lightweightsemaphore.h

// Provides an efficient implementation of a semaphore (LightweightSemaphore).
// This is an extension of Jeff Preshing's sempahore implementation (licensed 
// under the terms of its separate zlib license) that has been adapted and
// extended by Cameron Desrochers.

#pragma once

#include <cstddef> // For std::size_t
#include <atomic>
#include <type_traits> // For std::make_signed<T>

#if defined(_WIN32)
// Avoid including windows.h in a header; we only need a handful of
// items, so we'll redeclare them here (this is relatively safe since
// the API generally has to remain stable between Windows versions).
// I know this is an ugly hack but it still beats polluting the global
// namespace with thousands of generic names or adding a .cpp for nothing.
extern "C" {
	struct _SECURITY_ATTRIBUTES;
	__declspec(dllimport) void* __stdcall CreateSemaphoreW(_SECURITY_ATTRIBUTES* lpSemaphoreAttributes, long lInitialCount, long lMaximumCount, const wchar_t* lpName);
	__declspec(dllimport) int __stdcall CloseHandle(void* hObject);
	__declspec(dllimport) unsigned long __stdcall WaitForSingleObject(void* hHandle, unsigned long dwMilliseconds);
	__declspec(dllimport) int __stdcall ReleaseSemaphore(void* hSemaphore, long lReleaseCount, long* lpPreviousCount);
}
#elif defined(__MACH__)
#include <mach/mach.h>
#elif defined(__unix__)
#include <semaphore.h>
#endif

namespace moodycamel
{
namespace details
{

// Code in the mpmc_sema namespace below is an adaptation of Jeff Preshing's
// portable + lightweight semaphore implementations, originally from
// https://github.com/preshing/cpp11-on-multicore/blob/master/common/sema.h
// LICENSE:
// Copyright (c) 2015 Jeff Preshing
//
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
//
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
//
// 1. The origin of this software must not be misrepresented; you must not
//	claim that you wrote the original software. If you use this software
//	in a product, an acknowledgement in the product documentation would be
//	appreciated but is not required.
// 2. Altered source versions must be plainly marked as such, and must not be
//	misrepresented as being the original software.
// 3. This notice may not be removed or altered from any source distribution.
#if defined(_WIN32)
class Semaphore
{
private:
	void* m_hSema;
	
	Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
	Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;

public:
	Semaphore(int initialCount = 0)
	{
		assert(initialCount >= 0);
		const long maxLong = 0x7fffffff;
		m_hSema = CreateSemaphoreW(nullptr, initialCount, maxLong, nullptr);
		assert(m_hSema);
	}

	~Semaphore()
	{
		CloseHandle(m_hSema);
	}

	bool wait()
	{
		const unsigned long infinite = 0xffffffff;
		return WaitForSingleObject(m_hSema, infinite) == 0;
	}
	
	bool try_wait()
	{
		return WaitForSingleObject(m_hSema, 0) == 0;
	}
	
	bool timed_wait(std::uint64_t usecs)
	{
		return WaitForSingleObject(m_hSema, (unsigned long)(usecs / 1000)) == 0;
	}

	void signal(int count = 1)
	{
		while (!ReleaseSemaphore(m_hSema, count, nullptr));
	}
};
#elif defined(__MACH__)
//---------------------------------------------------------
// Semaphore (Apple iOS and OSX)
// Can't use POSIX semaphores due to http://lists.apple.com/archives/darwin-kernel/2009/Apr/msg00010.html
//---------------------------------------------------------
class Semaphore
{
private:
	semaphore_t m_sema;

	Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
	Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;

public:
	Semaphore(int initialCount = 0)
	{
		assert(initialCount >= 0);
		kern_return_t rc = semaphore_create(mach_task_self(), &m_sema, SYNC_POLICY_FIFO, initialCount);
		assert(rc == KERN_SUCCESS);
		(void)rc;
	}

	~Semaphore()
	{
		semaphore_destroy(mach_task_self(), m_sema);
	}

	bool wait()
	{
		return semaphore_wait(m_sema) == KERN_SUCCESS;
	}
	
	bool try_wait()
	{
		return timed_wait(0);
	}
	
	bool timed_wait(std::uint64_t timeout_usecs)
	{
		mach_timespec_t ts;
		ts.tv_sec = static_cast<unsigned int>(timeout_usecs / 1000000);
		ts.tv_nsec = static_cast<int>((timeout_usecs % 1000000) * 1000);

