unfair_mutex_intf.hpp

// Copyright Take Vos 2020-2022.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#pragma once
 
#include "../utility/utility.hpp"
#include "../macros.hpp"
#include <atomic>
#include <memory>
 
hi_export_module(hikogui.concurrency.unfair_mutex : intf);
 
hi_export namespace hi { inline namespace v1 {
 
template<bool UseDeadLockDetector>
class unfair_mutex_impl {
public:
    constexpr unfair_mutex_impl() noexcept {}
    unfair_mutex_impl(unfair_mutex_impl const&) = delete;
    unfair_mutex_impl(unfair_mutex_impl&&) = delete;
    unfair_mutex_impl& operator=(unfair_mutex_impl const&) = delete;
    unfair_mutex_impl& operator=(unfair_mutex_impl&&) = delete;
 
    ~unfair_mutex_impl();
 
    bool is_locked() const noexcept;
 
    void lock() noexcept;
 
    [[nodiscard]] bool try_lock() noexcept;
 
    void unlock() noexcept;
 
private:
    /*
     * semaphore value:
     *  0 - Unlocked, no other thread is waiting.
     *  1 - Locked, no other thread is waiting.
     *  2 - Locked, zero or more threads are waiting.
     */
    std::atomic_unsigned_lock_free semaphore = 0;
    using semaphore_value_type = typename decltype(semaphore)::value_type;
 
    bool holds_invariant() const noexcept;
 
    void lock_contended(semaphore_value_type expected) noexcept;
};
 
#ifndef NDEBUG
using unfair_mutex = unfair_mutex_impl<true>;
#else
using unfair_mutex = unfair_mutex_impl<false>;
#endif
 
}} // namespace hi::v1