Prevent a deadlock in plugin group handling

This was a nasty race condition that only seemed to pop up with Spitfire
plugins in REAPER, but it could also happen elsewhere. The
`active_plugins_mutex` was getting locked from the message loop, but the
plugin would block until `effOpen()` had been called. But because the
mutex was locked by the message loop we would never get to handling
`effOpen()`. Passing the pointer directly both removes this unnecessary
locking and fixes the issue.

CHANGELOG.md

@@ -6,6 +6,15 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic
 Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- Fixed some plugins, notably the _Spitfire Audio_ plugins, from causing a
+  deadlock when using plugin groups in _REAPER_. Even though this did not seem
+  to cause any issues in other hosts, the race condition that caused this issue
+  could also occur elsewhere.
+
 ## [2.2.0] - 2020-12-11
 
 ### Added

src/wine-host/bridges/group.cpp

@@ -105,17 +105,8 @@ GroupBridge::~GroupBridge() {
     stdio_context.stop();
 }
 
-void GroupBridge::handle_plugin_dispatch(size_t plugin_id) {
-    // At this point the `active_plugins` map will already contain the
-    // intialized plugin's `Vst2Bridge` instance and this thread's handle
-    Vst2Bridge* bridge;
-    {
-        std::lock_guard lock(active_plugins_mutex);
-        bridge = active_plugins[plugin_id].second.get();
-    }
-
-    // Blocks this thread until the plugin shuts down, handling all events on
-    // the main IO context
+void GroupBridge::handle_plugin_dispatch(size_t plugin_id, Vst2Bridge* bridge) {
+    // Blocks this thread until the plugin shuts down
     bridge->handle_dispatch();
     logger.log("'" + bridge->vst_plugin_path.string() + "' has exited");
 
@@ -202,16 +193,22 @@ void GroupBridge::accept_requests() {
                            "'");
 
                 // Start listening for dispatcher events sent to the plugin's
-                // socket on another thread. The actual event handling will
-                // still be posted to this IO context so that every plugin's
-                // primary event handling happens on the main thread. Since this
-                // is only used within this context we don't need any locks.
+                // socket on another thread. Parts of the actual event handling
+                // will still be posted to this IO context so that any events
+                // that potentially interact with the Win32 message loop are
+                // handled from the main thread. We also pass a raw pointer to
+                // the plugin so we don't have to immediately look the instance
+                // up in the map again, as this would require us to immediately
+                // lock the map again. This could otherwise result in a deadlock
+                // when using the Spitfire plugins, as they will block the
+                // message loop until `effOpen()` has been called and thus
+                // prevent this lock from happening.
                 const size_t plugin_id = next_plugin_id.fetch_add(1);
-                active_plugins[plugin_id] =
-                    std::pair(Win32Thread([this, plugin_id]() {
-                                  handle_plugin_dispatch(plugin_id);
-                              }),
-                              std::move(bridge));
+                active_plugins[plugin_id] = std::pair(
+                    Win32Thread([this, plugin_id, plugin_ptr = bridge.get()]() {
+                        handle_plugin_dispatch(plugin_id, plugin_ptr);
+                    }),
+                    std::move(bridge));
             } catch (const std::runtime_error& error) {
                 logger.log("Error while initializing '" + request.plugin_path +
                            "':");

src/wine-host/bridges/group.h

@@ -139,15 +139,14 @@ class GroupBridge {
      * This check is delayed by a few seconds to prevent having to constantly
      * restart the group process during plugin scanning.
      *
-     * @param plugin_id The ID of this plugin in the `active_plugins` map. The
-     *   thread can fetch the plugin's `Vst2Bridge` instance from that map using
-     *   this identifier.
+     * @param plugin_id The ID of this plugin in the `active_plugins` map. Used
+     *   to unload the plugin and join this thread again after the plugin exits.
      *
      * @note In the case that the process starts but no plugin gets initiated,
      *   then the process will never exit on its own. This should not happen
      *   though.
      */
-    void handle_plugin_dispatch(size_t plugin_id);
+    void handle_plugin_dispatch(size_t plugin_id, Vst2Bridge* bridge);
 
     /**
      * Listen for new requests to spawn plugins within this process and handle