Also handle mutual recursion in *::getState()

VST3 plugins could freeze in REAPER when the plugin sends
`IComponentHandler::performEdit()` followed by
`IPlugFrame::resizeView()` when REAPER simultaneously tries to call
`*::getState()`. Here `*::getState()` gets called from the GUI thread,
while `IPlugFrame::resizeView()` has to be handled on REAPER's GUI
thread, resulting in a deadlock unless we use the plugin-side mutual
recursion mechanism.

I've seen this cause issues with PSPaudioware InfiniStrip.

CHANGELOG.md

@@ -114,6 +114,9 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 - Fixed _UVI Plugsound Free_ crashing during initialization.
 - Fixed potential freezing when loading a VST3 preset that tries to resize an
   open editor window.
+- Fixed another potential freezing issue in REAPER that could happen when the
+  when the plugin resizes itself while sending a parameter change to the host,
+  if REAPER's 'disable saving full plug-in state' is not disabled.
 - Because of the new transport information prefetching, the excessive DSP usage
   in _SWAM Cello_ is now been fixed without requiring any manual compatibility
   options.

src/plugin/bridges/vst3-impls/plugin-proxy.cpp

@@ -362,32 +362,14 @@ tresult PLUGIN_API Vst3PluginProxyImpl::setState(Steinberg::IBStream* state) {
     if (state) {
         // Since both interfaces contain this function, this is used for both
         // `IComponent::setState()` as well as `IEditController::setState()`
-        const Vst3PluginProxy::SetState message{.instance_id = instance_id(),
-                                                .state = state};
-
         // NOTE: This will likely be called from the GUI thread, and some
         //       plugins will try to resize as part of setting their new state.
         //       That `IPlugFrame::resizeView()` _also_ has to be handled on the
         //       GUI thread. So if the GUI is active, we'll use the mutual
         //       recursion mechanism to allow this resize call to also be
         //       performed from the GUI thread.
-        if (last_created_plug_view_active) {
-            return last_created_plug_view->send_mutually_recursive_message(
-                std::move(message));
-        } else if (connected_instance_id) {
-            // We should also be able to handle the above situation when a
-            // `setState()` on a processor triggers a resize coming from the
-            // edit controller. To do that, we'll also check if the connected
-            // instance has an active plug view.
-            Vst3PluginProxyImpl& other_instance =
-                bridge.plugin_proxies.at(*connected_instance_id).get();
-            if (other_instance.last_created_plug_view_active) {
-                return other_instance.last_created_plug_view
-                    ->send_mutually_recursive_message(std::move(message));
-            }
-        }
-
-        return bridge.send_message(std::move(message));
+        return maybe_send_mutually_recursive_message(Vst3PluginProxy::SetState{
+            .instance_id = instance_id(), .state = state});
     } else {
         bridge.logger.log(
             "WARNING: Null pointer passed to "
@@ -400,8 +382,15 @@ tresult PLUGIN_API Vst3PluginProxyImpl::getState(Steinberg::IBStream* state) {
     if (state) {
         // Since both interfaces contain this function, this is used for both
         // `IComponent::getState()` as well as `IEditController::getState()`
+        // NOTE: This will likely be called from the GUI thread, if the plugin
+        //       somehow ends up sending a resize while this happens, we should
+        //       end up in a deadlock. Normally this wouldn't be an issue, but
+        //       REAPER will fetch the complete plugin state from time to time,
+        //       so when changing a parameter also resizes the GUI we can run
+        //       into a situation where we need mutually recursive function
+        //       calls.
         const GetStateResponse response =
-            bridge.send_message(Vst3PluginProxy::GetState{
+            maybe_send_mutually_recursive_message(Vst3PluginProxy::GetState{
                 .instance_id = instance_id(), .state = state});
 
         assert(response.state.write_back(state) == Steinberg::kResultOk);

src/plugin/bridges/vst3-impls/plugin-proxy.h

@@ -442,6 +442,40 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
      */
     void clear_parameter_cache();
 
+    /**
+     * If we have an active `IPlugView` instance, try to use the mutual
+     * recursion mechanism so that callbacks made by the plugin can be handled
+     * on this same thread. In case this is an audio processor with a separate
+     * edit controller, we'll also check if the object we're connected to has an
+     * active `IPlugView` instance. When there's no active `IPlugView` instance,
+     * we'll just send the event message like normal. This is needed to be able
+     * to handle function calls made by the host (which is mostly relevant for
+     * REAPER) on the GUI thread, when the plugin makes a callback to the host
+     * that should also be handled on that same thread (context menus and
+     * plugin-driven resizes).
+     */
+    template <typename T>
+    typename T::Response maybe_send_mutually_recursive_message(
+        const T& object) {
+        if (last_created_plug_view_active) {
+            return last_created_plug_view->send_mutually_recursive_message(
+                std::move(object));
+        } else if (connected_instance_id) {
+            // We should also be able to handle the above situation when a
+            // `setState()` on a processor triggers a resize coming from the
+            // edit controller. To do that, we'll also check if the connected
+            // instance has an active plug view.
+            Vst3PluginProxyImpl& other_instance =
+                bridge.plugin_proxies.at(*connected_instance_id).get();
+            if (other_instance.last_created_plug_view_active) {
+                return other_instance.last_created_plug_view
+                    ->send_mutually_recursive_message(std::move(object));
+            }
+        }
+
+        return bridge.send_message(std::move(object));
+    }
+
     Vst3PluginBridge& bridge;
 
     /**