Postpone clearing old MIDI events until next event

This fixes Native Instrument's FM7 crashing on MIDI input. The plugin
expects the last received MIDI event to always be alive during audio
processing, even if there have not been any new events in this
processing cycle.

CHANGELOG.md

@@ -14,6 +14,12 @@ Versioning](https://semver.org/spec/v2.0.0.html).
   at configure-time. This can make it a bit easier to diagnose Wine-related
   compilation issues.
 
+### Fixed
+
+- Prevent _Native Instruments' FM7_ from crashing when processing MIDI. As a
+  fix, MIDI events are now deallocated later then when they normally would have
+  to be.
+
 ## [3.1.0] - 2021-04-15
 
 ### Added

src/wine-host/bridges/vst2.cpp

@@ -278,7 +278,9 @@ Vst2Bridge::Vst2Bridge(MainContext& main_context,
                         }},
                     request.buffers);
 
-                next_audio_buffer_midi_events.clear();
+                // See the docstrong on `should_clear_midi_events` for why we
+                // don't just clear `next_buffer_midi_events` here
+                should_clear_midi_events = true;
             });
     });
 }
@@ -303,6 +305,14 @@ void Vst2Bridge::run() {
                 // plugin.
                 std::lock_guard lock(next_buffer_midi_events_mutex);
 
+                // See the docstring on `should_clear_midi_events` for why we
+                // only deallocate old MIDI events here instead of a at the end
+                // of every processing cycle
+                if (should_clear_midi_events) {
+                    next_audio_buffer_midi_events.clear();
+                    should_clear_midi_events = false;
+                }
+
                 next_audio_buffer_midi_events.push_back(
                     std::get<DynamicVstEvents>(event.payload));
                 DynamicVstEvents& events = next_audio_buffer_midi_events.back();

src/wine-host/bridges/vst2.h

@@ -168,6 +168,16 @@ class Vst2Bridge : public HostBridge {
      * at least until the next audio buffer gets processed.
      */
     std::vector<DynamicVstEvents> next_audio_buffer_midi_events;
+    /**
+     * Whether `next_audio_buffer_midi_events` should be cleared before
+     * inserting new events.
+     *
+     * HACK: Normally we should be able to clear these immediately after the
+     *       processing call, but Native Instruments' FM7 requires the last MIDI
+     *       event to stay alive if there have not been any new MIDI events
+     *       during the current processing cycle.
+     */
+    bool should_clear_midi_events = false;
     /**
      * Mutex for locking the above event queue, since recieving and processing
      * now happens in two different threads.