Library is not thread-safe: deadlock can happen

[sergey-shambir]

Here library calls foreign callbacks under mutex lock:

template<class TSlot, typename... TCallArgs>
    void call(TCallArgs&&... args)
    {
      lock_guard lock(*this);
      for (const auto& slot : slots_)
      {
        if (!slot.call)
          continue;

        TSlot(slot.key).call(std::forward<TCallArgs>(args)...);
      }
    }

    template<class TInit, class TSlot, typename... TCallArgs>
    TInit accumulate(TInit&& init, TCallArgs&&... args)
    {
      static_assert(std::is_same<typename TSlot::result_type, void>::value == false, "Cannot accumulate slot return values of type 'void'");

      lock_guard lock(*this);
      for (const auto& slot : slots_)
      {
        if (!slot.call)
          continue;

        init = init + TSlot(slot.key).call(std::forward<TCallArgs>(args)...);
      }

      return init;
}

Imagine that one thread connects to "signal1" own function "slot1" which fires "signal2". First, it locks "signal1" mutex, then attempts to lock "signal2" mutex.

Another thread connects to "signal2" own function "slot2" which fires "signal1". First, it locks "signal2" mutex, then attempts to lock "signal1" mutex.

In some cases, threads will enter into deadlock: thread1 always waits "signal2" mutex, while thread2 always waits "signal1" mutex.