mpmc_queue.h

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/*
  Copyright (c) 2018, 2025, Oracle and/or its affiliates.
 
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  it under the terms of the GNU General Public License, version 2.0,
  as published by the Free Software Foundation.
 
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  as designated in a particular file or component or in included license
  documentation.  The authors of MySQL hereby grant you an additional
  permission to link the program and your derivative works with the
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  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
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  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/
 
#ifndef MYSQL_HARNESS_MPMC_UNBOUND_QUEUE_INCLUDED
#define MYSQL_HARNESS_MPMC_UNBOUND_QUEUE_INCLUDED
 
#include <atomic>
#include <mutex>
 
#include "mysql/harness/waiting_queue_adaptor.h"
 
namespace mysql_harness {
 
/**
 * a unbounded multi-producer multi-consumer queue.
 *
 * multiple threads can read and write at the same time into the queue
 *
 * - enqueue and dequeue do not block each other
 *
 * see:
 *   - Micheal & Scott: two-lock concurrent queue
 *   - "An Unbounded Total Queue" in "The Art of MultiProcessor Programming"
 */
 
/*
 * Implementation works everywhere, where mutex are available. If atomics and
 * CAS/LL_SC are available faster implementations could be used:
 *
 * - https://github.com/cameron314/concurrentqueue
 *
 * Micheal & Scott's "lock-free" algorithm mentioned in the 1996 paper requires
 * GC and hazard pointers, but here is also:
 *
 * - https://idea.popcount.org/2012-09-11-concurrent-queue-in-c/
 */
template <typename T>
class MPMCQueueMS2Lock {
 public:
  using value_type = T;
 
  MPMCQueueMS2Lock() : head_{new Node}, tail_{head_} {}
 
  ~MPMCQueueMS2Lock() {
    T item;
 
    // free all items in the queue
    while (dequeue(item)) {
    }
 
    // free the head node
    delete head_;
  }
 
  /**
   * enqueue an element.
   *
   * @param item item to enqueue
   *
   * @returns if item was enqueued
   * @retval true item got assigned to queue
   */
  bool enqueue(const T &item) {
    Node *node = new Node;
    node->data = item;
 
    {
      std::unique_lock<std::mutex> lk(tail_mutex_);
 
      tail_->next = node;
      tail_ = node;
    }
 
    return true;
  }
 
  /**
   * enqueue an element.
   *
   * @param item item to enqueue
   *
   * @returns if item was enqueued
   * @retval true item got assigned to queue
   */
  bool enqueue(T &&item) {
    Node *node = new Node;
    node->data = std::move(item);
 
    {
      std::unique_lock<std::mutex> lk(tail_mutex_);
 
      tail_->next = node;
      tail_ = node;
    }
 
    return true;
  }
 
  /**
   * try to dequeue element.
   *
   * @param item location of dequeued item if dequeue() was successful
   *
   * @returns if item was written
   * @retval true first item removed from the queue and assigned to item
   * @retval false queue is empty
   */
  bool dequeue(T &item) {
    Node *node = nullptr;
    {
      std::unique_lock<std::mutex> lk(head_mutex_);
 
      node = head_;
      Node *new_head = node->next;
 
      if (new_head == nullptr) {
        return false;
      }
 
      item = std::move(new_head->data);
 
      head_ = new_head;
    }
 
    delete node;
 
    return true;
  }
 
 private:
  struct Node {
    T data;
    std::atomic<Node *> next{nullptr};
  };
 
  std::mutex head_mutex_;
  std::mutex tail_mutex_;
  Node *head_;
  Node *tail_;
};
 
// allow to switch implementation to a lock-free implementation later
template <typename T>
using MPMCQueue = MPMCQueueMS2Lock<T>;
 
template <typename T>
using WaitingMPMCQueue = WaitingQueueAdaptor<MPMCQueue<T>>;
 
}  // namespace mysql_harness
 
#endif