output/Thread: wake up the player thread periodically while playing

Without this, the pipe would run empty very often, which may result in
an xrun if the roundtrip to the PlayerThread and back takes too long.
By waking up the PlayerThread before the pipe runs empty, we make MPD
much more latency tolerant, which is a major optimization.

NEWS

@@ -52,6 +52,7 @@ ver 0.20 (not yet released)
   - soxr: allow multi-threaded resampling
 * player
   - reset song priority on playback
+  - reduce xruns
 * write database and state file atomically
 * always write UTF-8 to the log file.
 * remove dependency on GLib

src/output/OutputThread.cxx

@@ -493,10 +493,21 @@ AudioOutput::Play()
 		in_playback_loop = false;
 	};
 
+	unsigned n = 0;
+
 	do {
 		if (command != Command::NONE)
 			return true;
 
+		if (++n >= 64) {
+			/* wake up the player every now and then to
+			   give it a chance to refill the pipe before
+			   it runs empty */
+			const ScopeUnlock unlock(mutex);
+			player_control->LockSignal();
+			n = 0;
+		}
+
 		if (!PlayChunk(chunk))
 			break;