decoder/pcm: implement RFC 2586 (audio/L16)

https://bugs.musicpd.org/view.php?id=4525

NEWS

@@ -23,6 +23,7 @@ ver 0.20 (not yet released)
   - gme: add option "accuracy"
   - mad: reduce memory usage while scanning tags
   - mpcdec: read the bit rate
+  - pcm: support audio/L16 (RFC 2586)
 * playlist
   - cue: don't skip pregap
   - embcue: fix last track

doc/user.xml

@@ -1942,7 +1942,9 @@ buffer_size: 16384</programlisting>
         <title><varname>pcm</varname></title>
 
         <para>
-          Read raw PCM samples.
+          Read raw PCM samples.  It understands the "audio/L16" MIME
+          type with parameters "rate" and "channels" according to RFC
+          2586.
         </para>
       </section>
 

src/decoder/plugins/PcmDecoderPlugin.cxx

@@ -20,9 +20,12 @@
 #include "config.h"
 #include "PcmDecoderPlugin.hxx"
 #include "../DecoderAPI.hxx"
+#include "CheckAudioFormat.hxx"
 #include "input/InputStream.hxx"
 #include "util/Error.hxx"
 #include "util/ByteReverse.hxx"
+#include "util/NumberParser.hxx"
+#include "util/MimeType.hxx"
 #include "Log.hxx"
 
 #include <string.h>
@@ -30,7 +33,7 @@
 static void
 pcm_stream_decode(Decoder &decoder, InputStream &is)
 {
-	static constexpr AudioFormat audio_format = {
+	AudioFormat audio_format = {
 		44100,
 		SampleFormat::S16,
 		2,
@@ -40,6 +43,66 @@ pcm_stream_decode(Decoder &decoder, InputStream &is)
 	const bool reverse_endian = mime != nullptr &&
 		strcmp(mime, "audio/x-mpd-cdda-pcm-reverse") == 0;
 
+	const bool l16 = mime != nullptr &&
+		GetMimeTypeBase(mime) == "audio/L16";
+	if (l16) {
+		audio_format.sample_rate = 0;
+		audio_format.channels = 1;
+	}
+
+	{
+		const auto mime_parameters = ParseMimeTypeParameters(mime);
+		Error error;
+
+		/* MIME type parameters according to RFC 2586 */
+		auto i = mime_parameters.find("rate");
+		if (i != mime_parameters.end()) {
+			const char *s = i->second.c_str();
+			char *endptr;
+			unsigned value = ParseUnsigned(s, &endptr);
+			if (endptr == s || *endptr != 0) {
+				FormatWarning(audio_format_domain,
+					      "Failed to parse sample rate: %s",
+					      s);
+				return;
+			}
+
+			if (!audio_check_sample_rate(value, error)) {
+				LogError(error);
+				return;
+			}
+
+			audio_format.sample_rate = value;
+		}
+
+		i = mime_parameters.find("channels");
+		if (i != mime_parameters.end()) {
+			const char *s = i->second.c_str();
+			char *endptr;
+			unsigned value = ParseUnsigned(s, &endptr);
+			if (endptr == s || *endptr != 0) {
+				FormatWarning(audio_format_domain,
+					      "Failed to parse sample rate: %s",
+					      s);
+				return;
+			}
+
+			if (!audio_check_channel_count(value, error)) {
+				LogError(error);
+				return;
+			}
+
+			audio_format.channels = value;
+		}
+	}
+
+	if (audio_format.sample_rate == 0) {
+		FormatWarning(audio_format_domain,
+			      "Missing 'rate' parameter: %s",
+			      mime);
+		return;
+	}
+
 	const auto frame_size = audio_format.GetFrameSize();
 
 	const auto total_time = is.KnownSize()
@@ -88,6 +151,9 @@ pcm_stream_decode(Decoder &decoder, InputStream &is)
 }
 
 static const char *const pcm_mime_types[] = {
+	/* RFC 2586 */
+	"audio/L16",
+
 	/* for streams obtained by the cdio_paranoia input plugin */
 	"audio/x-mpd-cdda-pcm",