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 compactions    WriteBatch* 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 queue  if (!writers_.empty()) {    writers_.front()->cv.Signal();  }  return status;}