Work around thread safety issue in Melda plugins

This is super difficult to trigger on purpose, but I did run into it at
least once just now so it seems like a good idea to at least make sure
that this doesn't happen.

CHANGELOG.md

@@ -80,6 +80,9 @@ Versioning](https://semver.org/spec/v2.0.0.html).
   tries to use REAPER's [host function
   API](https://www.reaper.fm/sdk/vst/vst_ext.php#vst_host). This currently isn't
   supported by yabridge. We now explicitly ignore these requests.
+- Worked around a rare thread safety issue in _MeldaProduction_ VST3 plugins
+  where the plugin would deadlock when the host asks for the editor's size while
+  plugin is also being initialized form the audio thread.
 - Fixed JUCE VST3 plugins like Tokyo Dawn Records' _SlickEQ M_ causing the host
   to freeze when they send a parameter change from the audio thread using the
   wrong VST3 API while the plugin is also trying to resize the window from the

src/wine-host/bridges/vst3.cpp

@@ -800,13 +800,20 @@ void Vst3Bridge::run() {
                     .get();
             },
             [&](YaPlugView::GetSize& request) -> YaPlugView::GetSize::Response {
+                Vst3PluginInstance& instance =
+                    object_instances.at(request.owner_instance_id);
+
                 // Melda plugins will refuse to open dialogs of this function is
-                // not run from the GUI thread
+                // not run from the GUI thread. Oh and they also deadlock if
+                // audio processing gets initialized at the same time as this
+                // function, not sure why.
+                std::lock_guard lock(instance.get_size_mutex);
+
                 Steinberg::ViewRect size{};
                 const tresult result =
                     do_mutual_recursion_on_gui_thread([&]() -> tresult {
-                        return object_instances.at(request.owner_instance_id)
-                            .plug_view_instance->plug_view->getSize(&size);
+                        return instance.plug_view_instance->plug_view->getSize(
+                            &size);
                     });
 
                 return YaPlugView::GetSizeResponse{.result = result,
@@ -1419,9 +1426,18 @@ size_t Vst3Bridge::register_object_instance(
                     },
                     [&](const YaAudioProcessor::SetProcessing& request)
                         -> YaAudioProcessor::SetProcessing::Response {
-                        return object_instances.at(request.instance_id)
-                            .interfaces.audio_processor->setProcessing(
-                                request.state);
+                        Vst3PluginInstance& instance =
+                            object_instances.at(request.instance_id);
+
+                        // HACK: MeldaProduction plugins for some reason cannot
+                        //       handle it if this function is called from the
+                        //       audio thread while at the same time
+                        //       `IPlugView::getSize()` is being called from the
+                        //       GUI thread
+                        std::lock_guard lock(instance.get_size_mutex);
+
+                        return instance.interfaces.audio_processor
+                            ->setProcessing(request.state);
                     },
                     [&](MessageReference<YaAudioProcessor::Process>&
                             request_ref)

src/wine-host/bridges/vst3.h

@@ -216,6 +216,19 @@ struct Vst3PluginInstance {
      */
     std::optional<Vst3PlugViewInterfaces> plug_view_instance;
 
+    /**
+     * Used to make sure that `IPlugView::getSize()` can never be called at the
+     * same time as `IAudioProcessor::setProcessing()`.
+     *
+     * HACK: This really shouldn't be needed, but MeldaProduction plugins seem
+     *       to deadlock when this happens. It's pretty tricky to reproduce the
+     *       timing for making this happen (since opening the GUI also needs to
+     *       be delayed slightly, like when opening a plugin from Bitwig's popup
+     *       browser), but it seems like a good idea to make sure that this
+     *       doesn't cause any freezes.
+     */
+    std::mutex get_size_mutex;
+
     /**
      * This contains smart pointers to all VST3 plugin interfaces that can be
      * casted from `object`.