output/MultipleOutputs: parallelize AudioOutput destruction

src/output/Internal.hxx

@@ -283,7 +283,8 @@ public:
 	void StartThread();
 	void StopThread();
 
-	void Finish();
+	void BeginDestroy();
+	void FinishDestroy();
 
 	bool IsOpen() const {
 		return open;

src/output/MultipleOutputs.cxx

@@ -43,8 +43,11 @@ MultipleOutputs::MultipleOutputs(MixerListener &_mixer_listener)
 
 MultipleOutputs::~MultipleOutputs()
 {
+	/* parallel destruction */
 	for (auto i : outputs)
-		i->Finish();
+		i->BeginDestroy();
+	for (auto i : outputs)
+		i->FinishDestroy();
 }
 
 static AudioOutput *

src/output/OutputControl.cxx

@@ -274,13 +274,22 @@ AudioOutput::StopThread()
 }
 
 void
-AudioOutput::Finish()
+AudioOutput::BeginDestroy()
 {
 	if (mixer != nullptr)
 		mixer_auto_close(mixer);
 
+	if (thread.IsDefined()) {
+		const ScopeLock protect(mutex);
+		CommandAsync(Command::KILL);
+	}
+}
+
+void
+AudioOutput::FinishDestroy()
+{
 	if (thread.IsDefined())
-		StopThread();
+		thread.Join();
 
 	audio_output_free(this);
 }