Allow serializing CLAP plugin factory list message

src/common/serialization/clap.h

@@ -36,8 +36,10 @@
  * request of type `ClapControlRequest(T)` should send back a `T::Response`.
  * These messages are for main thread functions.
  */
-// TODO: Remove placeholder, add actual types
-using ClapMainThreadControlRequest = std::variant<WantsConfiguration>;
+// FIXME: Remove the `WantsConfiguration`. For some reason bitsery just won't
+//        serialize this without it.
+using ClapMainThreadControlRequest =
+    std::variant<WantsConfiguration, clap::plugin_factory::List>;
 
 template <typename S>
 void serialize(S& s, ClapMainThreadControlRequest& payload) {
@@ -117,13 +119,12 @@ struct ClapAudioThreadControlRequest {
 // TODO: Placeholder
 using ClapMainThreadCallbackRequest = std::variant<WantsConfiguration>;
 
-// TODO: Uncomment after changing `ClapMainThreadCallbackRequest`
-// template <typename S>
-// void serialize(S& s, ClapMainThreadCallbackRequest& payload) {
-//     // All of the objects in `ClapMainThreadCallbackRequest` should have their own
-//     // serialization function.
-//     s.ext(payload, bitsery::ext::InPlaceVariant{});
-// }
+template <typename S>
+void serialize(S& s, ClapMainThreadCallbackRequest& payload) {
+    // All of the objects in `ClapMainThreadCallbackRequest` should have their
+    // own serialization function.
+    s.ext(payload, bitsery::ext::InPlaceVariant{});
+}
 
 /**
  * Fetch the `std::variant<>` from an audio thread request object. This will

src/common/serialization/clap/plugin-factory.h

@@ -26,21 +26,27 @@ namespace plugin_factory {
 /**
  * The response to the `clap::plugin_factory::list` message defined below.
  */
-struct list_response {
-    std::vector<clap::plugin::descriptor> descriptors;
+struct ListResponse {
+    /**
+     * The descriptors for the plugins in the factory. This will be a nullopt if
+     * the plugin does not support the plugin factory.
+     */
+    std::optional<std::vector<clap::plugin::descriptor>> descriptors;
 
     template <typename S>
     void serialize(S& s) {
-        s.container(descriptors, 8192);
+        s.ext(descriptors, bitsery::ext::InPlaceOptional{},
+              [](S& s, auto& v) { s.container(v, 8192); });
     }
 };
 
 /**
  * Message combining `clap_plugin_factory::count()` with
- * `clap_plugin_factory::get()` to get all plugin descriptors in one go.
+ * `clap_plugin_factory::get()` to get all plugin descriptors in one go. Will
+ * return a nullopt if the plugin does not support the plugin factory.
  */
-struct list {
-    using Response = list_response;
+struct List {
+    using Response = ListResponse;
 
     // Since we send this to a specific CLAP plugin library, there are no
     // parameters here