DecoderAPI: stop decoder on MPD error

This commit adds the basic infrastructure for reporting bugs from
DecoderAPI.cxx via DecoderThread.cxx to DecoderControl.

src/DecoderAPI.cxx

@@ -134,6 +134,10 @@ gcc_pure
 static DecoderCommand
 decoder_get_virtual_command(Decoder &decoder)
 {
+	if (decoder.error.IsDefined())
+		/* an error has occurred: stop the decoder plugin */
+		return DecoderCommand::STOP;
+
 	const DecoderControl &dc = decoder.dc;
 	assert(dc.pipe != nullptr);
 

src/DecoderInternal.hxx

@@ -22,6 +22,7 @@
 
 #include "DecoderCommand.hxx"
 #include "ReplayGainInfo.hxx"
+#include "util/Error.hxx"
 
 class PcmConvert;
 struct DecoderControl;
@@ -87,6 +88,12 @@ struct Decoder {
 	 */
 	unsigned replay_gain_serial;
 
+	/**
+	 * An error has occurred (in DecoderAPI.cxx), and the plugin
+	 * will be asked to stop.
+	 */
+	Error error;
+
 	Decoder(DecoderControl &_dc, bool _initial_seek_pending, Tag *_tag)
 		:dc(_dc),
 		 convert(nullptr),

src/DecoderThread.cxx

@@ -360,7 +360,12 @@ decoder_run_song(DecoderControl &dc,
 
 	dc.Lock();
 
-	if (ret)
+	if (decoder.error.IsDefined()) {
+		/* copy the Error from sruct Decoder to
+		   DecoderControl */
+		dc.state = DecoderState::ERROR;
+		dc.error = std::move(decoder.error);
+	} else if (ret)
 		dc.state = DecoderState::STOP;
 	else {
 		dc.state = DecoderState::ERROR;