Fix #9117, 04ce1f07: [Fluidsynth] Infinite wait when stopping song (#9181)

In FluidSynth 2.2.0 an extra state was added to denote stopping. To transition
from this state to a stopped state the rendering needs to be running. Since
04ce1f07 locking was added that skipped the rendering when something else held
a lock, so the state would never get to stopped and join would never return.

(cherry picked from commit 6bd7f8816d)

src/music/fluidsynth.cpp

@@ -163,14 +163,21 @@ void MusicDriver_FluidSynth::PlaySong(const MusicSongInfo &song)
 
 void MusicDriver_FluidSynth::StopSong()
 {
+	{
 		std::lock_guard<std::mutex> lock{ _midi.synth_mutex };
 
 		if (!_midi.player) return;
 
 		fluid_player_stop(_midi.player);
+	}
+
+	/* The join must be run without lock as the Music rendering needs to be
+	 * running so FluidSynth's internals can actually stop the playing. */
 	if (fluid_player_join(_midi.player) != FLUID_OK) {
 		DEBUG(driver, 0, "Could not join player");
 	}
+
+	std::lock_guard<std::mutex> lock{ _midi.synth_mutex };
 	delete_fluid_player(_midi.player);
 	fluid_synth_system_reset(_midi.synth);
 	fluid_synth_all_sounds_off(_midi.synth, -1);