output/snapcast: implement SendTag()

src/output/plugins/meson.build

@@ -119,7 +119,9 @@ if get_option('snapcast')
     'snapcast/SnapcastOutputPlugin.cxx',
     'snapcast/Client.cxx',
   ]
-  output_plugins_deps += [ event_dep, net_dep ]
+  output_plugins_deps += [ event_dep, net_dep, yajl_dep ]
+
+  output_features.set('HAVE_YAJL', yajl_dep.found())
 
   # TODO: the Snapcast plugin needs just the "wave" encoder, but this
   # enables all available encoders

src/output/plugins/snapcast/Client.cxx

@@ -236,6 +236,27 @@ SnapcastClient::SendWireChunk(ConstBuffer<void> payload,
 	return ::SendWireChunk(GetSocket(), next_id++, payload, t);
 }
 
+static bool
+SendStreamTags(SocketDescriptor s, const PackedBE16 id,
+	       const ConstBuffer<void> payload) noexcept
+{
+	const PackedLE32 payload_size = payload.size;
+
+	SnapcastBase base{};
+	base.type = uint16_t(SnapcastMessageType::STREAM_TAGS);
+	base.id = id;
+	base.sent = ToSnapcastTimestamp(std::chrono::steady_clock::now());
+	base.size = sizeof(payload_size) + payload.size;
+
+	return SendT(s, base) && SendT(s, payload_size) && Send(s, payload);
+}
+
+void
+SnapcastClient::SendStreamTags(ConstBuffer<void> payload) noexcept
+{
+	::SendStreamTags(GetSocket(), next_id++, payload);
+}
+
 BufferedSocket::InputResult
 SnapcastClient::OnSocketInput(void *data, size_t length) noexcept
 {

src/output/plugins/snapcast/Client.hxx

@@ -60,6 +60,8 @@ public:
 
 	void LockClose() noexcept;
 
+	void SendStreamTags(ConstBuffer<void> payload) noexcept;
+
 	/**
 	 * Caller must lock the mutex.
 	 */

src/output/plugins/snapcast/Internal.hxx

@@ -165,7 +165,7 @@ public:
 
 	std::chrono::steady_clock::duration Delay() const noexcept override;
 
-	// TODO: void SendTag(const Tag &tag) override;
+	void SendTag(const Tag &tag) override;
 
 	size_t Play(const void *chunk, size_t size) override;
 

src/output/plugins/snapcast/SnapcastOutputPlugin.cxx

@@ -21,6 +21,7 @@
 #include "Internal.hxx"
 #include "Client.hxx"
 #include "output/OutputAPI.hxx"
+#include "output/Features.h"
 #include "encoder/EncoderInterface.hxx"
 #include "encoder/Configured.hxx"
 #include "encoder/plugins/WaveEncoderPlugin.hxx"
@@ -31,6 +32,10 @@
 #include "util/DeleteDisposer.hxx"
 #include "config/Net.hxx"
 
+#ifdef HAVE_YAJL
+#include "lib/yajl/Gen.hxx"
+#endif
+
 #include <cassert>
 
 #include <string.h>
@@ -207,6 +212,79 @@ SnapcastOutput::Delay() const noexcept
 		: std::chrono::steady_clock::duration::zero();
 }
 
+#ifdef HAVE_YAJL
+
+static constexpr struct {
+	TagType type;
+	const char *name;
+} snapcast_tags[] = {
+	/* these tags are mentioned in an example in
+	   snapcast/common/message/stream_tags.hpp */
+	{ TAG_ARTIST, "artist" },
+	{ TAG_ALBUM, "album" },
+	{ TAG_TITLE, "track" },
+	{ TAG_MUSICBRAINZ_TRACKID, "musicbrainzid" },
+};
+
+static bool
+TranslateTagType(Yajl::Gen &gen, const Tag &tag, TagType type,
+		 const char *name) noexcept
+{
+	// TODO: support multiple values?
+	const char *value = tag.GetValue(type);
+	if (value == nullptr)
+		return false;
+
+	gen.String(name);
+	gen.String(value);
+	return true;
+}
+
+static std::string
+ToJson(const Tag &tag) noexcept
+{
+	Yajl::Gen gen(nullptr);
+	gen.OpenMap();
+
+	bool empty = true;
+
+	for (const auto [type, name] : snapcast_tags)
+		if (TranslateTagType(gen, tag, type, name))
+			empty = false;
+
+	if (empty)
+		return {};
+
+	gen.CloseMap();
+
+	const auto result = gen.GetBuffer();
+	return {(const char *)result.data, result.size};
+}
+
+#endif
+
+void
+SnapcastOutput::SendTag(const Tag &tag)
+{
+#ifdef HAVE_YAJL
+	if (!LockHasClients())
+		return;
+
+	const auto json = ToJson(tag);
+	if (json.empty())
+		return;
+
+	const ConstBuffer payload(json.data(), json.size());
+
+	const std::lock_guard<Mutex> protect(mutex);
+	// TODO: enqueue StreamTags, don't send directly
+	for (auto &client : clients)
+		client.SendStreamTags(payload.ToVoid());
+#else
+	(void)tag;
+#endif
+}
+
 size_t
 SnapcastOutput::Play(const void *chunk, size_t size)
 {