Move areas:save() into async

On newer Minetest servers, handles saving jobs in async environment. To prevent conflicts, the save file is locked whie saving, and if a code requests saving while the file is locked, data is saved again immediately after finishing the current save.

internal.lua

@@ -4,26 +4,38 @@ function areas:player_exists(name)
 	return minetest.get_auth_handler().get_auth(name) ~= nil
 end
 
-local safe_file_write = minetest.safe_file_write
-if safe_file_write == nil then
-	function safe_file_write(path, content)
-		local file, err = io.open(path, "w")
-		if err then
-			return err
-		end
-		file:write(content)
-		file:close()
-	end
-end
+-- When saving is done in an async thread, the function will not be present in this global namespace.
+if not areas._internal_do_save then
+	local saving_requested = false
+	local saving_locked = false
 
--- Save the areas table to a file
-function areas:save()
-	local datastr = minetest.write_json(self.areas)
-	if not datastr then
-		minetest.log("error", "[areas] Failed to serialize area data!")
-		return
+	-- Required cuz we are referring to _G.areas._internal_do_save *inside*
+	-- async env (it does not exist in the main thread)
+	local function async_func(...)
+		return areas._internal_do_save(...)
+	end
+
+	local function done_callback()
+		saving_locked = false
+		if saving_requested == true then
+			saving_requested = false
+			return areas:save()
+		end
+	end
+
+	function areas:save()
+		if saving_locked == true then
+			saving_requested = true
+		else
+			saving_locked = true
+			return core.handle_async(async_func, done_callback, self.areas, self.config.filename)
+		end
+	end
+else
+	-- Save the areas table to a file
+	function areas:save()
+		return areas._internal_do_save(self.areas, self.config.filename)
 	end
-	return safe_file_write(self.config.filename, datastr)
 end
 
 -- Load the areas table from the save file