timer_queue.h

//  Portions Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
//
// Borrowed from
// http://www.crazygaze.com/blog/2016/03/24/portable-c-timer-queue/
// Timer Queue
//
// License
//
// The source code in this article is licensed under the CC0 license, so feel
// free to copy, modify, share, do whatever you want with it.
// No attribution is required, but Ill be happy if you do.
// CC0 license

// The person who associated a work with this deed has dedicated the work to the
// public domain by waiving all of his or her rights to the work worldwide
// under copyright law, including all related and neighboring rights, to the
// extent allowed by law.  You can copy, modify, distribute and perform the
// work, even for commercial purposes, all without asking permission.

#pragma once

#include "port/port.h"

#include <assert.h>
#include <chrono>
#include <condition_variable>
#include <functional>
#include <queue>
#include <thread>
#include <utility>
#include <vector>

// Allows execution of handlers at a specified time in the future
// Guarantees:
//  - All handlers are executed ONCE, even if cancelled (aborted parameter will
// be set to true)
//      - If TimerQueue is destroyed, it will cancel all handlers.
//  - Handlers are ALWAYS executed in the Timer Queue worker thread.
//  - Handlers execution order is NOT guaranteed
//
////////////////////////////////////////////////////////////////////////////////
// borrowed from
// http://www.crazygaze.com/blog/2016/03/24/portable-c-timer-queue/
class TimerQueue {
 public:
  TimerQueue() : m_th(&TimerQueue::run, this) {}

  ~TimerQueue() {
    cancelAll();
    // Abusing the timer queue to trigger the shutdown.
    add(0, [this](bool) {
      m_finish = true;
      return std::make_pair(false, 0);
    });
    m_th.join();
  }

  // Adds a new timer
  // \return
  //  Returns the ID of the new timer. You can use this ID to cancel the
  // timer
  uint64_t add(int64_t milliseconds,
               std::function<std::pair<bool, int64_t>(bool)> handler) {
    WorkItem item;
    Clock::time_point tp = Clock::now();
    item.end = tp + std::chrono::milliseconds(milliseconds);
    item.period = milliseconds;
    item.handler = std::move(handler);

    std::unique_lock<std::mutex> lk(m_mtx);
    uint64_t id = ++m_idcounter;
    item.id = id;
    m_items.push(std::move(item));

    // Something changed, so wake up timer thread
    m_checkWork.notify_one();
    return id;
  }

  // Cancels the specified timer
  // \return
  //  1 if the timer was cancelled.
  //  0 if you were too late to cancel (or the timer ID was never valid to
  // start with)
  size_t cancel(uint64_t id) {
    // Instead of removing the item from the container (thus breaking the
    // heap integrity), we set the item as having no handler, and put
    // that handler on a new item at the top for immediate execution
    // The timer thread will then ignore the original item, since it has no
    // handler.
    std::unique_lock<std::mutex> lk(m_mtx);
    for (auto&& item : m_items.getContainer()) {
      if (item.id == id && item.handler) {
        WorkItem newItem;
        // Zero time, so it stays at the top for immediate execution
        newItem.end = Clock::time_point();
        newItem.id = 0;  // Means it is a canceled item
        // Move the handler from item to newitem (thus clearing item)
        newItem.handler = std::move(item.handler);
        m_items.push(std::move(newItem));

        // Something changed, so wake up timer thread
        m_checkWork.notify_one();
        return 1;
      }
    }
    return 0;
  }

  // Cancels all timers
  // \return
  //  The number of timers cancelled
  size_t cancelAll() {
    // Setting all "end" to 0 (for immediate execution) is ok,
    // since it maintains the heap integrity
    std::unique_lock<std::mutex> lk(m_mtx);
    m_cancel = true;
    for (auto&& item : m_items.getContainer()) {
      if (item.id && item.handler) {
        item.end = Clock::time_point();
        item.id = 0;
      }
    }
    auto ret = m_items.size();

    m_checkWork.notify_one();
    return ret;
  }

 private:
  using Clock = std::chrono::steady_clock;
  TimerQueue(const TimerQueue&) = delete;
  TimerQueue& operator=(const TimerQueue&) = delete;

  void run() {
    std::unique_lock<std::mutex> lk(m_mtx);
    while (!m_finish) {
      auto end = calcWaitTime_lock();
      if (end.first) {
        // Timers found, so wait until it expires (or something else
        // changes)
        m_checkWork.wait_until(lk, end.second);
      } else {
        // No timers exist, so wait forever until something changes
        m_checkWork.wait(lk);
      }

      // Check and execute as much work as possible, such as, all expired
      // timers
      checkWork(&lk);
    }

    // If we are shutting down, we should not have any items left,
    // since the shutdown cancels all items
    assert(m_items.size() == 0);
  }

  std::pair<bool, Clock::time_point> calcWaitTime_lock() {
    while (m_items.size()) {
      if (m_items.top().handler) {
        // Item present, so return the new wait time
        return std::make_pair(true, m_items.top().end);
      } else {
        // Discard empty handlers (they were cancelled)
        m_items.pop();
      }
    }

    // No items found, so return no wait time (causes the thread to wait
    // indefinitely)
    return std::make_pair(false, Clock::time_point());
  }

  void checkWork(std::unique_lock<std::mutex>* lk) {
    while (m_items.size() && m_items.top().end <= Clock::now()) {
      WorkItem item(m_items.top());
      m_items.pop();

      if (item.handler) {
        (*lk).unlock();
        auto reschedule_pair = item.handler(item.id == 0);
        (*lk).lock();
        if (!m_cancel && reschedule_pair.first) {
          int64_t new_period = (reschedule_pair.second == -1)
                                   ? item.period
                                   : reschedule_pair.second;

          item.period = new_period;
          item.end = Clock::now() + std::chrono::milliseconds(new_period);
          m_items.push(std::move(item));
        }
      }
    }
  }

  bool m_finish = false;
  bool m_cancel = false;
  uint64_t m_idcounter = 0;
  std::condition_variable m_checkWork;

  struct WorkItem {
    Clock::time_point end;
    int64_t period;
    uint64_t id;  // id==0 means it was cancelled
    std::function<std::pair<bool, int64_t>(bool)> handler;
    bool operator>(const WorkItem& other) const { return end > other.end; }
  };

  std::mutex m_mtx;
  // Inheriting from priority_queue, so we can access the internal container
  class Queue : public std::priority_queue<WorkItem, std::vector<WorkItem>,
                                           std::greater<WorkItem>> {
   public:
    std::vector<WorkItem>& getContainer() { return this->c; }
  } m_items;
  rocksdb::port::Thread m_th;
};