		// added in OSX 10.10: https://developer.apple.com/library/prerelease/mac/documentation/General/Reference/APIDiffsMacOSX10_10SeedDiff/modules/Darwin.html
		kern_return_t rc = semaphore_timedwait(m_sema, ts);
		return rc == KERN_SUCCESS;
	}

	void signal()
	{
		while (semaphore_signal(m_sema) != KERN_SUCCESS);
	}

	void signal(int count)
	{
		while (count-- > 0)
		{
			while (semaphore_signal(m_sema) != KERN_SUCCESS);
		}
	}
};
#elif defined(__unix__)
//---------------------------------------------------------
// Semaphore (POSIX, Linux)
//---------------------------------------------------------
class Semaphore
{
private:
	sem_t m_sema;

	Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
	Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;

public:
	Semaphore(int initialCount = 0)
	{
		assert(initialCount >= 0);
		int rc = sem_init(&m_sema, 0, static_cast<unsigned int>(initialCount));
		assert(rc == 0);
		(void)rc;
	}

	~Semaphore()
	{
		sem_destroy(&m_sema);
	}

	bool wait()
	{
		// http://stackoverflow.com/questions/2013181/gdb-causes-sem-wait-to-fail-with-eintr-error
		int rc;
		do {
			rc = sem_wait(&m_sema);
		} while (rc == -1 && errno == EINTR);
		return rc == 0;
	}

	bool try_wait()
	{
		int rc;
		do {
			rc = sem_trywait(&m_sema);
		} while (rc == -1 && errno == EINTR);
		return rc == 0;
	}

	bool timed_wait(std::uint64_t usecs)
	{
		struct timespec ts;
		const int usecs_in_1_sec = 1000000;
		const int nsecs_in_1_sec = 1000000000;
		clock_gettime(CLOCK_REALTIME, &ts);
		ts.tv_sec += (time_t)(usecs / usecs_in_1_sec);
		ts.tv_nsec += (long)(usecs % usecs_in_1_sec) * 1000;
		// sem_timedwait bombs if you have more than 1e9 in tv_nsec
		// so we have to clean things up before passing it in
		if (ts.tv_nsec >= nsecs_in_1_sec) {
			ts.tv_nsec -= nsecs_in_1_sec;
			++ts.tv_sec;
		}

		int rc;
		do {
			rc = sem_timedwait(&m_sema, &ts);
		} while (rc == -1 && errno == EINTR);
		return rc == 0;
	}

	void signal()
	{
		while (sem_post(&m_sema) == -1);
	}

	void signal(int count)
	{
		while (count-- > 0)
		{
			while (sem_post(&m_sema) == -1);
		}
	}
};
#else
#error Unsupported platform! (No semaphore wrapper available)
#endif

}	// end namespace details


//---------------------------------------------------------
// LightweightSemaphore
//---------------------------------------------------------
class LightweightSemaphore
{
public:
	typedef std::make_signed<std::size_t>::type ssize_t;

private:
	std::atomic<ssize_t> m_count;
	details::Semaphore m_sema;
	int m_maxSpins;

	bool waitWithPartialSpinning(std::int64_t timeout_usecs = -1)
	{
		ssize_t oldCount;
		int spin = m_maxSpins;
		while (--spin >= 0)
		{
			oldCount = m_count.load(std::memory_order_relaxed);
			if ((oldCount > 0) && m_count.compare_exchange_strong(oldCount, oldCount - 1, std::memory_order_acquire, std::memory_order_relaxed))
				return true;
			std::atomic_signal_fence(std::memory_order_acquire);	 // Prevent the compiler from collapsing the loop.
		}
		oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
		if (oldCount > 0)
			return true;
		if (timeout_usecs < 0)
		{
			if (m_sema.wait())
				return true;
		}
		if (timeout_usecs > 0 && m_sema.timed_wait((std::uint64_t)timeout_usecs))
			return true;
		// At this point, we've timed out waiting for the semaphore, but the
		// count is still decremented indicating we may still be waiting on
		// it. So we have to re-adjust the count, but only if the semaphore
		// wasn't signaled enough times for us too since then. If it was, we
		// need to release the semaphore too.
		while (true)
		{
			oldCount = m_count.load(std::memory_order_acquire);
			if (oldCount >= 0 && m_sema.try_wait())
				return true;
			if (oldCount < 0 && m_count.compare_exchange_strong(oldCount, oldCount + 1, std::memory_order_relaxed, std::memory_order_relaxed))
				return false;
		}
	}

