Pipeline: Add a side effects step after the transaction finishes

This is to run things like streaming notifications out, which will
sometimes need data that is created by the transaction, but is
streamed out asynchronously.

lib/pleroma/web/activity_pub/side_effects.ex

@@ -16,6 +16,7 @@ defmodule Pleroma.Web.ActivityPub.SideEffects do
   alias Pleroma.Web.ActivityPub.Pipeline
   alias Pleroma.Web.ActivityPub.Utils
   alias Pleroma.Web.Streamer
+  alias Pleroma.Web.Push
 
   def handle(object, meta \\ [])
 
@@ -37,7 +38,12 @@ defmodule Pleroma.Web.ActivityPub.SideEffects do
   # - Set up notifications
   def handle(%{data: %{"type" => "Create"}} = activity, meta) do
     with {:ok, _object, _meta} <- handle_object_creation(meta[:object_data], meta) do
-      Notification.create_notifications(activity)
+      {:ok, notifications} = Notification.create_notifications(activity, do_send: false)
+
+      meta =
+        meta
+        |> add_notifications(notifications)
+
       {:ok, activity, meta}
     else
       e -> Repo.rollback(e)
@@ -200,4 +206,26 @@ defmodule Pleroma.Web.ActivityPub.SideEffects do
   end
 
   def handle_undoing(object), do: {:error, ["don't know how to handle", object]}
+
+  defp send_notifications(meta) do
+    Keyword.get(meta, :created_notifications, [])
+    |> Enum.each(fn notification ->
+      Streamer.stream(["user", "user:notification"], notification)
+      Push.send(notification)
+    end)
+
+    meta
+  end
+
+  defp add_notifications(meta, notifications) do
+    existing = Keyword.get(meta, :created_notifications, [])
+
+    meta
+    |> Keyword.put(:created_notifications, notifications ++ existing)
+  end
+
+  def handle_after_transaction(meta) do
+    meta
+    |> send_notifications()
+  end
 end