Don't use STL smart pointers with VST3 interfaces

This would cause double frees since those objects are supposed to clean
up after themselves.

src/common/serialization/vst3/README.md

@@ -62,4 +62,7 @@ TODO: Explain how we implement `createInstance()`
 - Since everything behind the scenes makes use of these `addRef()` and
   `release()` reference counting functions, we can't use the standard library's
   smart pointers when dealing with objects that are shared with the host or with
-  the Windows VST3 plugin.
+  the Windows VST3 plugin. In `IPtr<T>`'s destructor it will call release, and
+  the objects will clean themselfs up with a `delete this;` when the reference
+  count reaches 0. Combining this with the STL cmart pointers this would result
+  in a double free.

src/plugin/bridges/vst3.cpp

@@ -86,7 +86,10 @@ Vst3PluginBridge::Vst3PluginBridge()
 Steinberg::IPluginFactory* Vst3PluginBridge::get_plugin_factory() {
     // Even though we're working with raw pointers here, we should pretend that
     // we're `IPtr<Steinberg::IPluginFactory>` and do the reference counting
-    // ourselves because you can't always safely pass those around
+    // ourselves because you can't always safely pass those around The VST3
+    // interface can't pass smart pointers around because of binary
+    // compatibility, so we'll have to do the reference counting by hand like in
+    // the implementation in `public.sdk/source/main/pluginfactory.h`.
     if (plugin_factory) {
         plugin_factory->addRef();
     } else {
@@ -94,7 +97,7 @@ Steinberg::IPluginFactory* Vst3PluginBridge::get_plugin_factory() {
         // will request after loading the module. Host callback handlers should
         // have started before this since the Wine plugin host will request a
         // copy of the configuration during its initialization.
-        plugin_factory = std::make_unique<YaPluginFactoryPluginImpl>(*this);
+        plugin_factory = Steinberg::owned(new YaPluginFactoryPluginImpl(*this));
         sockets.host_vst_control.receive_into(
             WantsPluginFactory{}, *plugin_factory,
             std::pair<Vst3Logger&, bool>(logger, true));

src/plugin/bridges/vst3.h

@@ -80,15 +80,11 @@ class Vst3PluginBridge : PluginBridge<Vst3Sockets<std::jthread>> {
      * Our plugin factory. All information about the plugin and its supported
      * classes are copied directly from the Windows VST3 plugin's factory on the
      * Wine side, and we'll provide an implementation that can send control
-     * messages to the Wine plugin host. Even though we're passign plain
-     * pointers around, we should pretend that they're wrapped in the VST3 SDK's
-     * reference counting p pointers so we should do the reference counting
-     * ourselves.
+     * messages to the Wine plugin host. The VST3 interface only passes raw
+     * pointers around and the receiving side should then use `IPtr<T>::adopt()`
+     * or `owned()` to get back the orignal smart pointer.
      *
      * @related get_plugin_factory
-     *
-     * FIXME: We can't use `std::unique_ptr` here because that breaks VST3's
-     *        reference counting mechanism.
      */
-    std::unique_ptr<YaPluginFactory> plugin_factory;
+    Steinberg::IPtr<YaPluginFactory> plugin_factory;
 };