Add the ability to override writing back data

src/common/communication.h

@@ -109,25 +109,38 @@ inline T read_object(Socket& socket) {
 }
 
 /**
- * Encodes the base behavior for encoding the contents of the `data` argument
+ * Encodes the base behavior for reading from and writing to the `data` argument
  * for event dispatch functions. This is sufficient for host callbacks
  * (`audioMaster()`). The `dispatch()` function will require some more specific
  * structs.
  */
 struct DefaultDataConverter {
-    EventPayload operator()(int /*opcode*/, void* data) {
+    EventPayload read(const int /*opcode*/, const void* data) {
         if (data == nullptr) {
             return nullptr;
         }
 
         // Assume buffers are zeroed out, this is probably not the case
-        char* c_string = static_cast<char*>(data);
+        const char* c_string = static_cast<const char*>(data);
         if (c_string[0] != 0) {
             return std::string(c_string);
         } else {
             return WantsString{};
         }
     }
+
+    void write(const int /*opcode*/, void* data, const EventResult& response) {
+        if (response.data.has_value()) {
+            char* output = static_cast<char*>(data);
+
+            // For correctness we will copy the entire buffer and add a
+            // terminating null byte ourselves. In practice `response.data` will
+            // only ever contain C-style strings, but this would work with any
+            // other data format that can contain null bytes.
+            std::copy(response.data->begin(), response.data->end(), output);
+            output[response.data->size()] = 0;
+        }
+    }
 };
 
 /**
@@ -136,21 +149,21 @@ struct DefaultDataConverter {
  * since they follow the same format. See one of those functions for details on
  * the parameters and return value of this function.
  *
- * @tparam DataConverter how the `data` void pointer should be converted to a
- *   serializable type. For host callbacks this parameter contains either a
- *   string or a null pointer while `dispatch()` calls might contain opcode
- *   specific structs. See the documentation for `EventPayload` for more
- *   information. The `DefaultDataConverter` defined above handles the basic
- *   behavior that's sufficient for hsot callbacks.
- *
+ * @param data_converter Some struct that knows how to read data from and write
+ *   data back to the `data` void pointer. For host callbacks this parameter
+ *   contains either a string or a null pointer while `dispatch()` calls might
+ *   contain opcode specific structs. See the documentation for `EventPayload`
+ *   for more information. The `DefaultDataConverter` defined above handles the
+ *   basic behavior that's sufficient for hsot callbacks.
  * @param logging A pair containing a logger instance and whether or not this is
  *   for sending `dispatch()` events or host callbacks. Optional since it
  *   doesn't have to be done on both sides.
  *
  * @relates passthrough_event
  */
-template <typename DataConverter>
+template <typename D>
 intptr_t send_event(boost::asio::local::stream_protocol::socket& socket,
+                    D& data_converter,
                     int opcode,
                     int index,
                     intptr_t value,
@@ -159,7 +172,7 @@ intptr_t send_event(boost::asio::local::stream_protocol::socket& socket,
                     std::optional<std::pair<Logger&, bool>> logging) {
     // Encode the right payload type for this event. Check the documentation for
     // `EventPayload` for more information.
-    EventPayload payload = DataConverter{}(opcode, data);
+    const EventPayload payload = data_converter.read(opcode, data);
 
     if (logging.has_value()) {
         auto [logger, is_dispatch] = *logging;
@@ -170,21 +183,14 @@ intptr_t send_event(boost::asio::local::stream_protocol::socket& socket,
     write_object(socket, event);
 
     const auto response = read_object<EventResult>(socket);
+
     if (logging.has_value()) {
         auto [logger, is_dispatch] = *logging;
         logger.log_event_response(is_dispatch, response.return_value,
                                   response.data);
     }
-    if (response.data.has_value()) {
-        char* output = static_cast<char*>(data);
 
-        // For correctness we will copy the entire buffer and add a terminating
-        // null byte ourselves. In practice `response.data` will only ever
-        // contain C-style strings, but this would work with any other data
-        // format that can contain null bytes.
-        std::copy(response.data->begin(), response.data->end(), output);
-        output[response.data->size()] = 0;
-    }
+    data_converter.write(opcode, data, response);
 
     return response.return_value;
 }

src/plugin/host-bridge.cpp

@@ -138,16 +138,16 @@ HostBridge::HostBridge(audioMasterCallback host_callback)
 }
 
 struct DispatchDataConverter : DefaultDataConverter {
-    EventPayload operator()(int opcode, void* data) {
+    EventPayload read(const int opcode, const void* data) {
         // There are some events that need specific structs that we can't simply
         // serialize as a string because they might contain null bytes
         // TODO: More of these structs
         switch (opcode) {
             case effProcessEvents:
-                return DynamicVstEvents(*static_cast<VstEvents*>(data));
+                return DynamicVstEvents(*static_cast<const VstEvents*>(data));
                 break;
             default:
-                return DefaultDataConverter{}(opcode, data);
+                return DefaultDataConverter::read(opcode, data);
                 break;
         }
     }
@@ -183,9 +183,9 @@ intptr_t HostBridge::dispatch(AEffect* /*plugin*/,
     }
 
     // TODO: Maybe reuse buffers here when dealing with chunk data
-    return send_event<DispatchDataConverter>(
-        host_vst_dispatch, opcode, index, value, data, option,
-        std::pair<Logger&, bool>(logger, true));
+    DispatchDataConverter converter;
+    return send_event(host_vst_dispatch, converter, opcode, index, value, data,
+                      option, std::pair<Logger&, bool>(logger, true));
 }
 
 void HostBridge::process_replacing(AEffect* /*plugin*/,

src/wine-host/plugin-bridge.cpp

@@ -195,8 +195,9 @@ intptr_t PluginBridge::host_callback(AEffect* /*plugin*/,
                                      intptr_t value,
                                      void* data,
                                      float option) {
-    return send_event<DefaultDataConverter>(vst_host_callback, opcode, index,
-                                            value, data, option, std::nullopt);
+    DefaultDataConverter converter;
+    return send_event(vst_host_callback, converter, opcode, index, value, data,
+                      option, std::nullopt);
 }
 
 intptr_t VST_CALL_CONV host_callback_proxy(AEffect* effect,