Add some hard limits on inserted activities.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -56,10 +56,18 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
     end
   end
 
+  defp check_remote_limit(%{"object" => %{"content" => content}}) do
+    limit = Pleroma.Config.get([:instance, :remote_limit])
+    String.length(content) <= limit
+  end
+
+  defp check_remote_limit(_), do: true
+
   def insert(map, local \\ true) when is_map(map) do
     with nil <- Activity.normalize(map),
          map <- lazy_put_activity_defaults(map),
          :ok <- check_actor_is_active(map["actor"]),
+         {_, true} <- {:remote_limit_error, check_remote_limit(map)},
          {:ok, map} <- MRF.filter(map),
          :ok <- insert_full_object(map) do
       {recipients, _, _} = get_recipients(map)