output/alsa: schedule a timer to generate silence

Without this timer, DispatchSockets() may disable the
MultiSocketMonitor and if Play() doesn't get called soon, it never
gets a chance to generate silence.  However if Play() gets called,
generating silence isn't necessary anymore...

Resulting from this misdesign (added by commit ccafe3f3cf3 in 0.21.3),
the silence generator didn't work reliably.

2 files changed, 40 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 542cd5d04..354491ea5 100644
--- a/NEWS
+++ b/NEWS
@@ -6,6 +6,7 @@ ver 0.21.11 (not yet released)
 * output
   - alsa: fix busy loop while draining
   - alsa: fix missing drain call
+  - alsa: improve xrun-avoiding silence generator
   - alsa, osx: fix distortions with DSD_U32 and DoP on 32 bit CPUs
 * protocol
   - fix "list" with multiple "group" levels
diff --git a/src/output/plugins/AlsaOutputPlugin.cxx b/src/output/plugins/AlsaOutputPlugin.cxx
index 96daac192..7e1919e43 100644
--- a/src/output/plugins/AlsaOutputPlugin.cxx
+++ b/src/output/plugins/AlsaOutputPlugin.cxx
@@ -56,6 +56,15 @@ class AlsaOutput final
 
 	DeferEvent defer_invalidate_sockets;
 
+	/**
+	 * This timer is used to re-schedule the #MultiSocketMonitor
+	 * after it had been disabled to wait for the next Play() call
+	 * to deliver more data.  This timer is necessary to start
+	 * generating silence if Play() doesn't get called soon enough
+	 * to avoid the xrun.
+	 */
+	TimerEvent silence_timer;
+
 	Manual<PcmExport> pcm_export;
 
 	/**
@@ -109,6 +118,8 @@ class AlsaOutput final
 	 */
 	snd_pcm_uframes_t period_frames;
 
+	std::chrono::steady_clock::duration effective_period_duration;
+
 	/**
 	 * If snd_pcm_avail() goes above this value and no more data
 	 * is available in the #ring_buffer, we need to play some
@@ -348,6 +359,19 @@ private:
 		cond.signal();
 	}
 
+	/**
+	 * Callback for @silence_timer
+	 */
+	void OnSilenceTimer() noexcept {
+		{
+			const std::lock_guard<Mutex> lock(mutex);
+			assert(active);
+			waiting = false;
+		}
+
+		MultiSocketMonitor::InvalidateSockets();
+	}
+
 	/* virtual methods from class MultiSocketMonitor */
 	std::chrono::steady_clock::duration PrepareSockets() noexcept override;
 	void DispatchSockets() noexcept override;
@@ -359,6 +383,7 @@ AlsaOutput::AlsaOutput(EventLoop &_loop, const ConfigBlock &block)
 	:AudioOutput(FLAG_ENABLE_DISABLE),
 	 MultiSocketMonitor(_loop),
 	 defer_invalidate_sockets(_loop, BIND_THIS_METHOD(InvalidateSockets)),
+	 silence_timer(_loop, BIND_THIS_METHOD(OnSilenceTimer)),
 	 device(block.GetBlockValue("device", "")),
 #ifdef ENABLE_DSD
 	 dop_setting(block.GetBlockValue("dop", false) ||
@@ -515,6 +540,7 @@ AlsaOutput::Setup(AudioFormat &audio_format,
 		alsa_period_size = 1;
 
 	period_frames = alsa_period_size;
+	effective_period_duration = audio_format.FramesToTime<decltype(effective_period_duration)>(period_frames);
 
 	/* generate silence if there's less than one period of data
 	   in the ALSA-PCM buffer */
@@ -865,6 +891,7 @@ AlsaOutput::CancelInternal() noexcept
 
 	MultiSocketMonitor::Reset();
 	defer_invalidate_sockets.Cancel();
+	silence_timer.Cancel();
 }
 
 void
@@ -893,6 +920,7 @@ AlsaOutput::Close() noexcept
 	BlockingCall(GetEventLoop(), [this](){
 			MultiSocketMonitor::Reset();
 			defer_invalidate_sockets.Cancel();
+			silence_timer.Cancel();
 		});
 
 	period_buffer.Free();
@@ -1029,6 +1057,17 @@ try {
 			if (!CopyRingToPeriodBuffer()) {
 				MultiSocketMonitor::Reset();
 				defer_invalidate_sockets.Cancel();
+
+				/* just in case Play() doesn't get
+				   called soon enough, schedule a
+				   timer which generates silence
+				   before the xrun occurs */
+				/* the timer fires in half of a
+				   period; this short duration may
+				   produce a few more wakeups than
+				   necessary, but should be small
+				   enough to avoid the xrun */
+				silence_timer.Schedule(effective_period_duration / 2);
 			}
 
 			return;