fix for Latch race (#617)

Because the counter is decreased in `CountDown()` and subsequently checked against zero to determine if the completion event should be set:

`Latch::Wait` checks the counter against zero on entry and would exit early, before waiting on the completion event. This could then lead to the `Latch` instance being torn down before the `CountDown()` does a `Set()` which could then lead to unexpected and unwanted things happening.

2 files changed, 19 insertions, 29 deletions

diff --git a/src/zencore/include/zencore/thread.h b/src/zencore/include/zencore/thread.h
index 11ef01a35..9fe6dfb9b 100644
--- a/src/zencore/include/zencore/thread.h
+++ b/src/zencore/include/zencore/thread.h
@@ -103,16 +103,12 @@ public:
 	ZENCORE_API Event();
 	ZENCORE_API ~Event();
 
-	Event(Event&& Rhs) noexcept : m_EventHandle(Rhs.m_EventHandle) { Rhs.m_EventHandle = nullptr; }
+	Event(Event&& Rhs) noexcept : m_EventHandle(Rhs.m_EventHandle.load()) { Rhs.m_EventHandle = nullptr; }
 
 	Event(const Event& Rhs) = delete;
 	Event& operator=(const Event& Rhs) = delete;
 
-	inline Event& operator=(Event&& Rhs) noexcept
-	{
-		std::swap(m_EventHandle, Rhs.m_EventHandle);
-		return *this;
-	}
+	Event& operator=(Event&& Rhs) = delete;
 
 	ZENCORE_API void Set();
 	ZENCORE_API void Reset();
@@ -126,7 +122,7 @@ public:
 protected:
 	explicit Event(void* EventHandle) : m_EventHandle(EventHandle) {}
 
-	void* m_EventHandle = nullptr;
+	std::atomic<void*> m_EventHandle{nullptr};
 };
 
 /** Basic abstraction of an IPC mechanism (aka 'binary semaphore')
@@ -141,16 +137,18 @@ public:
 	ZENCORE_API std::error_code Set();
 	ZENCORE_API bool			Wait(int TimeoutMs = -1);
 
-	NamedEvent(NamedEvent&& Rhs) noexcept : m_EventHandle(Rhs.m_EventHandle) { Rhs.m_EventHandle = nullptr; }
+	NamedEvent(NamedEvent&& Rhs) noexcept : m_EventHandle(Rhs.m_EventHandle.load()) { Rhs.m_EventHandle = nullptr; }
 
-	inline NamedEvent& operator=(NamedEvent&& Rhs) noexcept
+	NamedEvent& operator=(NamedEvent&& Rhs)
 	{
-		std::swap(m_EventHandle, Rhs.m_EventHandle);
+		Close();
+		m_EventHandle	  = Rhs.m_EventHandle.load();
+		Rhs.m_EventHandle = nullptr;
 		return *this;
-	}
+	};
 
 protected:
-	void* m_EventHandle = nullptr;
+	std::atomic<void*> m_EventHandle{nullptr};
 
 private:
 	NamedEvent(const NamedEvent& Rhs) = delete;
@@ -201,15 +199,7 @@ public:
 		ZEN_ASSERT(Old > 0);
 	}
 
-	bool Wait(int TimeoutMs = -1)
-	{
-		std::ptrdiff_t Old = Counter.load();
-		if (Old == 0)
-		{
-			return true;
-		}
-		return Complete.Wait(TimeoutMs);
-	}
+	bool Wait(int TimeoutMs = -1) { return Complete.Wait(TimeoutMs); }
 
 private:
 	std::atomic_ptrdiff_t Counter;
diff --git a/src/zencore/thread.cpp b/src/zencore/thread.cpp
index 7bd21a229..9e3486e49 100644
--- a/src/zencore/thread.cpp
+++ b/src/zencore/thread.cpp
@@ -200,7 +200,7 @@ Event::Set()
 	SetEvent(m_EventHandle);
 #else
 	std::atomic_thread_fence(std::memory_order_acquire);
-	auto* Inner = (EventInner*)m_EventHandle;
+	auto* Inner = (EventInner*)m_EventHandle.load();
 	{
 		std::unique_lock Lock(Inner->Mutex);
 		Inner->bSet.store(true);
@@ -216,7 +216,7 @@ Event::Reset()
 	ResetEvent(m_EventHandle);
 #else
 	std::atomic_thread_fence(std::memory_order_acquire);
-	auto* Inner = (EventInner*)m_EventHandle;
+	auto* Inner = (EventInner*)m_EventHandle.load();
 	{
 		std::unique_lock Lock(Inner->Mutex);
 		Inner->bSet.store(false);
@@ -232,7 +232,7 @@ Event::Close()
 	m_EventHandle = nullptr;
 #else
 	std::atomic_thread_fence(std::memory_order_acquire);
-	auto* Inner = (EventInner*)m_EventHandle;
+	auto* Inner = (EventInner*)m_EventHandle.load();
 	{
 		std::unique_lock Lock(Inner->Mutex);
 		Inner->bSet.store(true);
@@ -260,7 +260,7 @@ Event::Wait(int TimeoutMs)
 	return (Result == WAIT_OBJECT_0);
 #else
 	std::atomic_thread_fence(std::memory_order_acquire);
-	auto* Inner = reinterpret_cast<EventInner*>(m_EventHandle);
+	auto* Inner = reinterpret_cast<EventInner*>(m_EventHandle.load());
 
 	if (Inner->bSet.load())
 	{
@@ -372,7 +372,7 @@ NamedEvent::Close()
 #if ZEN_PLATFORM_WINDOWS
 	CloseHandle(m_EventHandle);
 #elif ZEN_PLATFORM_LINUX || ZEN_PLATFORM_MAC
-	int Fd		  = int(intptr_t(m_EventHandle) & 0xffff'ffff);
+	int Fd		  = int(intptr_t(m_EventHandle.load()) & 0xffff'ffff);
 
 	if (flock(Fd, LOCK_EX | LOCK_NB) == 0)
 	{
@@ -390,7 +390,7 @@ NamedEvent::Close()
 		flock(Fd, LOCK_UN | LOCK_NB);
 		close(Fd);
 
-		int Sem = int(intptr_t(m_EventHandle) >> 32);
+		int Sem = int(intptr_t(m_EventHandle.load()) >> 32);
 		semctl(Sem, 0, IPC_RMID);
 	}
 #endif
@@ -408,7 +408,7 @@ NamedEvent::Set()
 		return MakeErrorCodeFromLastError();
 	}
 #elif ZEN_PLATFORM_LINUX || ZEN_PLATFORM_MAC
-	int Sem = int(intptr_t(m_EventHandle) >> 32);
+	int Sem = int(intptr_t(m_EventHandle.load()) >> 32);
 	if (semctl(Sem, 0, SETVAL, 0) == -1)
 	{
 		return MakeErrorCodeFromLastError();
@@ -434,7 +434,7 @@ NamedEvent::Wait(int TimeoutMs)
 
 	return (Result == WAIT_OBJECT_0);
 #elif ZEN_PLATFORM_LINUX || ZEN_PLATFORM_MAC
-	int Sem = int(intptr_t(m_EventHandle) >> 32);
+	int Sem = int(intptr_t(m_EventHandle.load()) >> 32);
 
 	int			  Result;
 	struct sembuf SemOp = {};