From 5e963736cee55aa8f4bb9d9fba451ff3864ddaa8 Mon Sep 17 00:00:00 2001
From: Alex Gleason <alex@alexgleason.me>
Date: Sun, 21 May 2023 15:26:02 -0500
Subject: [PATCH] Add AntiMentionSpamPolicy

---
 .../mrf/anti_mention_spam_policy.ex           | 87 +++++++++++++++++++
 .../mrf/anti_mention_spam_policy_test.exs     | 65 ++++++++++++++
 2 files changed, 152 insertions(+)
 create mode 100644 lib/pleroma/web/activity_pub/mrf/anti_mention_spam_policy.ex
 create mode 100644 test/pleroma/web/activity_pub/mrf/anti_mention_spam_policy_test.exs

diff --git a/lib/pleroma/web/activity_pub/mrf/anti_mention_spam_policy.ex b/lib/pleroma/web/activity_pub/mrf/anti_mention_spam_policy.ex
new file mode 100644
index 000000000..ad97a1552
--- /dev/null
+++ b/lib/pleroma/web/activity_pub/mrf/anti_mention_spam_policy.ex
@@ -0,0 +1,87 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.AntiMentionSpamPolicy do
+  alias Pleroma.User
+  require Pleroma.Constants
+
+  @behaviour Pleroma.Web.ActivityPub.MRF.Policy
+
+  defp user_has_followers?(%User{} = u), do: u.follower_count > 0
+  defp user_has_posted?(%User{} = u), do: u.note_count > 0
+
+  defp user_has_age?(%User{} = u) do
+    now = NaiveDateTime.utc_now()
+    diff = u.inserted_at |> NaiveDateTime.diff(now, :second)
+    diff > :timer.seconds(30)
+  end
+
+  defp good_reputation?(%User{} = u) do
+    user_has_age?(u) and user_has_followers?(u) and user_has_posted?(u)
+  end
+
+  # copied from HellthreadPolicy
+  defp get_recipient_count(message) do
+    recipients = (message["to"] || []) ++ (message["cc"] || [])
+
+    follower_collection =
+      User.get_cached_by_ap_id(message["actor"] || message["attributedTo"]).follower_address
+
+    if Enum.member?(recipients, Pleroma.Constants.as_public()) do
+      recipients =
+        recipients
+        |> List.delete(Pleroma.Constants.as_public())
+        |> List.delete(follower_collection)
+
+      {:public, length(recipients)}
+    else
+      recipients =
+        recipients
+        |> List.delete(follower_collection)
+
+      {:not_public, length(recipients)}
+    end
+  end
+
+  defp object_has_recipients?(%{"object" => object} = activity) do
+    {_, object_count} = get_recipient_count(object)
+    {_, activity_count} = get_recipient_count(activity)
+    object_count + activity_count > 0
+  end
+
+  defp object_has_recipients?(object) do
+    {_, count} = get_recipient_count(object)
+    count > 0
+  end
+
+  @impl true
+  def filter(%{"type" => "Create", "actor" => actor} = activity) do
+    with {:ok, %User{local: false} = u} <- User.get_or_fetch_by_ap_id(actor),
+         {:has_mentions, true} <- {:has_mentions, object_has_recipients?(activity)},
+         {:good_reputation, true} <- {:good_reputation, good_reputation?(u)} do
+      {:ok, activity}
+    else
+      {:ok, %User{local: true}} ->
+        {:ok, activity}
+
+      {:has_mentions, false} ->
+        {:ok, activity}
+
+      {:good_reputation, false} ->
+        {:reject, "[AntiMentionSpamPolicy] User rejected"}
+
+      {:error, _} ->
+        {:reject, "[AntiMentionSpamPolicy] Failed to get or fetch user by ap_id"}
+
+      e ->
+        {:reject, "[AntiMentionSpamPolicy] Unhandled error #{inspect(e)}"}
+    end
+  end
+
+  # in all other cases, pass through
+  def filter(message), do: {:ok, message}
+
+  @impl true
+  def describe, do: {:ok, %{}}
+end
diff --git a/test/pleroma/web/activity_pub/mrf/anti_mention_spam_policy_test.exs b/test/pleroma/web/activity_pub/mrf/anti_mention_spam_policy_test.exs
new file mode 100644
index 000000000..63947858c
--- /dev/null
+++ b/test/pleroma/web/activity_pub/mrf/anti_mention_spam_policy_test.exs
@@ -0,0 +1,65 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.AntiMentionSpamPolicyTest do
+  use Pleroma.DataCase
+  import Pleroma.Factory
+  alias Pleroma.Web.ActivityPub.MRF.AntiMentionSpamPolicy
+
+  test "it allows posts without mentions" do
+    user = insert(:user, local: false)
+    assert user.note_count == 0
+
+    message = %{
+      "type" => "Create",
+      "actor" => user.ap_id
+    }
+
+    {:ok, _message} = AntiMentionSpamPolicy.filter(message)
+  end
+
+  test "it allows posts from users with followers, posts, and age" do
+    user =
+      insert(:user,
+        local: false,
+        follower_count: 1,
+        note_count: 1,
+        inserted_at: ~N[1970-01-01 00:00:00]
+      )
+
+    message = %{
+      "type" => "Create",
+      "actor" => user.ap_id
+    }
+
+    {:ok, _message} = AntiMentionSpamPolicy.filter(message)
+  end
+
+  test "it allows posts from local users" do
+    user = insert(:user, local: true)
+
+    message = %{
+      "type" => "Create",
+      "actor" => user.ap_id
+    }
+
+    {:ok, _message} = AntiMentionSpamPolicy.filter(message)
+  end
+
+  test "it rejects posts with mentions from users without followers" do
+    user = insert(:user, local: false, follower_count: 0)
+
+    message = %{
+      "type" => "Create",
+      "actor" => user.ap_id,
+      "object" => %{
+        "to" => ["https://pleroma.soykaf.com/users/1"],
+        "cc" => ["https://pleroma.soykaf.com/users/1"],
+        "actor" => user.ap_id
+      }
+    }
+
+    {:reject, _message} = AntiMentionSpamPolicy.filter(message)
+  end
+end