decoder/plugin: add method protocols()

Similar to commit 4e2a551f30 but for
decoder plugins.  This is tailored for the FFmpeg decoder plugin which
implements some protocols (e.g. RTSP) as demuxer plugin.

src/decoder/Thread.cxx

@@ -46,6 +46,42 @@
 
 static constexpr Domain decoder_thread_domain("decoder_thread");
 
+/**
+ * Decode a URI with the given decoder plugin.
+ *
+ * Caller holds DecoderControl::mutex.
+ */
+static bool
+DecoderUriDecode(const DecoderPlugin &plugin,
+		 DecoderBridge &bridge, const char *uri)
+{
+	assert(plugin.uri_decode != nullptr);
+	assert(bridge.stream_tag == nullptr);
+	assert(bridge.decoder_tag == nullptr);
+	assert(uri != nullptr);
+	assert(bridge.dc.state == DecoderState::START);
+
+	FormatDebug(decoder_thread_domain, "probing plugin %s", plugin.name);
+
+	if (bridge.dc.command == DecoderCommand::STOP)
+		throw StopDecoder();
+
+	{
+		const ScopeUnlock unlock(bridge.dc.mutex);
+
+		FormatThreadName("decoder:%s", plugin.name);
+
+		plugin.UriDecode(bridge, uri);
+
+		SetThreadName("decoder");
+	}
+
+	assert(bridge.dc.state == DecoderState::START ||
+	       bridge.dc.state == DecoderState::DECODE);
+
+	return bridge.dc.state != DecoderState::START;
+}
+
 /**
  * Decode a stream with the given decoder plugin.
  *
@@ -236,6 +272,24 @@ MaybeLoadReplayGain(DecoderBridge &bridge, InputStream &is)
 	LoadReplayGain(bridge, is);
 }
 
+/**
+ * Try decoding a URI.
+ *
+ * DecoderControl::mutex is not be locked by caller.
+ */
+static bool
+TryUriDecode(DecoderBridge &bridge, const char *uri)
+{
+	return decoder_plugins_try([&bridge, uri](const DecoderPlugin &plugin){
+		if (!plugin.SupportsUri(uri))
+			return false;
+
+		std::unique_lock<Mutex> lock(bridge.dc.mutex);
+		bridge.Reset();
+		return DecoderUriDecode(plugin, bridge, uri);
+	});
+}
+
 /**
  * Try decoding a stream.
  *
@@ -244,6 +298,9 @@ MaybeLoadReplayGain(DecoderBridge &bridge, InputStream &is)
 static bool
 decoder_run_stream(DecoderBridge &bridge, const char *uri)
 {
+	if (TryUriDecode(bridge, uri))
+		return true;
+
 	DecoderControl &dc = bridge.dc;
 
 	auto input_stream = bridge.OpenUri(uri);