{output,mixer}/alsa: use snd_pcm_poll_descriptors_revents()

This call was missing, causing very high CPU usage when the ALSA
output plugin was used with dmix.

Closes #391

NEWS

@@ -3,6 +3,7 @@ ver 0.21.2 (not yet released)
   - ffmpeg: require FFmpeg 3.1 or later
   - ffmpeg: fix broken sound with certain codecs
 * output
+  - alsa: fix high CPU usage with dmix
   - httpd: fix two crash bugs
 * mixer
   - alsa: fix more rounding errors

src/input/plugins/AlsaInputPlugin.cxx

@@ -185,6 +185,8 @@ AlsaInputStream::PrepareSockets() noexcept
 void
 AlsaInputStream::DispatchSockets() noexcept
 {
+	non_block.DispatchSockets(*this, capture_handle);
+
 	const std::lock_guard<Mutex> protect(mutex);
 
 	auto w = PrepareWriteBuffer();

src/lib/alsa/NonBlock.cxx

@@ -49,6 +49,30 @@ AlsaNonBlockPcm::PrepareSockets(MultiSocketMonitor &m, snd_pcm_t *pcm)
 	return std::chrono::steady_clock::duration(-1);
 }
 
+void
+AlsaNonBlockPcm::DispatchSockets(MultiSocketMonitor &m,
+				 snd_pcm_t *pcm) noexcept
+{
+	int count = snd_pcm_poll_descriptors_count(pcm);
+	if (count <= 0)
+		return;
+
+	const auto pfds = pfd_buffer.Get(count), end = pfds + count;
+
+	auto *i = pfds;
+	m.ForEachReturnedEvent([&i, end](SocketDescriptor s, unsigned events){
+			if (i >= end)
+				return;
+
+			i->fd = s.Get();
+			i->events = i->revents = events;
+			++i;
+		});
+
+	unsigned short dummy;
+	snd_pcm_poll_descriptors_revents(pcm, pfds, i - pfds, &dummy);
+}
+
 std::chrono::steady_clock::duration
 AlsaNonBlockMixer::PrepareSockets(MultiSocketMonitor &m, snd_mixer_t *mixer) noexcept
 {
@@ -67,3 +91,27 @@ AlsaNonBlockMixer::PrepareSockets(MultiSocketMonitor &m, snd_mixer_t *mixer) noe
 	m.ReplaceSocketList(pfds, count);
 	return std::chrono::steady_clock::duration(-1);
 }
+
+void
+AlsaNonBlockMixer::DispatchSockets(MultiSocketMonitor &m,
+				   snd_mixer_t *mixer) noexcept
+{
+	int count = snd_mixer_poll_descriptors_count(mixer);
+	if (count <= 0)
+		return;
+
+	const auto pfds = pfd_buffer.Get(count), end = pfds + count;
+
+	auto *i = pfds;
+	m.ForEachReturnedEvent([&i, end](SocketDescriptor s, unsigned events){
+			if (i >= end)
+				return;
+
+			i->fd = s.Get();
+			i->events = i->revents = events;
+			++i;
+		});
+
+	unsigned short dummy;
+	snd_mixer_poll_descriptors_revents(mixer, pfds, i - pfds, &dummy);
+}

src/lib/alsa/NonBlock.hxx

@@ -42,6 +42,12 @@ public:
 	 */
 	std::chrono::steady_clock::duration PrepareSockets(MultiSocketMonitor &m,
 							   snd_pcm_t *pcm);
+
+	/**
+	 * Wrapper for snd_pcm_poll_descriptors_revents(), to be
+	 * called from MultiSocketMonitor::DispatchSockets().
+	 */
+	void DispatchSockets(MultiSocketMonitor &m, snd_pcm_t *pcm) noexcept;
 };
 
 /**
@@ -54,6 +60,12 @@ class AlsaNonBlockMixer {
 public:
 	std::chrono::steady_clock::duration PrepareSockets(MultiSocketMonitor &m,
 							   snd_mixer_t *mixer) noexcept;
+
+	/**
+	 * Wrapper for snd_mixer_poll_descriptors_revents(), to be
+	 * called from MultiSocketMonitor::DispatchSockets().
+	 */
+	void DispatchSockets(MultiSocketMonitor &m, snd_mixer_t *mixer) noexcept;
 };
 
 #endif

src/mixer/plugins/AlsaMixerPlugin.cxx

@@ -117,6 +117,8 @@ AlsaMixerMonitor::DispatchSockets() noexcept
 {
 	assert(mixer != nullptr);
 
+	non_block.DispatchSockets(*this, mixer);
+
 	int err = snd_mixer_handle_events(mixer);
 	if (err < 0) {
 		FormatError(alsa_mixer_domain,

src/output/plugins/AlsaOutputPlugin.cxx

@@ -889,6 +889,8 @@ AlsaOutput::PrepareSockets() noexcept
 void
 AlsaOutput::DispatchSockets() noexcept
 try {
+	non_block.DispatchSockets(*this, pcm);
+
 	{
 		const std::lock_guard<Mutex> lock(mutex);