activitypub: add support for filtering broken threads out of timelines

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -787,4 +787,37 @@ def visible_for_user?(activity, user) do
     y = activity.data["to"] ++ (activity.data["cc"] || [])
     visible_for_user?(activity, nil) || Enum.any?(x, &(&1 in y))
   end
+
+  # guard
+  def entire_thread_visible_for_user?(nil, user), do: false
+
+  # child
+  def entire_thread_visible_for_user?(
+        %Activity{data: %{"object" => %{"inReplyTo" => _parent_id}}} = tail,
+        user
+      ) do
+    parent = Activity.get_in_reply_to_activity(tail)
+    visible_for_user?(tail, user) && entire_thread_visible_for_user?(parent, user)
+  end
+
+  # root
+  def entire_thread_visible_for_user?(tail, user), do: visible_for_user?(tail, user)
+
+  # filter out broken threads
+  def contain_broken_threads(%Activity{} = activity, %User{} = user) do
+    entire_thread_visible_for_user?(activity, user)
+  end
+
+  # do post-processing on a specific activity
+  def contain_activity(%Activity{} = activity, %User{} = user) do
+    contain_broken_threads(activity, user)
+  end
+
+  # do post-processing on a timeline
+  def contain_timeline(timeline, user) do
+    timeline
+    |> Enum.filter(fn activity ->
+      contain_activity(activity, user)
+    end)
+  end
 end

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -223,6 +223,7 @@ def home_timeline(%{assigns: %{user: user}} = conn, params) do
 
     activities =
       ActivityPub.fetch_activities([user.ap_id | user.following], params)
+      |> ActivityPub.contain_timeline(user)
       |> Enum.reverse()
 
     conn

lib/pleroma/web/twitter_api/twitter_api_controller.ex

@@ -79,7 +79,9 @@ def friends_timeline(%{assigns: %{user: user}} = conn, params) do
       |> Map.put("blocking_user", user)
       |> Map.put("user", user)
 
-    activities = ActivityPub.fetch_activities([user.ap_id | user.following], params)
+    activities =
+      ActivityPub.fetch_activities([user.ap_id | user.following], params)
+      |> ActivityPub.contain_timeline(user)
 
     conn
     |> render(ActivityView, "index.json", %{activities: activities, for: user})