player/Thread: fix "single" mode race condition

If the decoder finishes decoding the current song between the two
IsIdle() checks, MPD stops playback instead of starting the decoder
for the next song.

This is usually not visible problem, because the main thread restarts
it via playlist::ResumePlayback(), but that way it, ignores "single"
mode.

As a workaround, this commit adds another "queued" check which
re-enters the player loop and checks again whether to start the
decoder.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/556

NEWS

@@ -4,6 +4,7 @@ ver 0.21.9 (not yet released)
 * Android
   - fix crash on ARMv7
   - request storage permission on Android 6+
+* fix spurious "single" mode bug
 
 ver 0.21.8 (2019/04/23)
 * input

src/player/Thread.cxx

@@ -1058,6 +1058,16 @@ Player::Run() noexcept
 
 			SongBorder();
 		} else if (dc.IsIdle()) {
+			if (queued)
+				/* the decoder has just stopped,
+				   between the two IsIdle() checks,
+				   probably while UnlockCheckOutputs()
+				   left the mutex unlocked; to restart
+				   the decoder instead of stopping
+				   playback completely, let's re-enter
+				   this loop */
+				continue;
+
 			/* check the size of the pipe again, because
 			   the decoder thread may have added something
 			   since we last checked */