output/alsa: use a new I/O thread with real-time scheduling

The normal I/O event thread can have a large latency, e.g. when
libgnutls loads all TLS CA certificates for a https connect.  This
makes it unreliable for the ALSA I/O notifications, and causes ring
buffer xruns.  To avoid interfering with high latency events such as
CURL's, we move the ALSA I/O events to a separate I/O thread which
also obtains real-time scheduling (if possible).

Closes #221

src/Instance.cxx

@@ -40,7 +40,8 @@
 #include <exception>
 
 Instance::Instance()
-	:idle_monitor(event_loop, BIND_THIS_METHOD(OnIdle))
+	:rtio_thread(true),
+	 idle_monitor(event_loop, BIND_THIS_METHOD(OnIdle))
 {
 }
 

src/Instance.hxx

@@ -76,8 +76,19 @@ struct Instance final
 	, public RemoteTagCacheHandler
 #endif
 {
+	/**
+	 * A thread running an #EventLoop for non-blocking (bulk) I/O.
+	 */
 	EventThread io_thread;
 
+	/**
+	 * Another thread running an #EventLoop for non-blocking
+	 * (real-time) I/O.  This is used instead of #io_thread for
+	 * events which require low latency, e.g. for filling hardware
+	 * ring buffers.
+	 */
+	EventThread rtio_thread;
+
 	MaskMonitor idle_monitor;
 
 #ifdef ENABLE_NEIGHBOR_PLUGINS

src/Main.cxx

@@ -604,7 +604,7 @@ try {
 	command_init();
 
 	for (auto &partition : instance->partitions) {
-		partition.outputs.Configure(instance->io_thread.GetEventLoop(),
+		partition.outputs.Configure(instance->rtio_thread.GetEventLoop(),
 					    config.replay_gain,
 					    partition.pc);
 		partition.UpdateEffectiveReplayGainMode();
@@ -625,6 +625,7 @@ try {
 #endif
 
 	instance->io_thread.Start();
+	instance->rtio_thread.Start();
 
 #ifdef ENABLE_NEIGHBOR_PLUGINS
 	if (instance->neighbors != nullptr)
@@ -736,6 +737,7 @@ try {
 	archive_plugin_deinit_all();
 #endif
 	config_global_finish();
+	instance->rtio_thread.Stop();
 	instance->io_thread.Stop();
 #ifndef ANDROID
 	SignalHandlersFinish();

src/event/Thread.cxx

@@ -20,6 +20,8 @@
 #include "config.h"
 #include "Thread.hxx"
 #include "thread/Name.hxx"
+#include "thread/Util.hxx"
+#include "Log.hxx"
 
 void
 EventThread::Start()
@@ -41,7 +43,16 @@ EventThread::Stop() noexcept
 void
 EventThread::Run() noexcept
 {
-	SetThreadName("io");
+	SetThreadName(realtime ? "rtio" : "io");
+
+	if (realtime) {
+		try {
+			SetThreadRealtime();
+		} catch (...) {
+			LogError(std::current_exception(),
+				 "RTIOThread could not get realtime scheduling, continuing anyway");
+		}
+	}
 
 	event_loop.Run();
 }

src/event/Thread.hxx

@@ -32,9 +32,12 @@ class EventThread final {
 
 	Thread thread;
 
+	const bool realtime;
+
 public:
-	EventThread()
-		:event_loop(ThreadId::Null()), thread(BIND_THIS_METHOD(Run)) {}
+	explicit EventThread(bool _realtime=false)
+		:event_loop(ThreadId::Null()), thread(BIND_THIS_METHOD(Run)),
+		 realtime(_realtime) {}
 
 	~EventThread() noexcept {
 		Stop();