Prevent data races in host callbacks

src/wine-host/plugin-bridge.cpp

@@ -268,8 +268,8 @@ intptr_t PluginBridge::host_callback(AEffect* /*plugin*/,
                                      void* data,
                                      float option) {
     HostCallbackDataConverter converter(plugin, time_info);
-    return send_event(vst_host_callback, converter, opcode, index, value, data,
-                      option, std::nullopt);
+    return send_event(vst_host_callback, host_callback_semaphore, converter,
+                      opcode, index, value, data, option, std::nullopt);
 }
 
 intptr_t VST_CALL_CONV host_callback_proxy(AEffect* effect,

src/wine-host/plugin-bridge.h

@@ -27,6 +27,7 @@
 #include <windows.h>
 
 #include <boost/asio/local/stream_protocol.hpp>
+#include <mutex>
 #include <thread>
 
 #include "../common/logging.h"
@@ -115,6 +116,13 @@ class PluginBridge {
      */
     std::thread process_replacing_handler;
 
+    /**
+     * A binary semaphore to prevent race conditions from the host callback
+     * function being called by two threads at once. See `send_event()` for more
+     * information.
+     */
+    std::mutex host_callback_semaphore;
+
     /**
      * A scratch buffer for sending and receiving data during `process` and
      * `processReplacing` calls.