Add missing const qualifier and add checks on RB retrieve

src/engine/bufferscalers/rubberbandworkerpool.cpp

@@ -14,10 +14,10 @@ RubberBandWorkerPool::RubberBandWorkerPool(UserSettingsPointer pConfig)
     m_channelPerWorker = multiThreadedOnStereo
             ? mixxx::audio::ChannelCount::mono()
             : mixxx::audio::ChannelCount::stereo();
-    DEBUG_ASSERT(0 == mixxx::kEngineChannelCount % m_channelPerWorker);
+    DEBUG_ASSERT(mixxx::kEngineChannelCount % m_channelPerWorker == 0);
 
     setThreadPriority(QThread::HighPriority);
-    setMaxThreadCount(mixxx::kEngineChannelCount / m_channelPerWorker);
+    setMaxThreadCount(mixxx::kEngineChannelCount / m_channelPerWorker - 1);
 
     // We allocate one runner less than the total of maximum supported channel,
     // so the engine thread will also perform a stretching operation, instead of
@@ -27,7 +27,3 @@ RubberBandWorkerPool::RubberBandWorkerPool(UserSettingsPointer pConfig)
         reserveThread();
     }
 }
-
-RubberBandWorkerPool::~RubberBandWorkerPool() {
-    waitForDone();
-}

src/engine/bufferscalers/rubberbandworkerpool.h

@@ -11,8 +11,6 @@
 // can be distributed stretching job
 class RubberBandWorkerPool : public QThreadPool, public Singleton<RubberBandWorkerPool> {
   public:
-    ~RubberBandWorkerPool();
-
     const mixxx::audio::ChannelCount& channelPerWorker() const {
         return m_channelPerWorker;
     }

src/engine/bufferscalers/rubberbandwrapper.cpp

@@ -33,14 +33,14 @@ void RubberBandWrapper::setTimeRatio(double ratio) {
 }
 size_t RubberBandWrapper::getSamplesRequired() const {
     size_t require = 0;
-    for (auto& stretcher : m_pInstances) {
+    for (const auto& stretcher : m_pInstances) {
         require = qMax(require, stretcher->getSamplesRequired());
     }
     return require;
 }
 int RubberBandWrapper::available() const {
     int available = std::numeric_limits<int>::max();
-    for (auto& stretcher : m_pInstances) {
+    for (const auto& stretcher : m_pInstances) {
         available = qMin(available, stretcher->available());
     }
     return available == std::numeric_limits<int>::max() ? 0 : available;
@@ -49,14 +49,17 @@ size_t RubberBandWrapper::retrieve(float* const* output, size_t samples) const {
     if (m_pInstances.size() == 1) {
         return m_pInstances[0]->retrieve(output, samples);
     } else {
-        size_t ret = 0;
+        // ensure we don't fetch more samples than we really have available.
+        samples = std::min(static_cast<size_t>(available()), samples);
         for (const auto& stretcher : m_pInstances) {
-            size_t thisRet = stretcher->retrieve(output, samples);
-            DEBUG_ASSERT(!ret || thisRet == ret);
-            ret = qMax(thisRet, ret);
+            size_t numSamplesRequested = stretcher->retrieve(output, samples);
+            ;
+            // there is something very wrong if we got a different of amount
+            // of samples than we requested
+            DEBUG_ASSERT(numSamplesRequested == samples);
             output += getChannelPerWorker();
         }
-        return ret;
+        return samples;
     }
 }
 size_t RubberBandWrapper::getInputIncrement() const {
@@ -151,6 +154,7 @@ void RubberBandWrapper::setup(mixxx::audio::SampleRate sampleRate,
         return;
     }
 
+    m_pInstances.reserve(chCount / channelPerWorker);
     for (int c = 0; c < chCount; c += channelPerWorker) {
         m_pInstances.emplace_back(
                 std::make_unique<RubberBandTask>(