std::shared_ptr AsyncTableResultScanner::Next()

in src/hbase/client/async-table-result-scanner.cc [46:82]

• 29 lines of code
• 7 McCabe index (conditional complexity)

std::shared_ptr<Result> AsyncTableResultScanner::Next() {
  VLOG(5) << "AsyncTableResultScanner: Next()";

  std::shared_ptr<Result> result = nullptr;
  std::shared_ptr<ScanResumer> local_resumer = nullptr;
  {
    std::unique_lock<std::mutex> mlock(mutex_);
    while (queue_.empty()) {
      if (closed_) {
        return nullptr;
      }
      if (error_) {
        throw error_;
      }
      cond_.wait(mlock);
    }
    result = queue_.front();
    queue_.pop();

    cache_size_ -= EstimatedSizeWithSharedPtr(result);
    if (resumer_ != nullptr && cache_size_ <= max_cache_size_ / 2) {
      VLOG(1) << std::this_thread::get_id() << " resume scan prefetching";
      local_resumer = resumer_;
      resumer_ = nullptr;
    }
  }

  // Need to call ScanResumer::Resume() outside of the scope of the mutex. The reason is that
  // folly/wangle event loop might end up running the attached logic(.then()) at the Scan RPC
  // in the same event thread before returning from the previous call. This seems like the
  // wrong thing to do(™), but we cannot fix that now. Since the call back can end up calling
  // this::OnNext(), we should unlock the mutex.
  if (local_resumer != nullptr) {
    local_resumer->Resume();
  }
  return result;
}