Add allowed_mapgens option in game.conf. (#9263)

The game.conf has a disallowed_mapgens option. However, some games
require a certain mapgen to be used, like the CTF plugin. This change
adds an option to specify allowed mapgens so that the setting can be
specified in a way that needn't be updated as map generators are added
to Minetest.

builtin/mainmenu/dlg_create_world.lua

@@ -39,11 +39,24 @@ local function create_world_formspec(dialogdata)
 		local gamepath = game_by_gameidx.path
 		local gameconfig = Settings(gamepath.."/game.conf")
 
+		local allowed_mapgens = (gameconfig:get("allowed_mapgens") or ""):split()
+		for key, value in pairs(allowed_mapgens) do
+			allowed_mapgens[key] = value:trim()
+		end
+
 		local disallowed_mapgens = (gameconfig:get("disallowed_mapgens") or ""):split()
 		for key, value in pairs(disallowed_mapgens) do
 			disallowed_mapgens[key] = value:trim()
 		end
 
+		if #allowed_mapgens > 0 then
+			for i = #mapgens, 1, -1 do
+				if table.indexof(allowed_mapgens, mapgens[i]) == -1 then
+					table.remove(mapgens, i)
+				end
+			end
+		end
+
 		if disallowed_mapgens then
 			for i = #mapgens, 1, -1 do
 				if table.indexof(disallowed_mapgens, mapgens[i]) > 0 then

doc/lua_api.txt

@@ -64,9 +64,17 @@ The game directory can contain the following files:
 * `game.conf`, with the following keys:
     * `name`: Required, human readable name  e.g. `name = Minetest`
     * `description`: Short description to be shown in the content tab
+    * `allowed_mapgens = <comma-separated mapgens>`
+      e.g. `allowed_mapgens = v5,v6,flat`
+      Mapgens not in this list are removed from the list of mapgens for
+      the game.
+      If not specified, all mapgens are allowed.
     * `disallowed_mapgens = <comma-separated mapgens>`
       e.g. `disallowed_mapgens = v5,v6,flat`
       These mapgens are removed from the list of mapgens for the game.
+      When both `allowed_mapgens` and `disallowed_mapgens` are
+      specified, `allowed_mapgens` is applied before
+      `disallowed_mapgens`.
 * `minetest.conf`:
   Used to set default settings when running this game.
 * `settingtypes.txt`: