akkoma/lib/pleroma/web/ostatus/handlers/follow_handler.ex

defmodule Pleroma.Web.OStatus.FollowHandler do
  alias Pleroma.Web.{XML, OStatus}
  alias Pleroma.Web.ActivityPub.ActivityPub
  alias Pleroma.User

  def handle(entry, doc) do
    with {:ok, actor} <- OStatus.find_make_or_update_user(doc),
         id when not is_nil(id) <- XML.string_from_xpath("/entry/id", entry),
         followed_uri when not is_nil(followed_uri) <-
           XML.string_from_xpath("/entry/activity:object/id", entry),
         {:ok, followed} <- OStatus.find_or_make_user(followed_uri),
         {:ok, activity} <- ActivityPub.follow(actor, followed, id, false) do
      User.follow(actor, followed)
      {:ok, activity}
    end
  end
end