Emit a metadata message as the first message on a raw connection.

Add --raw-legacy-port option to restore the old behaviour.

The initial message uses the new metadata format and identifies the
source dump978 version & that it has the FEC fixes.

Software based on older versions of dump978 and dump978-fa's parsers
(e.g. uat2esnt) will harmlessly ignore the initial message.

The --raw-legacy-port option configures a raw listening port which
does _not_ emit the metadata message, in case there is other 3rd party
software which is confused by the new message format.

dump978_main.cc

@@ -98,6 +98,7 @@ static int realmain(int argc, char **argv) {
         ("sdr-device-settings", po::value<std::string>(), "set SDR device key-value settings")
         ("stratuxv3", po::value<std::string>(), "read messages from Stratux v3 UAT dongle on given serial port")
         ("raw-port", po::value<std::vector<listen_option>>(), "listen for connections on [host:]port and provide raw messages")
+        ("raw-legacy-port", po::value<std::vector<listen_option>>(), "listen for connections on [host:]port and provide raw messages, with no initial metadata header")
         ("json-port", po::value<std::vector<listen_option>>(), "listen for connections on [host:]port and provide decoded json");
     // clang-format on
 
@@ -184,9 +185,29 @@ static int realmain(int argc, char **argv) {
         return ok;
     };
 
-    auto raw_ok = create_output_port("raw-port", &RawOutput::Create);
+    // Emit initial metadata-only message advertising our version etc
+    SharedMessageVector header;
+    if (!opts.count("raw-disable-header")) {
+        header = std::make_shared<MessageVector>();
+
+        // clang-format off
+        RawMessage::MetadataMap metadata = {
+            {"program", "dump978-fa"},
+            {"version", VERSION},
+            {"fecfix", "1"} // we have a fixed version of the FEC library
+        };
+        // clang-format on
+        header->push_back(RawMessage(std::move(metadata)));
+    }
+
+    auto raw_factory = std::bind(&RawOutput::Create, std::placeholders::_1, std::placeholders::_2, header);
+    auto raw_ok = create_output_port("raw-port", raw_factory);
+
+    auto raw_legacy_factory = std::bind(&RawOutput::Create, std::placeholders::_1, std::placeholders::_2, SharedMessageVector());
+    auto raw_legacy_ok = create_output_port("raw-legacy-port", raw_legacy_factory);
+
     auto json_ok = create_output_port("json-port", &JsonOutput::Create);
-    if (!raw_ok || !json_ok) {
+    if (!raw_ok || !raw_legacy_ok || !json_ok) {
         return 1;
     }
 

socket_output.cc

@@ -84,6 +84,12 @@ void SocketOutput::Close() {
 
 //////////////
 
+void RawOutput::Start() {
+    SocketOutput::Start();
+    if (header_)
+        Write(header_);
+}
+
 void RawOutput::InternalWrite(SharedMessageVector messages) {
     for (const auto &message : *messages) {
         Buf() << message << '\n';

socket_output.h

@@ -55,13 +55,17 @@ namespace flightaware::uat {
     class RawOutput : public SocketOutput {
       public:
         // factory method, this class must always be constructed via make_shared
-        static Pointer Create(boost::asio::io_service &service, boost::asio::ip::tcp::socket &&socket) { return Pointer(new RawOutput(service, std::move(socket))); }
+        static Pointer Create(boost::asio::io_service &service, boost::asio::ip::tcp::socket &&socket, SharedMessageVector header) { return Pointer(new RawOutput(service, std::move(socket), header)); }
+
+        void Start() override;
 
       protected:
         void InternalWrite(SharedMessageVector messages) override;
 
       private:
-        RawOutput(boost::asio::io_service &service_, boost::asio::ip::tcp::socket &&socket_) : SocketOutput(service_, std::move(socket_)) {}
+        RawOutput(boost::asio::io_service &service_, boost::asio::ip::tcp::socket &&socket_, SharedMessageVector header) : SocketOutput(service_, std::move(socket_)) { header_ = header; }
+
+        SharedMessageVector header_;
     };
 
     class JsonOutput : public SocketOutput {