sub_task_queue.h

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// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the Apache 2.0 License.
#pragma once
 
#include <atomic>
#include <deque>
#include <mutex>
 
namespace ccf::tasks
{
  // Helper type for OrderedTasks, containing a list of sub-tasks to be
  // performed in-order. Modifiers return bools indicating whether the caller
  // is responsible for scheduling a future flush of this queue.
  template <typename T>
  class SubTaskQueue
  {
  protected:
    std::mutex pending_mutex;
    std::deque<T> pending;
    std::atomic<bool> active;
    std::atomic<bool> paused;
 
  public:
    bool push(T&& t)
    {
      std::lock_guard<std::mutex> lock(pending_mutex);
      const bool ret = pending.empty() && !active.load();
      pending.emplace_back(std::forward<T>(t));
      return ret;
    }
 
    using Visitor = std::function<void(T&&)>;
    bool pop_and_visit(Visitor&& visitor)
    {
      decltype(pending) local;
      {
        std::lock_guard<std::mutex> lock(pending_mutex);
        active.store(true);
 
        std::swap(local, pending);
      }
 
      auto it = local.begin();
      while (!paused.load() && it != local.end())
      {
        visitor(std::forward<T>(*it));
        ++it;
      }
 
      {
        std::lock_guard<std::mutex> lock(pending_mutex);
        if (it != local.end())
        {
          // Paused mid-execution - some actions remain that need to be
          // spliced back onto the front of the pending pending
          pending.insert(pending.begin(), it, local.end());
        }
 
        active.store(false);
        return !pending.empty() && !paused.load();
      }
    }
 
    void pause()
    {
      std::lock_guard<std::mutex> lock(pending_mutex);
      paused.store(true);
    }
 
    bool unpause()
    {
      std::lock_guard<std::mutex> lock(pending_mutex);
      paused.store(false);
      return !pending.empty() && !active.load();
    }
 
    void get_queue_summary(size_t& num_pending, bool& is_active)
    {
      std::lock_guard<std::mutex> lock(pending_mutex);
      num_pending = pending.size();
      is_active = active.load();
    }
  };
}