add anti hellthread policy

lib/pleroma/web/activity_pub/mrf/anti_hellthread_policy.ex

@@ -0,0 +1,53 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.AntiHellthreadPolicy do
+  alias Pleroma.User
+  alias Pleroma.Web.CommonAPI
+
+  @moduledoc "Notify local users upon remote block."
+
+  @behaviour Pleroma.Web.ActivityPub.MRF
+
+  defp is_block_or_unblock(%{"type" => "Block", "object" => object}),
+    do: {true, "blocked", object}
+
+  defp is_block_or_unblock(%{
+         "type" => "Undo",
+         "object" => %{"type" => "Block", "object" => object}
+       }),
+       do: {true, "unblocked", object}
+
+  defp is_block_or_unblock(_), do: {false, nil, nil}
+
+  defp is_remote_or_displaying_local?(%User{local: false}), do: true
+
+  defp is_remote_or_displaying_local?(_),
+    do: Pleroma.Config.get([:mrf_blockpolicy, :display_local])
+
+  @impl true
+  def filter(message) do
+    with {true, action, object} <- is_block_or_unblock(message),
+         %User{} = actor <- User.get_cached_by_ap_id(message["actor"]),
+         %User{} = recipient <- User.get_cached_by_ap_id(object),
+         true <- recipient.local,
+         true <- is_remote_or_displaying_local?(actor),
+         false <- User.blocks_user?(recipient, actor) do
+      bot_user = Pleroma.Config.get([:mrf_blockpolicy, :user])
+
+      _reply =
+        CommonAPI.post(User.get_by_nickname(bot_user), %{
+          "status" =>
+            "@" <> recipient.nickname <> " you have been " <> action <> " by @" <> actor.nickname,
+          "visibility" => "direct"
+        })
+    end
+
+    {:ok, message}
+  end
+
+  @impl true
+  def describe, do: {:ok, %{}}
+end
+