filter/ffmpeg: detect the output sample format

Some FFmpeg filters change the sample format, and since MPD assumes
this never happens, this results in loud noise instead of music.  This
commit finally implements the TODO comment by sending one frame of
silence to the filter and checking the output frame's format.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1009

NEWS

@@ -1,6 +1,8 @@
 ver 0.22.4 (not yet released)
 * decoder
   - dsdiff: apply padding to odd-sized chunks
+* filter
+  - ffmpeg: detect the output sample format
 * output
   - moveoutput: fix always_on and tag lost on move
 

src/filter/plugins/FfmpegFilterPlugin.cxx

@@ -23,6 +23,7 @@
 #include "filter/Filter.hxx"
 #include "filter/Prepared.hxx"
 #include "lib/ffmpeg/Filter.hxx"
+#include "lib/ffmpeg/DetectFilterFormat.hxx"
 #include "config/Block.hxx"
 
 class PreparedFfmpegFilter final : public PreparedFilter {
@@ -60,7 +61,9 @@ PreparedFfmpegFilter::Open(AudioFormat &in_audio_format)
 
 	graph.CheckAndConfigure();
 
-	auto out_audio_format = in_audio_format; // TODO
+	const auto out_audio_format =
+		Ffmpeg::DetectFilterOutputFormat(in_audio_format, *buffer_src,
+						 *buffer_sink);
 
 	return std::make_unique<FfmpegFilter>(in_audio_format,
 					      out_audio_format,

src/lib/ffmpeg/DetectFilterFormat.cxx

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2003-2020 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.
+ */
+
+#include "DetectFilterFormat.hxx"
+#include "Frame.hxx"
+#include "SampleFormat.hxx"
+#include "pcm/Silence.hxx"
+#include "pcm/CheckAudioFormat.hxx"
+#include "util/WritableBuffer.hxx"
+
+extern "C" {
+#include <libavfilter/buffersrc.h>
+#include <libavfilter/buffersink.h>
+}
+
+#include <cassert>
+
+namespace Ffmpeg {
+
+AudioFormat
+DetectFilterOutputFormat(const AudioFormat &in_audio_format,
+			 AVFilterContext &buffer_src,
+			 AVFilterContext &buffer_sink)
+{
+	uint_least64_t silence[MAX_CHANNELS];
+	const size_t silence_size = in_audio_format.GetFrameSize();
+	assert(sizeof(silence) >= silence_size);
+
+	PcmSilence(WritableBuffer<void>{&silence, silence_size},
+		   in_audio_format.format);
+
+	Frame frame;
+	frame->format = ToFfmpegSampleFormat(in_audio_format.format);
+	frame->sample_rate = in_audio_format.sample_rate;
+	frame->channels = in_audio_format.channels;
+	frame->nb_samples = 1;
+
+	frame.GetBuffer();
+
+	memcpy(frame.GetData(0), silence, silence_size);
+
+	int err = av_buffersrc_add_frame(&buffer_src, frame.get());
+	if (err < 0)
+		throw MakeFfmpegError(err, "av_buffersrc_add_frame() failed");
+
+	frame.Unref();
+
+	err = av_buffersink_get_frame(&buffer_sink, frame.get());
+	if (err < 0)
+		throw MakeFfmpegError(err, "av_buffersink_get_frame() failed");
+
+	const SampleFormat sample_format = FromFfmpegSampleFormat(AVSampleFormat(frame->format));
+	if (sample_format == SampleFormat::UNDEFINED)
+		throw std::runtime_error("Unsupported FFmpeg sample format");
+
+	return CheckAudioFormat(frame->sample_rate, sample_format,
+				frame->channels);
+}
+
+} // namespace Ffmpeg

src/lib/ffmpeg/DetectFilterFormat.hxx

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2003-2020 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_DETECT_FILTER_FORMAT_HXX
+#define MPD_FFMPEG_DETECT_FILTER_FORMAT_HXX
+
+struct AVFilterContext;
+struct AudioFormat;
+
+namespace Ffmpeg {
+
+/**
+ * Attempt to detect the output format of the given FFmpeg filter by
+ * sending one frame of silence and checking what format comes back
+ * from the filter.
+ *
+ * This is a kludge because MPD needs to know the output format of a
+ * filter while initializing and cannot cope with format changes in
+ * between.
+ *
+ * This function can throw if the FFmpeg filter fails.
+ */
+AudioFormat
+DetectFilterOutputFormat(const AudioFormat &in_audio_format,
+			 AVFilterContext &buffer_src,
+			 AVFilterContext &buffer_sink);
+
+} // namespace Ffmpeg
+
+#endif

src/lib/ffmpeg/meson.build

@@ -20,7 +20,10 @@ endif
 
 ffmpeg_sources = []
 if libavfilter_dep.found()
-  ffmpeg_sources += 'Filter.cxx'
+  ffmpeg_sources += [
+    'Filter.cxx',
+    'DetectFilterFormat.cxx',
+  ]
 endif
 
 ffmpeg = static_library(