output_all: moved code to audio_output_all_wait()

Synchronization with the output thread will be implemented in
output_all.c, not in player_thread.c.  Currently, that's just a simple
g_usleep(1ms).

src/output_all.c

@@ -408,6 +408,18 @@ audio_output_all_check(void)
 	return 0;
 }
 
+bool
+audio_output_all_wait(unsigned threshold)
+{
+	if (audio_output_all_check() < threshold)
+		return true;
+
+	/* XXX synchronize in a better way */
+	g_usleep(1000);
+
+	return audio_output_all_check() < threshold;
+}
+
 void
 audio_output_all_pause(void)
 {

src/output_all.h

@@ -104,6 +104,17 @@ audio_output_all_play(struct music_chunk *chunk);
 unsigned
 audio_output_all_check(void);
 
+/**
+ * Checks if the size of the #music_pipe is below the #threshold.  If
+ * not, it attempts to synchronize with all output threads, and waits
+ * until another #music_chunk is finished.
+ *
+ * @param threshold the maximum number of chunks in the pipe
+ * @return true if there are less than #threshold chunks in the pipe
+ */
+bool
+audio_output_all_wait(unsigned threshold);
+
 /**
  * Puts all audio outputs into pause mode.  Most implementations will
  * simply close it then.

src/player_thread.c

@@ -192,14 +192,10 @@ player_check_decoder_startup(struct player *player)
 		/* the decoder is ready and ok */
 
 		if (audio_format_defined(&player->play_audio_format) &&
-		    audio_output_all_check() > 0) {
+		    !audio_output_all_wait(1))
 			/* the output devices havn't finished playing
 			   all chunks yet - wait for that */
-
-			/* XXX synchronize in a better way */
-			g_usleep(1000);
 			return true;
-		}
 
 		player->decoder_starting = false;
 
@@ -479,14 +475,10 @@ play_next_chunk(struct player *player)
 	unsigned cross_fade_position;
 	bool success;
 
-	if (audio_output_all_check() >= 64) {
+	if (!audio_output_all_wait(64))
 		/* the output pipe is still large enough, don't send
 		   another chunk */
-
-		/* XXX synchronize in a better way */
-		g_usleep(1000);
 		return true;
-	}
 
 	if (player->xfade == XFADE_ENABLED &&
 	    dc.pipe != NULL && dc.pipe != player->pipe &&