decoder/API: add class StopDecoder

src/decoder/DecoderAPI.hxx

@@ -46,6 +46,13 @@
 
 class Error;
 
+/**
+ * Throw an instance of this class to stop decoding the current song
+ * (successfully).  It can be used to jump out of all of a decoder's
+ * stack frames.
+ */
+class StopDecoder {};
+
 /**
  * Notify the player thread that it has finished initialization and
  * that it has read the song's meta data.

src/decoder/DecoderThread.cxx

@@ -356,6 +356,8 @@ try {
 	return !path_fs.IsNull()
 		? decoder_run_file(decoder, real_uri, path_fs)
 		: decoder_run_stream(decoder, real_uri);
+} catch (StopDecoder) {
+	return true;
 } catch (const std::runtime_error &e) {
 	/* copy the exception to decoder.error */
 

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -310,7 +310,7 @@ MPDOpusDecoder::HandleEOS()
 		return decoder_get_command(decoder);
 	}
 
-	return DecoderCommand::STOP;
+	throw StopDecoder();
 }
 
 inline DecoderCommand