filter/ffmpeg: support double-precision samples

Insert an "aformat" filter which converts double-precision to
single-precision.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1235

NEWS

@@ -4,6 +4,7 @@ ver 0.22.11 (not yet released)
 * filter
   - ffmpeg: pass "channel_layout" instead of "channels" to buffersrc
   - ffmpeg: fix "av_buffersink_get_frame() failed: Resource temporarily unavailable"
+  - ffmpeg: support double-precision samples (by converting to single precision)
 * Android
   - build with NDK r23
 

src/filter/plugins/FfmpegFilterPlugin.cxx

@@ -81,7 +81,16 @@ PreparedFfmpegFilter::Open(AudioFormat &in_audio_format)
 
 	auto &buffer_sink = Ffmpeg::MakeAudioBufferSink(*graph);
 
-	graph.ParseSingleInOut(graph_string, buffer_sink, buffer_src);
+	/* if the filter's output format is not supported by MPD, this
+	   "aformat" filter is inserted at the end and takes care for
+	   the required conversion */
+	auto &aformat = Ffmpeg::MakeAutoAformat(*graph);
+
+	int error = avfilter_link(&aformat, 0, &buffer_sink, 0);
+	if (error < 0)
+		throw MakeFfmpegError(error, "avfilter_link() failed");
+
+	graph.ParseSingleInOut(graph_string, aformat, buffer_src);
 	graph.CheckAndConfigure();
 
 	const auto out_audio_format =

src/lib/ffmpeg/Filter.cxx

@@ -130,6 +130,14 @@ MakeAformat(AudioFormat &audio_format,
 			    args, nullptr, graph_ctx);
 }
 
+AVFilterContext &
+MakeAutoAformat(AVFilterGraph &graph_ctx)
+{
+	return CreateFilter(RequireFilterByName("aformat"), "aformat",
+			    "sample_fmts=flt|s32|s16",
+			    nullptr, graph_ctx);
+}
+
 void
 FilterGraph::ParseSingleInOut(const char *filters, AVFilterContext &in,
 			      AVFilterContext &out)

src/lib/ffmpeg/Filter.hxx

@@ -103,6 +103,13 @@ AVFilterContext &
 MakeAformat(AudioFormat &audio_format,
 	    AVFilterGraph &graph_ctx);
 
+/**
+ * Create an "aformat" filter which automatically converts the output
+ * to a format supported by MPD.
+ */
+AVFilterContext &
+MakeAutoAformat(AVFilterGraph &graph_ctx);
+
 class FilterGraph {
 	AVFilterGraph *graph = nullptr;