[4.7] Fix #35454: Do not activate a VST before its processing is set up

src/framework/vst/internal/vstaudioclient.cpp

@@ -437,11 +437,13 @@ void VstAudioClient::updateProcessSetup()
     setup.sampleRate = m_outputSpec.sampleRate;
 
     if (processor->setupProcessing(setup) != Steinberg::kResultOk) {
+        m_isProcessingSetUp = false;
         return;
     }
 
     setUpProcessData();
     flushBuffers();
+    m_isProcessingSetUp = true;
 
     ensureActivity();
 }
@@ -551,6 +553,13 @@ void VstAudioClient::ensureActivity()
         return;
     }
 
+    //! NOTE: The plugin may finish loading after the track has already been
+    //! switched on (e.g. right before an export). Activating it before
+    //! setupProcessing() would let process() run without output buffers
+    if (!m_isProcessingSetUp) {
+        return;
+    }
+
     IAudioProcessorPtr processor = pluginProcessor();
     if (!processor) {
         return;

src/framework/vst/internal/vstaudioclient.h

@@ -101,6 +101,7 @@ class VstAudioClient : public muse::Contextable
 
     bool m_needUnprepareProcessData = false;
     bool m_needUpdateState = false;
+    bool m_isProcessingSetUp = false;
 
     audioplugins::AudioPluginType m_type = audioplugins::AudioPluginType::Undefined;
     audio::OutputSpec m_outputSpec;