Merge #4713

4713: Give the condition waits proper conditions r=def- a=Jupeyy

<!-- What is the motivation for the changes of this pull request -->

## Checklist

- [ ] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test if it works standalone, system.c especially
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: Jupeyy <jupjopjap@gmail.com>

src/engine/client/backend_sdl.cpp

@@ -61,6 +61,7 @@ void CGraphicsBackend_Threaded::ThreadFunc(void *pUser)
 	auto *pSelf = (CGraphicsBackend_Threaded *)pUser;
 	std::unique_lock<std::mutex> Lock(pSelf->m_BufferSwapMutex);
 	// notify, that the thread started
+	pSelf->m_Started = true;
 	pSelf->m_BufferDoneCond.notify_all();
 	while(!pSelf->m_Shutdown)
 	{
@@ -75,11 +76,12 @@ void CGraphicsBackend_Threaded::ThreadFunc(void *pUser)
 			pSelf->m_pBuffer = nullptr;
 			pSelf->m_BufferInProcess.store(false, std::memory_order_relaxed);
 			pSelf->m_BufferDoneCond.notify_all();
-		}
+
 #if defined(CONF_VIDEORECORDER)
-		if(IVideo::Current())
-			IVideo::Current()->NextVideoFrameThread();
+			if(IVideo::Current())
+				IVideo::Current()->NextVideoFrameThread();
 #endif
+		}
 	}
 }
 
@@ -97,7 +99,7 @@ void CGraphicsBackend_Threaded::StartProcessor(ICommandProcessor *pProcessor)
 	std::unique_lock<std::mutex> Lock(m_BufferSwapMutex);
 	m_Thread = thread_init(ThreadFunc, this, "Graphics thread");
 	// wait for the thread to start
-	m_BufferDoneCond.wait(Lock);
+	m_BufferDoneCond.wait(Lock, [this]() -> bool { return m_Started; });
 }
 
 void CGraphicsBackend_Threaded::StopProcessor()
@@ -127,7 +129,7 @@ bool CGraphicsBackend_Threaded::IsIdle() const
 void CGraphicsBackend_Threaded::WaitForIdle()
 {
 	std::unique_lock<std::mutex> Lock(m_BufferSwapMutex);
-	if(m_pBuffer != nullptr)
+	while(m_pBuffer != nullptr)
 		m_BufferDoneCond.wait(Lock);
 }
 

src/engine/client/backend_sdl.h

@@ -72,6 +72,7 @@ private:
 	std::condition_variable m_BufferDoneCond;
 	CCommandBuffer *m_pBuffer;
 	std::atomic_bool m_Shutdown;
+	bool m_Started = false;
 	std::atomic_bool m_BufferInProcess;
 	void *m_Thread;