OutputControl: reduce the number of OutputThread wakeups

src/OutputControl.cxx

@@ -248,8 +248,11 @@ audio_output_play(struct audio_output *ao)
 
 	assert(ao->allow_play);
 
-	if (audio_output_is_open(ao))
+	if (audio_output_is_open(ao) && !ao->in_playback_loop &&
+	    !ao->woken_for_play) {
+		ao->woken_for_play = true;
 		ao->cond.signal();
+	}
 }
 
 void audio_output_pause(struct audio_output *ao)

src/OutputInit.cxx

@@ -171,6 +171,8 @@ ao_base_init(struct audio_output *ao,
 	ao->open = false;
 	ao->pause = false;
 	ao->allow_play = true;
+	ao->in_playback_loop = false;
+	ao->woken_for_play = false;
 	ao->fail_timer = nullptr;
 
 	/* set up the filter chain */

src/OutputInternal.hxx

@@ -128,6 +128,21 @@ struct audio_output {
 	bool allow_play;
 
 	/**
+	 * True while the OutputThread is inside ao_play().  This
+	 * means the PlayerThread does not need to wake up the
+	 * OutputThread when new chunks are added to the MusicPipe,
+	 * because the OutputThread is already watching that.
+	 */
+	bool in_playback_loop;
+
+	/**
+	 * Has the OutputThread been woken up to play more chunks?
+	 * This is set by audio_output_play() and reset by ao_play()
+	 * to reduce the number of duplicate wakeups.
+	 */
+	bool woken_for_play;
+
+	/**
 	 * If not nullptr, the device has failed, and this timer is used
 	 * to estimate how long it should stay disabled (unless
 	 * explicitly reopened with "play").

src/OutputThread.cxx

@@ -510,6 +510,9 @@ ao_play(struct audio_output *ao)
 
 	ao->chunk_finished = false;
 
+	assert(!ao->in_playback_loop);
+	ao->in_playback_loop = true;
+
 	while (chunk != nullptr && ao->command == AO_COMMAND_NONE) {
 		assert(!ao->chunk_finished);
 
@@ -525,6 +528,9 @@ ao_play(struct audio_output *ao)
 		chunk = chunk->next;
 	}
 
+	assert(ao->in_playback_loop);
+	ao->in_playback_loop = false;
+
 	ao->chunk_finished = true;
 
 	ao->mutex.unlock();
@@ -656,8 +662,10 @@ audio_output_task(void *arg)
 			   chunks in the pipe */
 			continue;
 
-		if (ao->command == AO_COMMAND_NONE)
+		if (ao->command == AO_COMMAND_NONE) {
+			ao->woken_for_play = false;
 			ao->cond.wait(ao->mutex);
+		}
 	}
 }