in fcn expand_mp_events, also collect addon_ids, for mp_settings

At the same time that we expand multiplayer events, by looking up
the tags of the given ids from the game config, we also note their
addon_id field if present and write this list to the mp_settings,
which will become an attribute of the [multiplayer] tag for the
scenario. When the scenario is hosted, this will tell other
clients what addons they can download to be able to play.

src/mp_game_settings.cpp

@@ -32,6 +32,7 @@ mp_game_settings::mp_game_settings() :
 	mp_campaign(),
 	difficulty_define(),
 	active_mods(),
+	addon_ids(),
 	side_users(),
 	show_configure(true),
 	show_connect(true),
@@ -66,6 +67,7 @@ mp_game_settings::mp_game_settings(const config& cfg)
 	, mp_campaign(cfg["mp_campaign"].str())
 	, difficulty_define(cfg["difficulty_define"].str())
 	, active_mods(utils::split(cfg["active_mods"], ','))
+	, addon_ids(utils::split(cfg["addon_ids"], ','))
 	, side_users(utils::map_split(cfg["side_users"]))
 	, show_configure(cfg["show_configure"].to_bool(true))
 	, show_connect(cfg["show_connect"].to_bool(true))
@@ -102,6 +104,7 @@ config mp_game_settings::to_config() const
 	cfg["mp_campaign"] = mp_campaign;
 	cfg["difficulty_define"] = difficulty_define;
 	cfg["active_mods"] = utils::join(active_mods, ",");
+	cfg["addon_ids"] = utils::join(addon_ids, ",");
 	cfg["side_users"] = utils::join_map(side_users);
 	cfg["show_configure"] = show_configure;
 	cfg["show_connect"] = show_connect;

src/mp_game_settings.hpp

@@ -40,6 +40,7 @@ struct mp_game_settings : public savegame::savegame_config
 	std::string mp_campaign;
 	std::string difficulty_define;
 	std::vector<std::string> active_mods;
+	std::vector<std::string> addon_ids;
 	std::map<std::string, std::string> side_users;
 
 	bool show_configure;

src/saved_game.cpp

@@ -221,12 +221,15 @@ struct modevents_entry_for
 	std::string type_;
 };
 
+// Gets the ids of the mp_era and modifications which were set to be active, then fetches these configs from the game_config and copies their [event] and [lua] to the starting_pos_.
+// At this time, also collect the addon_id attributes which appeared in them and put this list in the addon_ids attribute of the mp_settings.
 void saved_game::expand_mp_events()
 {
 	expand_scenario();
 	if(this->starting_pos_type_ == STARTINGPOS_SCENARIO && !this->starting_pos_["has_mod_events"].to_bool(false))
 	{
 		std::vector<modevents_entry> mods;
+		std::vector<std::string> addon_ids;
 
 		boost::copy( mp_settings_.active_mods
 			| boost::adaptors::transformed(modevents_entry_for("modification"))
@@ -239,10 +242,17 @@ void saved_game::expand_mp_events()
 			if(const config& cfg = game_config_manager::get()->
 				game_config().find_child(mod.type, "id", mod.id))
 			{
+				// Note the addon_id
+				if (cfg.has_attribute("addon_id") && !cfg["addon_id"].empty()) {
+					addon_ids.push_back(cfg["addon_id"].str());
+				}
+
+				// Copy events
 				BOOST_FOREACH(const config& modevent, cfg.child_range("event"))
 				{
 					this->starting_pos_.add_child("event", modevent);
 				}
+				// Copy lua
 				BOOST_FOREACH(const config& modlua, cfg.child_range("lua"))
 				{
 					this->starting_pos_.add_child("lua", modlua);
@@ -255,6 +265,7 @@ void saved_game::expand_mp_events()
 			}
 		}
 
+		mp_settings_.addon_ids = addon_ids;
 		this->starting_pos_["has_mod_events"] = true;
 	}
 }