path: root/third_party/nix/src/libutil/sync.hh

#pragma once

#include <cassert>
#include <condition_variable>
#include <cstdlib>
#include <mutex>

namespace nix {

/* This template class ensures synchronized access to a value of type
   T. It is used as follows:

     struct Data { int x; ... };

     Sync<Data> data;

     {
       auto data_(data.lock());
       data_->x = 123;
     }

   Here, "data" is automatically unlocked when "data_" goes out of
   scope.
*/

template <class T, class M = std::mutex>
class Sync {
 private:
  M mutex;
  T data;

 public:
  Sync() {}
  Sync(const T& data) : data(data) {}
  Sync(T&& data) noexcept : data(std::move(data)) {}

  class Lock {
   private:
    Sync* s;
    std::unique_lock<M> lk;
    friend Sync;
    Lock(Sync* s) : s(s), lk(s->mutex) {}

   public:
    Lock(Lock&& l) : s(l.s) { abort(); }
    Lock(const Lock& l) = delete;
    ~Lock() {}
    T* operator->() { return &s->data; }
    T& operator*() { return s->data; }

    void wait(std::condition_variable& cv) {
      assert(s);
      cv.wait(lk);
    }

    template <class Rep, class Period>
    std::cv_status wait_for(
        std::condition_variable& cv,
        const std::chrono::duration<Rep, Period>& duration) {
      assert(s);
      return cv.wait_for(lk, duration);
    }

    template <class Rep, class Period, class Predicate>
    bool wait_for(std::condition_variable& cv,
                  const std::chrono::duration<Rep, Period>& duration,
                  Predicate pred) {
      assert(s);
      return cv.wait_for(lk, duration, pred);
    }

    template <class Clock, class Duration>
    std::cv_status wait_until(
        std::condition_variable& cv,
        const std::chrono::time_point<Clock, Duration>& duration) {
      assert(s);
      return cv.wait_until(lk, duration);
    }
  };

  Lock lock() { return Lock(this); }
};

}  // namespace nix