// *****************************************************************************
// * This file is part of the FreeFileSync project. It is distributed under    *
// * GNU General Public License: http://www.gnu.org/licenses/gpl-3.0           *
// * Copyright (C) Zenju (zenju AT freefilesync DOT org) - All Rights Reserved *
// *****************************************************************************

#ifndef GLOBALS_H_8013740213748021573485
#define GLOBALS_H_8013740213748021573485

#include <atomic>
#include <memory>
#include "scope_guard.h"

namespace zen
{
//solve static destruction order fiasco by providing shared ownership and serialized access to global variables
template <class T>
class Global
{
public:
	Global() { static_assert(std::is_trivially_destructible<Pod>::value, "this memory needs to live forever"); }
	explicit Global(std::unique_ptr<T>&& newInst) { set(std::move(newInst)); }
	~Global() { set(nullptr); }
	
    std::shared_ptr<T> get() //=> return std::shared_ptr to let instance life time be handled by caller (MT usage!)
    {
        while (pod.spinLock.exchange(true)) ;
        ZEN_ON_SCOPE_EXIT(pod.spinLock = false);
        if (pod.inst)
            return *pod.inst;
        return nullptr;
    }

    void set(std::unique_ptr<T>&& newInst)
    {
        std::shared_ptr<T>* tmpInst = nullptr;
        if (newInst)
            tmpInst = new std::shared_ptr<T>(std::move(newInst));
        {
            while (pod.spinLock.exchange(true)) ;
            ZEN_ON_SCOPE_EXIT(pod.spinLock = false);
            std::swap(pod.inst, tmpInst);
        }
        delete tmpInst;
    }

private:
	//avoid static destruction order fiasco: there may be accesses to "Global<T>::get()" during process shutdown
	//e.g. _("") used by message in debug_minidump.cpp or by some detached thread assembling an error message!
	//=> use trivially-destructible POD only!!!
	struct Pod
	{
		std::shared_ptr<T>* inst = nullptr;
		//serialize access; can't use std::mutex: has non-trival destructor
		std::atomic<bool> spinLock { false };
	} pod;
};

#if defined _MSC_VER && _MSC_VER < 1900
#error function scope static initialization is not yet thread-safe!
#endif
}

#endif //GLOBALS_H_8013740213748021573485