BackgroundQueue.h

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// -*- mode: c++; c-file-style: "linux"; c-basic-offset: 2; indent-tabs-mode: nil -*-
//
//  Copyright (C) 2024 the wxMaxima team
//
//  This program is free software; you can redistribute it and/or modify
//  it under the terms of the GNU General Public License as published by
//  the Free Software Foundation; either version 2 of the License, or
//  (at your option) any later version.
//
//  This program is distributed in the hope that it will be useful,
//  but WITHOUT ANY WARRANTY; without even the implied warranty of
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//  GNU General Public License for more details.
//
//
//  You should have received a copy of the GNU General Public License
//  along with this program; if not, write to the Free Software
//  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
//
//  SPDX-License-Identifier: GPL-2.0+
 
#ifndef BACKGROUNDQUEUE_H
#define BACKGROUNDQUEUE_H
 
#include "Version.h" // stop_token / stop_source (real ones or shims)
#include <condition_variable>
#include <deque>
#include <functional>
#include <memory>
#include <mutex>
#include <thread>
#include <utility>
#include <vector>
 
class BackgroundTask
{
public:
  enum class Priority
  {
    High, 
    Low   
  };
 
  void RequestStop();
 
  void Wait();
 
  bool Pending() const;
 
  ~BackgroundTask() = default;
 
private:
  friend class BackgroundQueue;
  explicit BackgroundTask(std::function<void(stop_token)> fn)
    : m_fn(std::move(fn)) {}
 
  void Run();
 
  enum class State { Queued, Running, Done };
 
  std::function<void(stop_token)> m_fn;
  stop_source m_stopSource;
  mutable std::mutex m_mutex;
  std::condition_variable m_doneCond;
  State m_state = State::Queued;
};
 
class BackgroundQueue
{
public:
  static std::shared_ptr<BackgroundTask>
    Add(BackgroundTask::Priority priority, std::function<void(stop_token)> task);
 
  static int Workers();
 
  BackgroundQueue(const BackgroundQueue &) = delete;
  BackgroundQueue &operator=(const BackgroundQueue &) = delete;
 
private:
  BackgroundQueue();
  ~BackgroundQueue();
 
  static BackgroundQueue &Get();
 
  void Enqueue(const std::shared_ptr<BackgroundTask> &task,
               BackgroundTask::Priority priority);
  void Drop(BackgroundTask *task);
  void WorkerLoop();
 
  friend class BackgroundTask;
 
  std::mutex m_mutex;
  std::condition_variable m_cond;
  std::deque<std::shared_ptr<BackgroundTask>> m_high;
  std::deque<std::shared_ptr<BackgroundTask>> m_low;
  std::vector<std::thread> m_workers;
  bool m_shutdown = false;
};
 
#endif // BACKGROUNDQUEUE_H