Allow client and server negotiate on savegame format if both are patched

src/network/network_client.cpp

@@ -355,6 +355,7 @@ NetworkRecvStatus ClientNetworkGameSocketHandler::SendJoin()
 	p->Send_string(_settings_client.network.client_name); // Client name
 	p->Send_uint8 (_network_join_as);     // PlayAs
 	p->Send_uint8 (NETLANG_ANY);          // Language
+	p->Send_uint8 (citymania::GetAvailableLoadFormats());  // Compressnion formats that we can decompress
 	my_client->SendPacket(p);
 	return NETWORK_RECV_STATUS_OKAY;
 }

src/network/network_server.cpp

@@ -592,7 +592,7 @@ NetworkRecvStatus ServerNetworkGameSocketHandler::SendMap()
 		sent_packets = 4; // We start with trying 4 packets
 
 		/* Make a dump of the current game */
-		if (SaveWithFilter(this->savegame, true) != SL_OK) usererror("network savedump failed");
+		if (SaveWithFilter(this->savegame, true, this->cm_preset) != SL_OK) usererror("network savedump failed");
 	}
 
 	if (this->status == STATUS_MAP) {
@@ -908,9 +908,15 @@ NetworkRecvStatus ServerNetworkGameSocketHandler::Receive_CLIENT_JOIN(Packet *p)
 	p->Recv_string(name, sizeof(name));
 	playas = (Owner)p->Recv_uint8();
 	client_lang = (NetworkLanguage)p->Recv_uint8();
+	uint8 savegame_formats = p->CanReadFromPacket(1) ? p->Recv_uint8() : 23u /* assume non-modded has everything but zstd */;
 
 	if (this->HasClientQuit()) return NETWORK_RECV_STATUS_CONN_LOST;
 
+	/* Find common savegame compression format to use */
+	auto preset = citymania::FindCompatibleSavePreset("", savegame_formats);
+	if (!preset) return this->SendError(NETWORK_ERROR_NOT_EXPECTED);
+	this->cm_preset = *preset;
+
 	/* join another company does not affect these values */
 	switch (playas) {
 		case COMPANY_NEW_COMPANY: // New company

src/network/network_server.h

@@ -12,6 +12,7 @@
 
 #include "network_internal.h"
 #include "core/tcp_listen.h"
+#include "../saveload/saveload.h"
 
 class ServerNetworkGameSocketHandler;
 /** Make the code look slightly nicer/simpler. */
@@ -72,6 +73,7 @@ public:
 
 	struct PacketWriter *savegame; ///< Writer used to write the savegame.
 	NetworkAddress client_address; ///< IP-address of the client (so he can be banned)
+	citymania::SavePreset cm_preset; ///< Preset to use for the savegame
 
 	ServerNetworkGameSocketHandler(SOCKET s);
 	~ServerNetworkGameSocketHandler();