Revamp synchronization with the audio engine (#6881)

The revamp consists of one lock. When the audio thread needs to render audio or another thread wants to run a change, acquiring the lock grants mutual exclusion to do one of the two. The intention is that this will provide stronger guarantees that changes do not run concurrently with the audio thread, as well as having the synchronization mechanism itself be free of data races (verified with TSan).

include/AudioEngine.h

@@ -25,14 +25,13 @@
 #ifndef LMMS_AUDIO_ENGINE_H
 #define LMMS_AUDIO_ENGINE_H
 
-#include <QMutex>
-
-#if (QT_VERSION >= QT_VERSION_CHECK(5,14,0))
-	#include <QRecursiveMutex>
+#ifdef __MINGW32__
+#include <mingw.mutex.h>
+#else
+#include <mutex>
 #endif
 
 #include <QThread>
-#include <QWaitCondition>
 #include <samplerate.h>
 
 #include <vector>
@@ -420,10 +419,6 @@ private:
 
 	void clearInternal();
 
-	//! Called by the audio thread to give control to other threads,
-	//! such that they can do changes in the model (like e.g. removing effects)
-	void runChangesInModel();
-
 	bool m_renderOnly;
 
 	std::vector<AudioPort *> m_audioPorts;
@@ -453,8 +448,6 @@ private:
 	struct qualitySettings m_qualitySettings;
 	float m_masterGain;
 
-	bool m_isProcessing;
-
 	// audio device stuff
 	void doSetAudioDevice( AudioDevice *_dev );
 	AudioDevice * m_audioDev;
@@ -476,19 +469,7 @@ private:
 
 	bool m_clearSignal;
 
-	bool m_changesSignal;
-	unsigned int m_changes;
-	QMutex m_changesMutex;
-#if (QT_VERSION >= QT_VERSION_CHECK(5,14,0))
-	QRecursiveMutex m_doChangesMutex;
-#else
-	QMutex m_doChangesMutex;
-#endif
-	QMutex m_waitChangesMutex;
-	QWaitCondition m_changesAudioEngineCondition;
-	QWaitCondition m_changesRequestCondition;
-
-	bool m_waitingForWrite;
+	std::mutex m_changeMutex;
 
 	friend class Engine;
 	friend class AudioEngineWorkerThread;