From e55f69a676eab6ab30f15bdab2a4141a5f6ccf75 Mon Sep 17 00:00:00 2001
From: Francis Dinh <normandy@firemail.cc>
Date: Tue, 8 May 2018 23:50:19 -0400
Subject: [PATCH] Handle Undo activites containing an Announce

---
 lib/pleroma/web/activity_pub/transmogrifier.ex | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/lib/pleroma/web/activity_pub/transmogrifier.ex b/lib/pleroma/web/activity_pub/transmogrifier.ex
index 2871a2544..8785ddaa9 100644
--- a/lib/pleroma/web/activity_pub/transmogrifier.ex
+++ b/lib/pleroma/web/activity_pub/transmogrifier.ex
@@ -220,9 +220,27 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     end
   end
 
+  def handle_incoming(
+        %{
+          "type" => "Undo",
+          "object" => %{"type" => "Announce", "id" => object_id},
+          "actor" => actor,
+          "id" => id
+        } = data
+      ) do
+    with %User{} = actor <- User.get_or_fetch_by_ap_id(actor),
+         {:ok, object} <-
+           get_obj_helper(object_id) || ActivityPub.fetch_object_from_id(object_id),
+         {:ok, activity} <- ActivityPub.unannounce(object, false) do
+      {:ok, activity}
+    else
+      e -> :error
+    end
+  end
+
   # TODO
   # Accept
-  # Undo
+  # Undo for non-Announce
 
   def handle_incoming(_), do: :error