Add mrf to force link tag of quoting posts

lib/pleroma/constants.ex

@@ -84,6 +84,10 @@ defmodule Pleroma.Constants do
 
   const(upload_object_types, do: ["Document", "Image"])
 
+  const(activity_json_canonical_mime_type,
+    do: "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\""
+  )
+
   const(activity_json_mime_types,
     do: [
       "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\"",

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

@@ -0,0 +1,49 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2023 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.QuoteToLinkTagPolicy do
+  @moduledoc "Force a Link tag for posts quoting another post. (may break outgoing federation of quote posts with older Pleroma versions)"
+  @behaviour Pleroma.Web.ActivityPub.MRF.Policy
+
+  alias Pleroma.Web.ActivityPub.ObjectValidators.CommonFixes
+
+  require Pleroma.Constants
+
+  @impl Pleroma.Web.ActivityPub.MRF.Policy
+  def filter(%{"object" => %{"quoteUrl" => _} = object} = activity) do
+    {:ok, Map.put(activity, "object", filter_object(object))}
+  end
+
+  @impl Pleroma.Web.ActivityPub.MRF.Policy
+  def filter(object), do: {:ok, object}
+
+  @impl Pleroma.Web.ActivityPub.MRF.Policy
+  def describe, do: {:ok, %{}}
+
+  @impl Pleroma.Web.ActivityPub.MRF.Policy
+  def history_awareness, do: :auto
+
+  defp filter_object(%{"quoteUrl" => quote_url} = object) do
+    tags = object["tag"] || []
+
+    if Enum.any?(tags, fn tag ->
+         CommonFixes.is_object_link_tag(tag) and tag["href"] == quote_url
+       end) do
+      object
+    else
+      object
+      |> Map.put(
+        "tag",
+        tags ++
+          [
+            %{
+              "type" => "Link",
+              "mediaType" => Pleroma.Constants.activity_json_canonical_mime_type(),
+              "href" => quote_url
+            }
+          ]
+      )
+    end
+  end
+end

lib/pleroma/web/activity_pub/object_validators/common_fixes.ex

@@ -112,16 +112,14 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.CommonFixes do
   def fix_quote_url(data), do: data
 
   # https://codeberg.org/fediverse/fep/src/branch/main/fep/e232/fep-e232.md
-  defp is_object_link_tag(
-         %{
-           "type" => "Link",
-           "mediaType" => media_type,
-           "href" => href
-         } = tag
-       )
-       when media_type in Pleroma.Constants.activity_json_mime_types() and is_binary(href) do
+  def is_object_link_tag(%{
+        "type" => "Link",
+        "mediaType" => media_type,
+        "href" => href
+      })
+      when media_type in Pleroma.Constants.activity_json_mime_types() and is_binary(href) do
     true
   end
 
-  defp is_object_link_tag(_), do: false
+  def is_object_link_tag(_), do: false
 end