Add a MRF Policy for appending re: to identical subjects in replies.

config/config.md

@@ -39,6 +39,7 @@ Note: `strip_exif` has been replaced by `Pleroma.Upload.Filter.Mogrify`.
   * `Pleroma.Web.ActivityPub.MRF.DropPolicy`: Drops all activities. It generally doesn’t makes sense to use in production
   * `Pleroma.Web.ActivityPub.MRF.SimplePolicy`: Restrict the visibility of activities from certains instances (See ``:mrf_simple`` section)
   * `Pleroma.Web.ActivityPub.MRF.RejectNonPublic`: Drops posts with non-public visibility settings (See ``:mrf_rejectnonpublic`` section)
+  * `Pleroma.Web.ActivityPub.MRF.EnsureRePrepended`: Rewrites posts to ensure that replies to posts with subjects do not have an identical subject and instead begin with re:.
 * `public`: Makes the client API in authentificated mode-only except for user-profiles. Useful for disabling the Local Timeline and The Whole Known Network.
 * `quarantined_instances`: List of ActivityPub instances where private(DMs, followers-only) activities will not be send.
 * `managed_config`: Whenether the config for pleroma-fe is configured in this config or in ``static/config.json``

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

@@ -0,0 +1,37 @@
+defmodule Pleroma.Web.ActivityPub.MRF.EnsureRePrepended do
+  alias Pleroma.Activity
+
+  @behaviour Pleroma.Web.ActivityPub.MRF
+
+  def filter_by_summary(
+        %{"summary" => parent_summary} = parent,
+        %{"summary" => child_summary} = child
+      )
+      when not is_nil(child_summary) and child_summary == parent_summary and
+             byte_size(child_summary) > 1 do
+    if not String.starts_with?(child_summary, "re:") do
+      Map.put(child, "summary", "re: " <> child_summary)
+    else
+      child
+    end
+  end
+
+  def filter_by_summary(parent, child), do: child
+
+  def filter(%{"type" => activity_type} = object) when activity_type == "Create" do
+    child = object["object"]
+    in_reply_to = Activity.get_create_activity_by_object_ap_id(child["inReplyTo"])
+
+    child =
+      if(in_reply_to,
+        do: filter_by_summary(in_reply_to.data["object"], child),
+        else: child
+      )
+
+    object = Map.put(object, "object", child)
+
+    {:ok, object}
+  end
+
+  def filter(object), do: {:ok, object}
+end