Fix VstTimeinfo responses and allow null responses

The host is allowed to return a null pointer if it doesn't support the
query.

README.md

@@ -7,8 +7,6 @@ Yet Another way to use Windows VST2 plugins in Linux VST hosts.
 There are a few things that should be done before releasing this, including:
 
 - Fix implementation bugs:
-  - `VstTimeInfo` doesn't get passed to the plugin correctly in
-    `audioMasterGetTime`.
   - Polish GUIs even further. There are some todos left in
     `src/wine-host/editor.{h,cpp}`.
   - There are likely some minor issues left.

src/common/events.h

@@ -285,8 +285,15 @@ void passthrough_event(boost::asio::local::stream_protocol::socket& socket,
                      // Not sure why the VST API has twenty different ways of
                      // returning structs, but in this case the value returned
                      // from the callback function is actually a pointer to a
-                     // `VstTimeInfo` struct!
+                     // `VstTimeInfo` struct! It can also be a null pointer if
-                     return *reinterpret_cast<const VstTimeInfo*>(return_value);
+                     // the host doesn't support this.
+                     const auto time_info =
+                         reinterpret_cast<const VstTimeInfo*>(return_value);
+                     if (time_info == nullptr) {
+                         return std::monostate{};
+                     } else {
+                         return *time_info;
+                     }
                  },
                  [&](WantsString&) -> EventResposnePayload {
                      return std::string(static_cast<char*>(data));

src/common/serialization.h

@@ -295,6 +295,9 @@ struct Event {
  * - A specific struct in response to an event such as `audioMasterGetTime` or
  *   `audioMasterIOChanged`.
  * - An X11 window pointer for the editor window.
+ *
+ * TODO: Replace `std::monostate` with `std::nullptr_t` as it's more expressive
+ *       in what it actually represents.
  */
 using EventResposnePayload = std::variant<std::monostate,
                                           std::string,

src/wine-host/editor.cpp

@@ -130,7 +130,9 @@ void Editor::handle_events() {
 
         // Handle X11 events
         // TODO: Check if we should forward other events mostly to prevent
-        //       unnecessary GUI processing in the background
+        //       unnecessary GUI processing in the background. Since
+        //       `effEditIdle` should only be called when the plugin's editor is
+        //       open this should not cause any different in CPU though.
         // TODO: Check whether drag and drop works out of the box
         xcb_generic_event_t* generic_event;
         while ((generic_event = xcb_poll_for_event(x11_connection.get())) !=

src/wine-host/plugin-bridge.cpp

@@ -272,7 +272,7 @@ class HostCallbackDataConverter : DefaultDataConverter {
    public:
     HostCallbackDataConverter(AEffect* plugin,
                               Editor& editor,
-                              VstTimeInfo& time_info)
+                              std::optional<VstTimeInfo>& time_info)
         : plugin(plugin), editor(editor), time_info(time_info) {}
 
     std::optional<EventPayload> read(const int opcode,
@@ -314,8 +314,15 @@ class HostCallbackDataConverter : DefaultDataConverter {
         switch (opcode) {
             case audioMasterGetTime:
                 // Write the returned `VstTimeInfo` struct into a field and make
-                // the function return a poitner to it in the function below
+                // the function return a poitner to it in the function below.
-                time_info = std::get<VstTimeInfo>(response.payload);
+                // Depending on whether the host supported the requested time
+                // information this operations returns either a null pointer or
+                // a pointer to a `VstTimeInfo` object.
+                if (std::holds_alternative<std::monostate>(response.payload)) {
+                    time_info = std::nullopt;
+                } else {
+                    time_info = std::get<VstTimeInfo>(response.payload);
+                }
                 break;
             default:
                 DefaultDataConverter::write(opcode, data, response);
@@ -325,12 +332,16 @@ class HostCallbackDataConverter : DefaultDataConverter {
 
     intptr_t return_value(const int opcode, const intptr_t original) {
         switch (opcode) {
-            case audioMasterGetTime:
+            case audioMasterGetTime: {
                 // Return a pointer to the `VstTimeInfo` object written in the
                 // function above
-                // TODO: This is incorrect!
+                VstTimeInfo* time_info_pointer = nullptr;
-                return reinterpret_cast<intptr_t>(&time);
+                if (time_info.has_value()) {
-                break;
+                    time_info_pointer = &time_info.value();
+                }
+
+                return reinterpret_cast<intptr_t>(time_info_pointer);
+            } break;
             default:
                 return DefaultDataConverter::return_value(opcode, original);
                 break;
@@ -341,7 +352,7 @@ class HostCallbackDataConverter : DefaultDataConverter {
     AEffect* plugin;
     // TODO: Clean up
     Editor& editor;
-    VstTimeInfo& time_info;
+    std::optional<VstTimeInfo>& time_info;
 };
 
 intptr_t PluginBridge::host_callback(AEffect* effect,

src/wine-host/plugin-bridge.h

@@ -67,9 +67,11 @@ class PluginBridge {
 
     /**
      * With the `audioMasterGetTime` host callback the plugin expects the return
-     * value from the calblack to be a pointer to a VstTimeInfo struct.
+     * value from the calblack to be a pointer to a VstTimeInfo struct. If the
+     * host did not support a certain time info query, than we'll store the
+     * returned null pointer as a nullopt.
      */
-    VstTimeInfo time_info;
+    std::optional<VstTimeInfo> time_info;
 
    private:
     /**