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// Dummy head of doubly-linked list of snapshots
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SnapshotImpl head_;
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};
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_DB_SNAPSHOT_H_
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