Inhibit event loop during VST3 offline processing

This prevents T-RackS 5 from causing the export in Bitwig Studio 4.1.0
beta 2 to freeze.

CHANGELOG.md

@@ -27,6 +27,11 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 - The socket endpoint used by a plugin group host process to accept new
   connections now gets removed when the group host process shuts down.
   Previously this would leave behind a file in the temporary directory.
+- Fixed the VST3 version of _IK Multimedia's T-RackS 5_ causing offline
+  rendering to stall indefinitely. This could happen when exporting or bouncing
+  audio in **Bitwig Studio 4.1** or in **REAPER**. That plugin deadlocks when it
+  receives timer events while doing offline audio processing, so we now prevent
+  that from happening.
 
 ## [3.6.0] - 2021-10-15
 

src/wine-host/bridges/vst3.cpp

@@ -114,8 +114,11 @@ Vst3Bridge::Vst3Bridge(MainContext& main_context,
 bool Vst3Bridge::inhibits_event_loop() noexcept {
     std::lock_guard lock(object_instances_mutex);
 
-    for (const auto& [instance_id, object] : object_instances) {
-        if (!object.is_initialized) {
+    for (const auto& [instance_id, instance] : object_instances) {
+        // HACK: IK Multimedia's T-RackS 5 will deadlock if it receives a timer
+        //       proc during offline processing, so we need to prevent that from
+        //       happening.
+        if (!instance.is_initialized || instance.is_offline_processing) {
             return true;
         }
     }
@@ -1429,6 +1432,15 @@ size_t Vst3Bridge::register_object_instance(
                     },
                     [&](YaAudioProcessor::SetupProcessing& request)
                         -> YaAudioProcessor::SetupProcessing::Response {
+                        Vst3PluginInstance& instance =
+                            object_instances.at(request.instance_id);
+
+                        // See the comment in
+                        // `Vst3Bridge::inhibits_event_loop()`
+                        instance.is_offline_processing =
+                            request.setup.processMode ==
+                            Steinberg::Vst::kOffline;
+
                         const tresult result =
                             object_instances.at(request.instance_id)
                                 .interfaces.audio_processor->setupProcessing(

src/wine-host/bridges/vst3.h

@@ -244,6 +244,14 @@ struct Vst3PluginInstance {
      * have, but we'll just do this out of precaution.
      */
     bool is_initialized = false;
+
+    /**
+     * Whether the plugin instance is currently in offline processing mode or
+     * not. Needed as a HACK for IK Multimedia's T-RackS 5 because those plugins
+     * will deadlock if they receive a timer proc on the Win32 message loop
+     * while doing offline processing.
+     */
+    bool is_offline_processing = false;
 };
 
 /**