construct/include/ircd/ctx/mutex.h

// Matrix Construct
//
// Copyright (C) Matrix Construct Developers, Authors & Contributors
// Copyright (C) 2016-2018 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.

#pragma once
#define HAVE_IRCD_CTX_MUTEX_H

namespace ircd::ctx
{
	class mutex;

	template<class queue> void release_sequence(queue &);
}

//
// The mutex only allows one context to lock it and continue,
// additional contexts are queued. This can be used with std::
// locking concepts.
//
class ircd::ctx::mutex
{
	bool m;
	list q;

  public:
	size_t waiting() const;

	bool try_lock();
	template<class time_point> bool try_lock_until(const time_point &);
	template<class duration> bool try_lock_for(const duration &);

	void lock();
	void unlock();

	mutex();
	mutex(mutex &&) noexcept;
	mutex(const mutex &) = delete;
	mutex &operator=(mutex &&) noexcept;
	mutex &operator=(const mutex &) = delete;
	~mutex() noexcept;
};

inline
ircd::ctx::mutex::mutex()
:m{false}
{
}

inline
ircd::ctx::mutex::mutex(mutex &&o)
noexcept
:m{std::move(o.m)}
{
	o.m = false;
}

inline
ircd::ctx::mutex &
ircd::ctx::mutex::operator=(mutex &&o)
noexcept
{
	this->~mutex();
	std::swap(m, o.m);
	return *this;
}

inline
ircd::ctx::mutex::~mutex()
noexcept
{
	assert(!m);
}

inline void
ircd::ctx::mutex::unlock()
{
	assert(m);
	m = false;
	release_sequence(q);
}

inline void
ircd::ctx::mutex::lock()
{
	if(likely(try_lock()))
		return;

	q.push_back(current);
	while(!try_lock())
		wait();
}

template<class duration>
bool
ircd::ctx::mutex::try_lock_for(const duration &d)
{
	return try_lock_until(steady_clock::now() + d);
}

template<class time_point>
bool
ircd::ctx::mutex::try_lock_until(const time_point &tp)
{
	if(likely(try_lock()))
		return true;

	q.push_back(current);
	while(!try_lock())
	{
		if(unlikely(wait_until<std::nothrow_t>(tp)))
		{
			q.remove(current);
			return false;
		}
	}

	return true;
}

inline bool
ircd::ctx::mutex::try_lock()
{
	if(m)
		return false;

	m = true;
	return true;
}

inline size_t
ircd::ctx::mutex::waiting()
const
{
	return q.size();
}

template<class queue>
void
ircd::ctx::release_sequence(queue &q)
{
	ctx *next; do
	{
		if(!q.empty())
		{
			next = q.front();
			q.pop_front();
		}
		else next = nullptr;
	}
	while(next == current);

	if(next)
		yield(*next);
}