Also use rwlocks on the VST3 plugin side

src/plugin/bridges/vst3.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <shared_mutex>
 #include <thread>
 
 #include "../../common/communication/vst3.h"
@@ -73,6 +74,15 @@ class Vst3PluginBridge : PluginBridge<Vst3Sockets<std::jthread>> {
      */
     Steinberg::IPluginFactory* get_plugin_factory();
 
+    /**
+     * Fetch the plugin proxy instance along with a lock valid for the
+     * instance's lifetime. This is mostly just to save some boilerplate
+     * everywhere. Use C++17's structured binding as syntactic sugar to not have
+     * to deal with the lock handle.
+     */
+    std::pair<Vst3PluginProxyImpl&, std::shared_lock<std::shared_mutex>>
+    get_proxy(size_t instance_id) noexcept;
+
     /**
      * Add a `Vst3PluginProxyImpl` to the list of registered proxy objects so we
      * can handle host callbacks. This function is called in
@@ -194,21 +204,26 @@ class Vst3PluginBridge : PluginBridge<Vst3Sockets<std::jthread>> {
      */
     Steinberg::IPtr<Vst3PluginFactoryProxyImpl> plugin_factory = nullptr;
 
-   public:
     /**
      * All VST3 plugin objects we created from this plugin. We keep track of
      * these in case the plugin does a host callback, so we can associate that
      * call with the exact host context object passed to it during a call to
      * `initialize()`. The IDs here are the same IDs as generated by the Wine
      * plugin host. An instance is added here through a call by
-     * `register_plugin_proxy()` in the constractor, and an instance is then
-     * removed through a call to `unregister_plugin_proxy()` in the destructor.
+     * `register_plugin_proxy()` in `Vst3PluginProxyImpl`'s constructor, and an
+     * instance is then removed through a call to `unregister_plugin_proxy()` in
+     * the destructor.
      */
     std::unordered_map<size_t, std::reference_wrapper<Vst3PluginProxyImpl>>
         plugin_proxies;
 
-   private:
-    std::mutex plugin_proxies_mutex;
+    /**
+     * In theory all object handling is safe iff the host also doesn't do
+     * anything weird even without locks, but we'll still prevent adding or
+     * removing instances while accessing other instances at the same time
+     * anyways. See `Vst3Bridge::object_instances_mutex` for more details.
+     */
+    std::shared_mutex plugin_proxies_mutex;
 
     /**
      * Used in `Vst3Bridge::send_mutually_recursive_message()` to be able to