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/**
* @file lockstatic.h
* @author Nat Goodspeed
* @date 2019-12-03
* @brief LockStatic class provides mutex-guarded access to the specified
* static data.
*
* $LicenseInfo:firstyear=2019&license=viewerlgpl$
* Copyright (c) 2019, Linden Research, Inc.
* $/LicenseInfo$
*/
#if ! defined(LL_LOCKSTATIC_H)
#define LL_LOCKSTATIC_H
#include "mutex.h" // std::unique_lock
namespace llthread
{
// Instantiate this template to obtain a pointer to the canonical static
// instance of Static while holding a lock on that instance. Use of
// Static::mMutex presumes that Static declares some suitable mMutex.
template <typename Static>
class LockStatic
{
typedef std::unique_lock<decltype(Static::mMutex)> lock_t;
public:
LockStatic():
mData(getStatic()),
mLock(mData->mMutex)
{}
Static* get() const { return mData; }
operator Static*() const { return get(); }
Static* operator->() const { return get(); }
// sometimes we must explicitly unlock...
void unlock()
{
// but once we do, access is no longer permitted
mData = nullptr;
mLock.unlock();
}
protected:
Static* mData;
lock_t mLock;
private:
Static* getStatic()
{
// Static::mMutex must be function-local static rather than class-
// static. Some of our consumers must function properly (therefore
// lock properly) even when the containing module's static variables
// have not yet been runtime-initialized. A mutex requires
// construction. A static class member might not yet have been
// constructed.
//
// We could store a dumb mutex_t*, notice when it's NULL and allocate a
// heap mutex -- but that's vulnerable to race conditions. And we can't
// defend the dumb pointer with another mutex.
//
// We could store a std::atomic<mutex_t*> -- but a default-constructed
// std::atomic<T> does not contain a valid T, even a default-constructed
// T! Which means std::atomic, too, requires runtime initialization.
//
// But a function-local static is guaranteed to be initialized exactly
// once: the first time control reaches that declaration.
static Static sData;
return &sData;
}
};
} // llthread namespace
#endif /* ! defined(LL_LOCKSTATIC_H) */
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