allow non hosts to change factions later.

previously players had to leave an rejoin the game to change their faction.
this was most annoying in coop games where you want might want to choose
your faction in consultation with the other players. Also you previously could
not even checkout the different available factions again after you joined
the game.

(cherry-picked from commit 5ba389111e)

data/gui/window/mp_join_game.cfg

@@ -61,6 +61,18 @@
 						[/label]
 					[/column]
 
+					[column]
+						grow_factor = 0
+						vertical_alignment = "center"
+
+						[button]
+							id = "select_leader"
+							definition = "settings_mp_staging"
+							tooltip = _"Select the faction and leader for this side"
+						[/button]
+
+					[/column]
+
 					[column]
 						grow_factor = 1
 						horizontal_grow = true

src/gui/dialogs/multiplayer/mp_join_game.cpp

@@ -154,6 +154,8 @@ bool mp_join_game::fetch_game_config()
 
 	int side_num = 0, nb_sides = 0;
 	for(const config& side : get_scenario().child_range("side")) {
+		// TODO: it can happen that the scenario specifies that the controller
+		//       of a side should also gain control of another side.
 		if(side["controller"] == "reserved" && side["current_player"] == preferences::login()) {
 			side_choice = &side;
 			side_num = nb_sides;
@@ -188,6 +190,11 @@ bool mp_join_game::fetch_game_config()
 
 	// If the client is allowed to choose their team, do that here instead of having it set by the server
 	if((*side_choice)["allow_changes"].to_bool(true)) {
+		// TODO: show_flg_select does basicially the same,
+		//       i didn't want to risk breakig this for stable (1.14)
+		//       but id's be better if we didnt have this code dublication.
+		//       what i am in particular unsure about is why the '!era' and
+		//       'possible_sides.empty()' cases below are handled differntly here.
 		const config& era = level_.child("era");
 		// TODO: Check whether we have the era. If we don't, inform the player
 		if(!era) {
@@ -304,6 +311,59 @@ void mp_join_game::pre_show(window& window)
 	plugins_context_->set_callback("chat",   [&chat](const config& cfg) { chat.send_chat_message(cfg["message"], false); }, true);
 }
 
+void mp_join_game::show_flg_select(int side_num)
+{
+	if(const config& side_choice = get_scenario().child("side", side_num - 1)) {
+		if(!side_choice["allow_changes"].to_bool(true)) {
+			return;
+		}
+		const config& era = level_.child("era");
+		if(!era) {
+			ERR_MP << "no era information\n";
+			return;
+		}
+
+		config::const_child_itors possible_sides = era.child_range("multiplayer_side");
+		if(possible_sides.empty()) {
+			WRN_MP << "no [multiplayer_side] found in era '" << era["id"] << "'.\n";
+			return;
+		}
+
+		const std::string color = side_choice["color"].str();
+
+		std::vector<const config*> era_factions;
+		for(const config& side : possible_sides) {
+			era_factions.push_back(&side);
+		}
+
+		const bool is_mp = state_.classification().is_normal_mp_game();
+		const bool lock_settings = get_scenario()["force_lock_settings"].to_bool(!is_mp);
+		const bool use_map_settings = level_.child("multiplayer")["mp_use_map_settings"].to_bool();
+		const bool saved_game = level_.child("multiplayer")["savegame"].to_bool();
+
+		ng::flg_manager flg(era_factions, side_choice, lock_settings, use_map_settings, saved_game);
+
+		gui2::dialogs::faction_select dlg(flg, color, side_num);
+		dlg.show();
+
+		if(dlg.get_retval() != gui2::retval::OK) {
+			return;
+		}
+
+		config faction;
+		config& change = faction.add_child("change_faction");
+		change["change_faction"] = true;
+		change["name"] = preferences::login();
+		change["faction"] = flg.current_faction()["id"];
+		change["leader"] = flg.current_leader();
+		change["gender"] = flg.current_gender();
+		//TODO: the host cannot yet handle this and always uses the first side owned by that player.
+		change["side_num"] = side_num;
+
+		network_connection_.send_data(faction);
+	}
+}
+
 void mp_join_game::generate_side_list(window& window)
 {
 	if(stop_updates_) {
@@ -318,7 +378,9 @@ void mp_join_game::generate_side_list(window& window)
 	team_tree_map_.clear();
 	const std::map<std::string, string_map> empty_map;
 
+	int side_num = 0;
 	for(const auto& side : get_scenario().child_range("side")) {
+		++side_num;
 		if(!side["allow_player"].to_bool(true)) {
 			continue;
 		}
@@ -411,6 +473,16 @@ void mp_join_game::generate_side_list(window& window)
 
 		grid& row_grid = node.get_grid();
 
+		auto* select_leader_button = find_widget<button>(&row_grid, "select_leader", false, false);
+		if(select_leader_button) {
+			if(side["player_id"] == preferences::login() && side["allow_changes"].to_bool(true)) {
+				connect_signal_mouse_left_click(*select_leader_button, std::bind(&mp_join_game::show_flg_select, this, side_num));
+			}
+			else {
+				select_leader_button->set_visible(widget::visibility::hidden);
+			}
+		}
+
 		if(income_amt == 0) {
 			find_widget<image>(&row_grid, "income_icon", false).set_visible(widget::visibility::invisible);
 			find_widget<label>(&row_grid, "side_income", false).set_visible(widget::visibility::invisible);

src/gui/dialogs/multiplayer/mp_join_game.hpp

@@ -52,6 +52,8 @@ private:
 	/** Inherited from modal_dialog. */
 	virtual void post_show(window& window) override;
 
+	void show_flg_select(int side_num);
+
 	void generate_side_list(window& window);
 
 	void network_handler(window& window);