Transmogrifier: federate quotes with _misskey_quote field

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -660,6 +660,14 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
 
   def set_reply_to_uri(obj), do: obj
 
+  # Misskey quotes
+  # Despite being underscored, it's potentially more reliable for interop.
+  def set_quote_url(%{"quoteUrl" => quote_url} = object) when is_binary(quote_url) do
+    Map.put(object, "_misskey_quote", quote_url)
+  end
+
+  def set_quote_url(obj), do: obj
+
   @doc """
   Serialized Mastodon-compatible `replies` collection containing _self-replies_.
   Based on Mastodon's ActivityPub::NoteSerializer#replies.
@@ -714,6 +722,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     |> prepare_attachments
     |> set_conversation
     |> set_reply_to_uri
+    |> set_quote_url
     |> set_replies
     |> strip_internal_fields
     |> strip_internal_tags

priv/static/schemas/litepub-0.1.jsonld

@@ -17,6 +17,7 @@
             "ostatus": "http://ostatus.org#",
             "schema": "http://schema.org#",
             "toot": "http://joinmastodon.org/ns#",
+            "misskey": "https://misskey-hub.net/ns#",
             "value": "schema:value",
             "sensitive": "as:sensitive",
             "litepub": "http://litepub.social/ns#",
@@ -27,6 +28,7 @@
                 "@type": "@id"
             },
             "quoteUrl": "as:quoteUrl",
+            "_misskey_quote": "misskey:_misskey_quote",
             "oauthRegistrationEndpoint": {
                 "@id": "litepub:oauthRegistrationEndpoint",
                 "@type": "@id"

test/pleroma/web/activity_pub/transmogrifier_test.exs

@@ -382,7 +382,11 @@ defmodule Pleroma.Web.ActivityPub.TransmogrifierTest do
       {:ok, modified} = Transmogrifier.prepare_outgoing(quote_post.data)
 
       quoted_post = Object.normalize(quoted_post)
+
       assert modified["object"]["quoteUrl"] == quoted_post.data["id"]
+
+      # Add Misskey's quote as a fallback
+      assert modified["object"]["_misskey_quote"] == quoted_post.data["id"]
     end
   end