Side Effects: On undoing, put information about the undone object.

lib/pleroma/web/activity_pub/side_effects.ex

@@ -174,6 +174,10 @@ def handle(%{data: %{"type" => "Announce"}} = object, meta) do
   def handle(%{data: %{"type" => "Undo", "object" => undone_object}} = object, meta) do
     with undone_object <- Activity.get_by_ap_id(undone_object),
          :ok <- handle_undoing(undone_object) do
+      meta =
+        meta
+        |> Keyword.put(:embedded_object, undone_object.data)
+
       {:ok, object, meta}
     end
   end

test/web/activity_pub/side_effects_test.exs

@@ -312,8 +312,13 @@ test "when activation is required", %{delete: delete, user: user} do
       }
     end
 
-    test "deletes the original block", %{block_undo: block_undo, block: block} do
-      {:ok, _block_undo, _} = SideEffects.handle(block_undo)
+    test "deletes the original block, but sets `embedded_object`", %{
+      block_undo: block_undo,
+      block: block
+    } do
+      {:ok, _block_undo, meta} = SideEffects.handle(block_undo)
+
+      assert meta[:embedded_object] == block.data
       refute Activity.get_by_id(block.id)
     end