filter/ffmpeg: pass "channel_layout" instead of "channels" to buffersrc

Fixes part 1 of https://github.com/MusicPlayerDaemon/MPD/issues/1235

NEWS

@@ -1,6 +1,8 @@
 ver 0.22.11 (not yet released)
 * protocol
   - fix "albumart" crash
+* filter
+  - ffmpeg: pass "channel_layout" instead of "channels" to buffersrc
 * Android
   - build with NDK r23
 

src/lib/ffmpeg/ChannelLayout.hxx

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2003-2021 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_FFMPEG_CHANNEL_LAYOUT_HXX
+#define MPD_FFMPEG_CHANNEL_LAYOUT_HXX
+
+extern "C" {
+#include <libavutil/channel_layout.h>
+}
+
+/**
+ * Convert a MPD channel count to a libavutil channel_layout bit mask.
+ */
+static constexpr uint64_t
+ToFfmpegChannelLayout(unsigned channels) noexcept
+{
+	switch (channels) {
+	case 1:
+		return AV_CH_LAYOUT_MONO;
+
+	case 2:
+		return AV_CH_LAYOUT_STEREO;
+
+	case 3:
+		return AV_CH_LAYOUT_SURROUND;
+
+	case 4:
+		// TODO is this AV_CH_LAYOUT_2_2?
+		return AV_CH_LAYOUT_QUAD;
+
+	case 5:
+		// TODO is this AV_CH_LAYOUT_5POINT0_BACK?
+		return AV_CH_LAYOUT_5POINT0;
+
+	case 6:
+		return AV_CH_LAYOUT_5POINT1;
+
+	case 7:
+		return AV_CH_LAYOUT_6POINT1;
+
+	case 8:
+		return AV_CH_LAYOUT_7POINT1;
+
+	default:
+		/* unreachable */
+		return 0;
+	}
+}
+
+#endif

src/lib/ffmpeg/Filter.cxx

@@ -18,10 +18,13 @@
  */
 
 #include "Filter.hxx"
+#include "ChannelLayout.hxx"
 #include "SampleFormat.hxx"
 #include "pcm/AudioFormat.hxx"
 #include "util/RuntimeError.hxx"
 
+#include <cinttypes>
+
 #include <stdio.h>
 
 namespace Ffmpeg {
@@ -57,10 +60,10 @@ FilterContext::MakeAudioBufferSource(AudioFormat &audio_format,
 
 	char abuffer_args[256];
 	sprintf(abuffer_args,
-		"sample_rate=%u:sample_fmt=%s:channels=%u:time_base=1/%u",
+		"sample_rate=%u:sample_fmt=%s:channel_layout=0x%" PRIx64 ":time_base=1/%u",
 		audio_format.sample_rate,
 		av_get_sample_fmt_name(src_format),
-		audio_format.channels,
+		ToFfmpegChannelLayout(audio_format.channels),
 		audio_format.sample_rate);
 
 	return {RequireFilterByName("abuffer"), "abuffer", abuffer_args, nullptr, graph_ctx};