Add JACK sample rate callback

This will be necessary once we start the decoding thread, which calls
GetSampleRate a lot.

src/arch/Sound/RageSoundDriver_JACK.cpp

@@ -31,7 +31,8 @@ RString RageSoundDriver_JACK::Init()
 	if (client == NULL)
 		return "Couldn't connect to JACK server";
 
-	LOG->Trace("JACK connected at %u Hz", jack_get_sample_rate(client));
+	sample_rate = jack_get_sample_rate(client);
+	LOG->Trace("JACK connected at %u Hz", sample_rate);
 
 	// Set callback for processing audio
 	if (jack_set_process_callback(client, ProcessTrampoline, this))
@@ -40,6 +41,12 @@ RString RageSoundDriver_JACK::Init()
 		goto out_close;
 	}
 
+	if (jack_set_sample_rate_callback(client, SampleRateTrampoline, this))
+	{
+		error = "Couldn't set JACK sample-rate callback";
+		goto out_close;
+	}
+
 	// TODO Set a jack_on_shutdown callback as well?  Probably just stop
 	// caring about sound altogether if that happens.
 
@@ -136,7 +143,9 @@ int64_t RageSoundDriver_JACK::GetPosition() const
 
 int RageSoundDriver_JACK::GetSampleRate() const
 {
-	return jack_get_sample_rate(client);
+	// For now, let's pretend there isn't a race condition between this and
+	// SampleRateCallback().
+	return sample_rate;
 }
 
 int RageSoundDriver_JACK::ProcessCallback(jack_nframes_t nframes)
@@ -152,12 +161,25 @@ int RageSoundDriver_JACK::ProcessCallback(jack_nframes_t nframes)
 	return 0;
 }
 
+int RageSoundDriver_JACK::SampleRateCallback(jack_nframes_t nframes)
+{
+	// For now, let's pretend there isn't a race condition between this and
+	// GetSampleRate().
+	sample_rate = jack_get_sample_rate(client);
+	return 0;
+}
+
 // Static callback trampoline
 int RageSoundDriver_JACK::ProcessTrampoline(jack_nframes_t nframes, void *arg)
 {
 	return ((RageSoundDriver_JACK *) arg)->ProcessCallback(nframes);
 }
 
+int RageSoundDriver_JACK::SampleRateTrampoline(jack_nframes_t nframes, void *arg)
+{
+	return ((RageSoundDriver_JACK *) arg)->SampleRateCallback(nframes);
+}
+
 /*
  * (c) 2013 Devin J. Pohly
  * All rights reserved.

src/arch/Sound/RageSoundDriver_JACK.h

@@ -22,12 +22,16 @@ private:
 	jack_port_t *port_l;
 	jack_port_t *port_r;
 
+	int sample_rate;
+
 	// Helper for Init()
 	RString ConnectPorts();
 
 	// JACK callbacks and trampolines
 	int ProcessCallback(jack_nframes_t nframes);
 	static int ProcessTrampoline(jack_nframes_t nframes, void *arg);
+	int SampleRateCallback(jack_nframes_t nframes);
+	static int SampleRateTrampoline(jack_nframes_t nframes, void *arg);
 };
 
 #endif