Add a bridge for the Wine host to connect back

This is equivalent to plugin/bridge.h but for communicating between the
native Linus VST plugin and the Windows VST plugin.

meson.build

@@ -43,6 +43,7 @@ shared_library(
 executable(
   'yabridge-host',
   [
+    'src/wine-host/bridge.cpp',
     'src/wine-host/vst-host.cpp',
   ],
   native : false,

src/wine-host/bridge.cpp

@@ -0,0 +1,53 @@
+#include "bridge.h"
+
+/**
+ * A function pointer to what should be the entry point of a VST plugin.
+ */
+using VstEntryPoint = AEffect*(VST_CALL_CONV*)(audioMasterCallback);
+
+intptr_t VST_CALL_CONV
+host_callback(AEffect*, int32_t, int32_t, intptr_t, void*, float);
+
+Bridge::Bridge(std::string plugin_dll_path, std::string socket_endpoint_path)
+    : plugin_handle(LoadLibrary(plugin_dll_path.c_str()), &FreeLibrary),
+      io_context(),
+      socket_endpoint(socket_endpoint_path),
+      host_vst_dispatch(io_context) {
+    // Got to love these C APIs
+
+    // VST plugin entry point functions should be called `VSTPluginMain`, but
+    // there are some older deprecated names that legacy plugins may still use
+    VstEntryPoint vst_entry_point = nullptr;
+    for (auto name : {"VSTPluginMain", "MAIN", "main_plugin"}) {
+        vst_entry_point =
+            reinterpret_cast<VstEntryPoint>(reinterpret_cast<size_t>(
+                GetProcAddress(plugin_handle.get(), name)));
+
+        if (name != nullptr) {
+            break;
+        }
+    }
+    if (vst_entry_point == nullptr) {
+        throw std::runtime_error(
+            "Could not find a valid VST entry point for '" + plugin_dll_path +
+            "'.");
+    }
+
+    plugin = vst_entry_point(host_callback);
+    if (plugin == nullptr) {
+        throw std::runtime_error("VST plugin at '" + plugin_dll_path +
+                                 "' failed to initialize.");
+    }
+
+    host_vst_dispatch.connect(socket_endpoint);
+}
+
+// // TODO: Placeholder
+// intptr_t VST_CALL_CONV host_callback(AEffect* plugin,
+//                                      int32_t opcode,
+//                                      int32_t index,
+//                                      intptr_t value,
+//                                      void* data,
+//                                      float option) {
+//     return 1;
+// }

src/wine-host/bridge.h

@@ -0,0 +1,72 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020  Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#pragma once
+
+#include "native-includes.h"
+
+// `native-includes.h` has to be included before any other files as otherwise we
+// might get the wrong version of certain libraries
+#define WIN32_LEAN_AND_MEAN
+#include <vestige/aeffect.h>
+#include <windows.h>
+
+/**
+ * This handles the communication between the Linux native VST plugin and the
+ * Wine VST host. The functions below should be used as callback functions in an
+ * `AEffect` object.
+ */
+class Bridge {
+   public:
+    /**
+     * Initializes the Windows VST plugin and set up communication with the
+     * native Linux VST plugin.
+     *
+     * @param plugin_dll_path A (Unix style) path to the VST plugin .dll file to
+     *   load.
+     * @param socket_endpoint_path A (Unix style) path to the Unix socket
+     *   endpoint the native VST plugin created to communicate over.
+     *
+     * @throw std::runtime_error Thrown when the VST plugin could not be loaded,
+     *   or if communication could not be set up.
+     */
+    Bridge(std::string plugin_dll_path, std::string socket_endpoint_path);
+
+    // TODO: Set up all callback handlers
+
+   private:
+    /**
+     * The shared library handle of the VST plugin. I sadly could not get
+     * Boost.DLL to work here, so we'll just load the VST plugisn by hand.
+     */
+    std::unique_ptr<std::remove_pointer_t<HMODULE>, decltype(&FreeLibrary)>
+        plugin_handle;
+
+    /**
+     * The loaded plugin's `AEffect` struct, obtained using the above library
+     * handle.
+     */
+    AEffect* plugin;
+
+    boost::asio::io_context io_context;
+    boost::asio::local::stream_protocol::endpoint socket_endpoint;
+
+    // The naming convention for these sockets is `<from>_<to>_<event>`. For
+    // instance the socket named `host_vst_dispatch` forwards
+    // `AEffect.dispatch()` calls from the native VST host to the Windows VST
+    // plugin (through the Wine VST host).
+    boost::asio::local::stream_protocol::socket host_vst_dispatch;
+};

src/wine-host/vst-host.cpp

@@ -25,6 +25,7 @@
 #include <windows.h>
 
 #include "../common/communication.h"
+#include "bridge.h"
 
 /**
  * A function pointer to what should be the entry point of a VST plugin.