leveldb源码分析21

// S1 锁mutex，防止并发，如果指定option则尝试获取snapshot；然后增加MemTable的引用值。  
MutexLock l(&mutex_);
SequenceNumber snapshot;
if (options.snapshot != NULL)
snapshot = reinterpret_cast<const SnapshotImpl*>(options.snapshot)->number_;
else snapshot = versions_->LastSequence(); // 取当前版本的最后Sequence  
MemTable *mem = mem_, *imm = imm_;
Version* current = versions_->current();
mem->Ref();
if (imm != NULL) imm->Ref();
current->Ref();
// S2 从sstable文件和MemTable中读取时，释放锁mutex；之后再次锁mutex。  
bool have_stat_update = false;
Version::GetStats stats;
{
    mutex_.Unlock();
    // 先从memtable中查询，再从immutable memtable中查询  
    LookupKey lkey(key, snapshot);
    if (mem->Get(lkey, value, &s)) {
    }
    else if (imm != NULL && imm->Get(lkey, value, &s)) {
    }
    else { // 需要从sstable文件中查询  
        s = current->Get(options, lkey, value, &stats);
        have_stat_update = true; // 记录之，用于compaction  
    }
    mutex_.Lock();
}
// S3 如果是从sstable文件查询出来的，检查是否需要做compaction。最后把MemTable的引用计数减1。  
if (have_stat_update &¤t->UpdateStats(stats)) {
    MaybeScheduleCompaction();
}
mem->Unref();
if (imm != NULL)imm->Unref();
current->Unref();

Iterator* DBImpl::NewIterator(const ReadOptions& options) {
    SequenceNumber latest_snapshot;
    Iterator* internal_iter = NewInternalIterator(options, &latest_snapshot);
    returnNewDBIterator(&dbname_, env_, user_comparator(), internal_iter,
        (options.snapshot != NULL
            ? reinterpret_cast<constSnapshotImpl*>(options.snapshot)->number_
            : latest_snapshot));
}

Iterator* NewDBIterator(const std::string* dbname, Env* env,
    const Comparator*user_key_comparator, Iterator* internal_iter,
    const SequenceNumber& sequence) {
    return new DBIter(dbname, env, user_key_comparator, internal_iter, sequence);
}

Iterator* DBImpl::NewInternalIterator(const ReadOptions& options,
    SequenceNumber*latest_snapshot) {
    IterState* cleanup = newIterState;
    mutex_.Lock();
    // 根据last sequence设置lastest snapshot，并收集所有的子iterator  
    *latest_snapshot = versions_->LastSequence();
    std::vector<Iterator*>list;
    list.push_back(mem_->NewIterator()); // >memtable  
    mem_->Ref();
    if (imm_ != NULL) {
        list.push_back(imm_->NewIterator()); // >immutablememtable  
        imm_->Ref();
    }
    versions_->current()->AddIterators(options, &list); // >current的所有sstable  
    Iterator* internal_iter = NewMergingIterator(&internal_comparator_, &list[0], list.size());
    versions_->current()->Ref();
    // 注册清理机制  
    cleanup->mu = &mutex_;
    cleanup->mem = mem_;
    cleanup->imm = imm_;
    cleanup->version = versions_->current();
    internal_iter->RegisterCleanup(CleanupIteratorState, cleanup, NULL);
    mutex_.Unlock();
    return internal_iter;
}

static void CleanupIteratorState(void* arg1, void* arg2) {
    IterState* state = reinterpret_cast<IterState*>(arg1);
    state->mu->Lock();
    state->mem->Unref();
    if (state->imm != NULL)state->imm->Unref();
    state->version->Unref();
    state->mu->Unlock();
    delete state;
}

virtual Slice key() const {
    assert(Valid());
    return current_->key();
}

virtual Slice value() const {
    assert(Valid());
    return current_->value();
}

virtual Status status() const {
    Status status;
    for (int i = 0; i < n_; i++) { // 只有所有内部Iterator都ok时，才返回ok  
        status = children_[i].status();
        if (!status.ok()) break;
    }
    return status;
}

virtual void SeekToFirst() {
    for (int i = 0; i < n_; i++) children_[i].SeekToFirst();
    FindSmallest();
    direction_ = kForward;
}

virtual void SeekToLast() {
    for (int i = 0; i < n_; i++) children_[i].SeekToLast();
    FindLargest();
    direction_ = kReverse;
}

virtual void Seek(constSlice& target) {
    for (int i = 0; i < n_; i++) children_[i].Seek(target);
    FindSmallest();
    direction_ = kForward;
}

virtual void Next() {
    assert(Valid());
    // 确保所有的子Iterator都定位在key()之后.  
    // 如果我们在正向移动，对于除current_外的所有子Iterator这点已然成立  
    // 因为current_是最小的子Iterator，并且key() = current_->key()。  
    // 否则，我们需要明确设置其它的子Iterator  
    if (direction_ != kForward) {
        for (int i = 0; i < n_; i++) { // 把所有current之外的Iterator定位到key()之后  
            IteratorWrapper* child = &children_[i];
            if (child != current_) {
                child->Seek(key());
                if (child->Valid() && comparator_->Compare(key(), child->key()) == 0)
                    child->Next(); // key等于current_->key()的，再向后移动一位  
            }
        }
        direction_ = kForward;
    }
    // current也向后移一位，然后再查找key最小的Iterator  
    current_->Next();
    FindSmallest();
}

virtual void Prev() {
    assert(Valid());
    // 确保所有的子Iterator都定位在key()之前.  
    // 如果我们在逆向移动，对于除current_外的所有子Iterator这点已然成立  
    // 因为current_是最大的，并且key() = current_->key()  
    // 否则，我们需要明确设置其它的子Iterator  
    if (direction_ != kReverse) {
        for (int i = 0; i < n_; i++) {
            IteratorWrapper* child = &children_[i];
            if (child != current_) {
                child->Seek(key());
                if (child->Valid()) {
                    // child位于>=key()的第一个entry上，prev移动一位到<key()  
                    child->Prev();
                }
                else { // child所有的entry都 < key()，直接seek到last即可  
                    child->SeekToLast();
                }
            }
        }
        direction_ = kReverse;
    }
    //current也向前移一位，然后再查找key最大的Iterator  
    current_->Prev();
    FindLargest();
}