Call optional destroy() callback before unloading plugin library.

include/plugin/callbacks.hpp

@@ -2,9 +2,17 @@
 #include <plugin/Plugin.hpp>
 
 
-/** Called once to initialize and return the Plugin instance.
-You must implement this in your plugin
+/** Called immediately after loading your plugin.
+
+Use this to save `plugin` to a global variable and add Models to it.
+Required in plugins.
 */
-extern "C" {
+extern "C"
 void init(rack::plugin::Plugin* plugin);
-}
+
+/** Called before your plugin library is unloaded.
+
+Optional in plugins.
+*/
+extern "C"
+void destroy();

src/plugin.cpp

@@ -247,8 +247,22 @@ void init() {
 
 void destroy() {
 	for (Plugin* plugin : plugins) {
-		// We must delete the plugin *before* freeing the library, because the vtable of Model subclasses are static in the plugin, and we need it for the virtual destructor.
 		void* handle = plugin->handle;
+
+		// Call destroy() if defined in the plugin library
+		typedef void (*DestroyCallback)();
+		DestroyCallback destroyCallback = NULL;
+		if (handle) {
+#if defined ARCH_WIN
+			destroyCallback = (DestroyCallback) GetProcAddress((HMODULE) handle, "destroy");
+#else
+			destroyCallback = (DestroyCallback) dlsym(handle, "destroy");
+#endif
+		}
+		if (destroyCallback)
+			destroyCallback();
+
+		// We must delete the Plugin instance *before* freeing the library, because the vtables of Model subclasses are defined in the library, which are needed in the Plugin destructor.
 		delete plugin;
 
 		// Free library handle