output/alsa: disable DoP if it fails

See http://bugs.musicpd.org/view.php?id=4496

NEWS

@@ -31,6 +31,7 @@ ver 0.20 (not yet released)
   - alsa: fix multi-channel order
   - alsa: remove option "use_mmap"
   - alsa: support DSD_U32
+  - alsa: disable DoP if it fails
   - jack: reduce CPU usage
   - pulse: set channel map to WAVE-EX
   - recorder: record tags

src/output/plugins/AlsaOutputPlugin.cxx

@@ -731,13 +731,25 @@ AlsaOutput::SetupOrDop(AudioFormat &audio_format, PcmExport::Params &params,
 		       Error &error)
 {
 #ifdef ENABLE_DSD
-	if (dop && audio_format.format == SampleFormat::DSD) {
+	Error dop_error;
+	if (dop && audio_format.format == SampleFormat::DSD &&
+	    SetupDop(audio_format, params, dop_error)) {
 		params.dop = true;
-		return SetupDop(audio_format, params, error);
+		return true;
 	}
 #endif
 
-	return AlsaSetup(this, audio_format, params, error);
+	if (AlsaSetup(this, audio_format, params, error))
+		return true;
+
+#ifdef ENABLE_DSD
+	if (dop_error.IsDefined())
+		/* if DoP was attempted, prefer returning the original
+		   DoP error instead of the fallback error */
+		error = std::move(dop_error);
+#endif
+
+	return false;
 }
 
 inline bool