Fix RubberBand segafult when changing audio setting

src/engine/bufferscalers/enginebufferscalerubberband.cpp

@@ -97,7 +97,6 @@ void EngineBufferScaleRubberBand::onSignalChanged() {
     // TODO: Resetting the sample rate will cause internal
     // memory allocations that may block the real-time thread.
     // When is this function actually invoked??
-    m_rubberBand.clear();
     if (!getOutputSignal().isValid()) {
         return;
     }

src/preferences/dialog/dlgprefsound.cpp

@@ -334,6 +334,14 @@ void DlgPrefSound::slotApply() {
         ScopedWaitCursor cursor;
         const auto keylockEngine =
                 keylockComboBox->currentData().value<EngineBuffer::KeylockEngine>();
+
+        // Temporary set an empty config to force the audio thread to stop and
+        // stay off while we are swapping the keylock settings. This is
+        // necessary because the audio thread doesn't have any synchronisation
+        // mechanism due to its realtime nature and editing the RubberBand
+        // config while it is running leads to race conditions.
+        m_pSoundManager->closeActiveConfig();
+
         m_pKeylockEngine.set(static_cast<double>(keylockEngine));
         m_pSettings->set(kKeylockEngingeCfgkey,
                 ConfigValue(static_cast<int>(keylockEngine)));

src/soundio/soundmanager.cpp

@@ -553,22 +553,20 @@ SoundManagerConfig SoundManager::getConfig() const {
     return m_config;
 }
 
-SoundDeviceStatus SoundManager::setConfig(const SoundManagerConfig& config) {
-    SoundDeviceStatus status = SoundDeviceStatus::Ok;
-    m_config = config;
-    checkConfig();
-
+void SoundManager::closeActiveConfig() {
     // Close open devices. After this call we will not get any more
     // onDeviceOutputCallback() or pushBuffer() calls because all the
     // SoundDevices are closed. closeDevices() blocks and can take a while.
     const bool sleepAfterClosing = true;
     closeDevices(sleepAfterClosing);
+}
+
+SoundDeviceStatus SoundManager::setConfig(const SoundManagerConfig& config) {
+    SoundDeviceStatus status = SoundDeviceStatus::Ok;
+    m_config = config;
+    checkConfig();
 
-    // certain parts of mixxx rely on this being here, for the time being, just
-    // letting those be -- bkgood
-    // Do this first so vinyl control gets the right samplerate -- Owen W.
-    m_pConfig->set(ConfigKey("[Soundcard]", "Samplerate"),
-            ConfigValue(static_cast<int>(m_config.getSampleRate().value())));
+    closeActiveConfig();
 
     status = setupDevices();
     if (status == SoundDeviceStatus::Ok) {

src/soundio/soundmanager.h

@@ -74,6 +74,7 @@ class SoundManager : public QObject {
     QList<QString> getHostAPIList() const;
     SoundManagerConfig getConfig() const;
     SoundDeviceStatus setConfig(const SoundManagerConfig& config);
+    void closeActiveConfig();
     void checkConfig();
 
     void onDeviceOutputCallback(const SINT iFramesPerBuffer);