implement anti link spam MRF

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

@@ -0,0 +1,46 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.AntiLinkSpamPolicy do
+  alias Pleroma.User
+
+  require Logger
+
+  # has the user successfully posted before?
+  defp user_has_posted_before?(%User{} = u) do
+    u.info.note_count > 0 || u.info.follower_count > 0
+  end
+
+  # does the post contain links?
+  defp contains_links?(%{"content" => content} = _object) do
+    content
+    |> Floki.filter_out("a.mention,a.hashtag,a[rel~=\"tag\"],a.zrl")
+    |> Floki.attribute("a", "href")
+    |> length() > 0
+  end
+
+  def filter(%{"type" => "Create", "actor" => actor, "object" => object} = message) do
+    with {:ok, %User{} = u} <- User.get_or_fetch_by_ap_id(actor),
+         {:contains_links, true} <- {:contains_links, contains_links?(object)},
+         {:posted_before, true} <- {:posted_before, user_has_posted_before?(u)} do
+      {:ok, message}
+    else
+      {:contains_links, false} ->
+        {:ok, message}
+
+      {:posted_before, false} ->
+        {:reject, nil}
+
+      {:error, _} ->
+        {:reject, nil}
+
+      e ->
+        Logger.warn("[MRF anti-link-spam] WTF: unhandled error #{inspect(e)}")
+        {:reject, nil}
+    end
+  end
+
+  # in all other cases, pass through
+  def filter(message), do: {:ok, message}
+end