godot/core/os/safe_binary_mutex.h

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/*  safe_binary_mutex.h                                                   */
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#ifndef SAFE_BINARY_MUTEX_H
#define SAFE_BINARY_MUTEX_H

#include "core/error/error_macros.h"
#include "core/os/mutex.h"
#include "core/typedefs.h"

#ifdef THREADS_ENABLED

// A very special kind of mutex, used in scenarios where these
// requirements hold at the same time:
// - Must be used with a condition variable (only binary mutexes are suitable).
// - Must have recursive semnantics (or simulate, as this one does).
// The implementation keeps the lock count in TS. Therefore, only
// one object of each version of the template can exists; hence the Tag argument.
// Tags must be unique across the Godot codebase.
// Also, don't forget to declare the thread_local variable on each use.
template <int Tag>
class SafeBinaryMutex {
	friend class MutexLock<SafeBinaryMutex>;

	using StdMutexType = THREADING_NAMESPACE::mutex;

	mutable THREADING_NAMESPACE::mutex mutex;
	static thread_local uint32_t count;

public:
	_ALWAYS_INLINE_ void lock() const {
		if (++count == 1) {
			mutex.lock();
		}
	}

	_ALWAYS_INLINE_ void unlock() const {
		DEV_ASSERT(count);
		if (--count == 0) {
			mutex.unlock();
		}
	}

	_ALWAYS_INLINE_ bool try_lock() const {
		if (count) {
			count++;
			return true;
		} else {
			if (mutex.try_lock()) {
				count++;
				return true;
			} else {
				return false;
			}
		}
	}

	~SafeBinaryMutex() {
		DEV_ASSERT(!count);
	}
};

// This specialization is needed so manual locking and MutexLock can be used
// at the same time on a SafeBinaryMutex.
template <int Tag>
class MutexLock<SafeBinaryMutex<Tag>> {
	friend class ConditionVariable;

	THREADING_NAMESPACE::unique_lock<THREADING_NAMESPACE::mutex> lock;

public:
	_ALWAYS_INLINE_ explicit MutexLock(const SafeBinaryMutex<Tag> &p_mutex) :
			lock(p_mutex.mutex) {
		SafeBinaryMutex<Tag>::count++;
	};
	_ALWAYS_INLINE_ ~MutexLock() {
		SafeBinaryMutex<Tag>::count--;
	};
};

#else // No threads.

template <int Tag>
class SafeBinaryMutex : public MutexImpl {
	static thread_local uint32_t count;
};

template <int Tag>
class MutexLock<SafeBinaryMutex<Tag>> {
public:
	MutexLock(const SafeBinaryMutex<Tag> &p_mutex) {}
	~MutexLock() {}
};

#endif // THREADS_ENABLED

#endif // SAFE_BINARY_MUTEX_H