Fix concurrency issue in plugin group shutdown

CHANGELOG.md

@@ -35,6 +35,8 @@ Versioning](https://semver.org/spec/v2.0.0.html).
   plugin to shut down. This could happen when using Kontakt in Bitwig, as Bitwig
   sets a limit on the amount of time a plugin may take to shut down when closing
   Bitwig.
+- Fixed a potential crash or freeze when removing a lot of plugins from a plugin
+  group at exactly the same time.
 
 ## [2.1.0] - 2020-11-20
 

src/wine-host/bridges/group.cpp

@@ -135,8 +135,9 @@ void GroupBridge::handle_plugin_dispatch(size_t plugin_id) {
 
     // Defer actually shutting down the process to allow for fast plugin
     // scanning by allowing plugins to reuse the same group host process
+    std::lock_guard lock(shutdown_timer_mutex);
     shutdown_timer.expires_after(2s);
-    shutdown_timer.async_wait([&](const boost::system::error_code& error) {
+    shutdown_timer.async_wait([this](const boost::system::error_code& error) {
         // A previous timer gets canceled automatically when another plugin
         // exits
         if (error.failed()) {

src/wine-host/bridges/group.h

@@ -276,4 +276,9 @@ class GroupBridge {
      * @see handle_plugin_dispatch
      */
     boost::asio::steady_timer shutdown_timer;
+    /**
+     * A mutex to prevent two threads from simultaneously modifying the shutdown
+     * timer when multiple plugins exit at the same time.
+     */
+    std::mutex shutdown_timer_mutex;
 };