single_waiting_task_mutex.hpp

#pragma once
 
/// @file userver/engine/single_waiter_mutex.hpp
/// @brief @copybrief engine::SingleWaitingTaskMutex
 
#include <atomic>
#include <chrono>
#include <mutex>  // for std locks
 
#include <userver/engine/deadline.hpp>
#include <userver/engine/impl/wait_list_fwd.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace engine {
 
/// @ingroup userver_concurrency
///
/// @brief Lighter version of Mutex with not more than 1 waiting task.
///
/// There are some situations when a resource is accessed
/// concurrently, but concurrency factor is limited by 2.
/// For instance: implications of socket r/w duality.
///
/// Ignores task cancellations (succeeds even if the current task is cancelled).
///
/// ## Example usage:
///
/// The class's API is the same as of engine::Mutex:
///
/// @snippet engine/mutex_test.cpp  Sample engine::Mutex usage
///
/// @see @ref scripts/docs/en/userver/synchronization.md
class SingleWaitingTaskMutex final {
 public:
  SingleWaitingTaskMutex();
  ~SingleWaitingTaskMutex();
 
  SingleWaitingTaskMutex(const SingleWaitingTaskMutex&) = delete;
  SingleWaitingTaskMutex(SingleWaitingTaskMutex&&) = delete;
  SingleWaitingTaskMutex& operator=(const SingleWaitingTaskMutex&) = delete;
  SingleWaitingTaskMutex& operator=(SingleWaitingTaskMutex&&) = delete;
 
  /// Locks the mutex. Blocks current coroutine if the mutex is locked by
  /// another coroutine.
  /// @note the behaviour is undefined if a coroutine tries to lock a mutex
  /// which is already locked by the current coroutine.
  /// @note the method waits for the mutex even if the current task is
  /// cancelled.
  void lock();
 
  /// Unlocks the mutex. Before calling this method the mutex should be locked
  /// by the current coroutine.
  void unlock();
 
  bool try_lock();
 
  template <typename Rep, typename Period>
  bool try_lock_for(const std::chrono::duration<Rep, Period>&);
 
  template <typename Clock, typename Duration>
  bool try_lock_until(const std::chrono::time_point<Clock, Duration>&);
 
  bool try_lock_until(Deadline deadline);
 
 private:
  class Impl;
 
  utils::FastPimpl<Impl, 32, 16> impl_;
};
 
template <typename Rep, typename Period>
bool SingleWaitingTaskMutex::try_lock_for(
    const std::chrono::duration<Rep, Period>& duration) {
  return try_lock_until(Deadline::FromDuration(duration));
}
 
template <typename Clock, typename Duration>
bool SingleWaitingTaskMutex::try_lock_until(
    const std::chrono::time_point<Clock, Duration>& until) {
  return try_lock_until(Deadline::FromTimePoint(until));
}
 
}  // namespace engine
 
USERVER_NAMESPACE_END