	ssize_t waitManyWithPartialSpinning(ssize_t max, std::int64_t timeout_usecs = -1)
	{
		assert(max > 0);
		ssize_t oldCount;
		int spin = m_maxSpins;
		while (--spin >= 0)
		{
			oldCount = m_count.load(std::memory_order_relaxed);
			if (oldCount > 0)
			{
				ssize_t newCount = oldCount > max ? oldCount - max : 0;
				if (m_count.compare_exchange_strong(oldCount, newCount, std::memory_order_acquire, std::memory_order_relaxed))
					return oldCount - newCount;
			}
			std::atomic_signal_fence(std::memory_order_acquire);
		}
		oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
		if (oldCount <= 0)
		{
			if ((timeout_usecs == 0) || (timeout_usecs < 0 && !m_sema.wait()) || (timeout_usecs > 0 && !m_sema.timed_wait((std::uint64_t)timeout_usecs)))
			{
				while (true)
				{
					oldCount = m_count.load(std::memory_order_acquire);
					if (oldCount >= 0 && m_sema.try_wait())
						break;
					if (oldCount < 0 && m_count.compare_exchange_strong(oldCount, oldCount + 1, std::memory_order_relaxed, std::memory_order_relaxed))
						return 0;
				}
			}
		}
		if (max > 1)
			return 1 + tryWaitMany(max - 1);
		return 1;
	}

public:
	LightweightSemaphore(ssize_t initialCount = 0, int maxSpins = 10000) : m_count(initialCount), m_maxSpins(maxSpins)
	{
		assert(initialCount >= 0);
		assert(maxSpins >= 0);
	}

	bool tryWait()
	{
		ssize_t oldCount = m_count.load(std::memory_order_relaxed);
		while (oldCount > 0)
		{
			if (m_count.compare_exchange_weak(oldCount, oldCount - 1, std::memory_order_acquire, std::memory_order_relaxed))
				return true;
		}
		return false;
	}

	bool wait()
	{
		return tryWait() || waitWithPartialSpinning();
	}

	bool wait(std::int64_t timeout_usecs)
	{
		return tryWait() || waitWithPartialSpinning(timeout_usecs);
	}

	// Acquires between 0 and (greedily) max, inclusive
	ssize_t tryWaitMany(ssize_t max)
	{
		assert(max >= 0);
		ssize_t oldCount = m_count.load(std::memory_order_relaxed);
		while (oldCount > 0)
		{
			ssize_t newCount = oldCount > max ? oldCount - max : 0;
			if (m_count.compare_exchange_weak(oldCount, newCount, std::memory_order_acquire, std::memory_order_relaxed))
				return oldCount - newCount;
		}
		return 0;
	}

	// Acquires at least one, and (greedily) at most max
	ssize_t waitMany(ssize_t max, std::int64_t timeout_usecs)
	{
		assert(max >= 0);
		ssize_t result = tryWaitMany(max);
		if (result == 0 && max > 0)
			result = waitManyWithPartialSpinning(max, timeout_usecs);
		return result;
	}
	
	ssize_t waitMany(ssize_t max)
	{
		ssize_t result = waitMany(max, -1);
		assert(result > 0);
		return result;
	}

	void signal(ssize_t count = 1)
	{
		assert(count >= 0);
		ssize_t oldCount = m_count.fetch_add(count, std::memory_order_release);
		ssize_t toRelease = -oldCount < count ? -oldCount : count;
		if (toRelease > 0)
		{
			m_sema.signal((int)toRelease);
		}
	}
	
	std::size_t availableApprox() const
	{
		ssize_t count = m_count.load(std::memory_order_relaxed);
		return count > 0 ? static_cast<std::size_t>(count) : 0;
	}
};

}   // end namespace moodycamel