Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {// A begin Writer w(&mutex_);w.batch = my_batch;w.sync = options.sync;w.done = false;// A end// B begin MutexLock l(&mutex_);writers_.push_back(&w);while (!w.done && &w != writers_.front()) {w.cv.Wait();}if (w.done) {return w.status;}// B end// May temporarily unlock and wait.Status status = MakeRoomForWrite(my_batch == NULL);uint64_t last_sequence = versions_->LastSequence();Writer* last_writer = &w;if (status.ok() && my_batch != NULL) {  // NULL batch is for compactionsWriteBatch* updates = BuildBatchGroup(&last_writer);WriteBatchInternal::SetSequence(updates, last_sequence + 1);last_sequence += WriteBatchInternal::Count(updates);// Add to log and apply to memtable.  We can release the lock// during this phase since &w is currently responsible for logging// and protects against concurrent loggers and concurrent writes// into mem_.{mutex_.Unlock();status = log_->AddRecord(WriteBatchInternal::Contents(updates));bool sync_error = false;if (status.ok() && options.sync) {status = logfile_->Sync();if (!status.ok()) {sync_error = true;}}if (status.ok()) {status = WriteBatchInternal::InsertInto(updates, mem_);}mutex_.Lock();if (sync_error) {// The state of the log file is indeterminate: the log record we// just added may or may not show up when the DB is re-opened.// So we force the DB into a mode where all future writes fail.RecordBackgroundError(status);}}if (updates == tmp_batch_) tmp_batch_->Clear();versions_->SetLastSequence(last_sequence);}while (true) {Writer* ready = writers_.front();writers_.pop_front();if (ready != &w) {ready->status = status;ready->done = true;ready->cv.Signal();}if (ready == last_writer) break;}// Notify new head of write queueif (!writers_.empty()) {writers_.front()->cv.Signal();}return status;
}