Add limited game auto-selection to dedicated server

src/main.cpp

@@ -830,9 +830,7 @@ static bool game_configure(GameParams *game_params, const Settings &cmd_args)
 		return false;
 	}
 
-	game_configure_subgame(game_params, cmd_args);
-
-	return true;
+	return game_configure_subgame(game_params, cmd_args);
 }
 
 static void game_configure_port(GameParams *game_params, const Settings &cmd_args)
@@ -1014,17 +1012,25 @@ static bool determine_subgame(GameParams *game_params)
 		if (game_params->game_spec.isValid()) {
 			gamespec = game_params->game_spec;
 			infostream << "Using commanded gameid [" << gamespec.id << "]" << std::endl;
-		} else {
-			if (game_params->is_dedicated_server) {
-				std::string contentdb_url = g_settings->get("contentdb_url");
-
-				// If this is a dedicated server and no gamespec has been specified,
-				// print a friendly error pointing to ContentDB.
-				errorstream << "To run a " PROJECT_NAME_C " server, you need to select a game using the '--gameid' argument." << std::endl
-				            << "Check out " << contentdb_url << " for a selection of games to pick from and download." << std::endl;
+		} else if (game_params->is_dedicated_server) {
+			auto games = getAvailableGameIds();
+			// If there's exactly one obvious choice then do the right thing
+			if (games.size() > 1)
+				games.erase("devtest");
+			if (games.size() == 1) {
+				gamespec = findSubgame(*games.begin());
+				infostream << "Automatically selecting gameid [" << gamespec.id << "]" << std::endl;
+			} else {
+				// Else, force the user to choose
+				auto &url = g_settings->get("contentdb_url");
+
+				errorstream << "To run a " PROJECT_NAME_C " server, you need to select a game using the '--gameid' argument." << std::endl;
+				if (games.empty())
+					errorstream << "Check out " << url << " for a selection of games to pick from and download." << std::endl;
+				else
+					errorstream << "Use '--gameid list' to print a list of all installed games." << std::endl;
+				return false;
 			}
-
-			return false;
 		}
 	} else { // World exists
 		std::string world_gameid = getWorldGameId(game_params->world_path, false);
@@ -1045,6 +1051,8 @@ static bool determine_subgame(GameParams *game_params)
 	}
 
 	if (!gamespec.isValid()) {
+		if (!game_params->is_dedicated_server)
+			return true; // not an error, this would prevent the main menu from running
 		errorstream << "Game [" << gamespec.id << "] could not be found."
 		            << std::endl;
 		return false;