Fix variant roll changing when fetching assets (#704)

lib/beacon/plug.ex

@@ -17,9 +17,32 @@ defmodule Beacon.Plug do
   """
   @behaviour Plug
 
+  @private_routes [
+    "__beacon_check__",
+    "__beacon_assets__",
+    "__beacon_media__"
+  ]
+
   @impl Plug
   def init(_opts), do: []
 
   @impl Plug
-  def call(conn, _opts), do: Plug.Conn.put_session(conn, :beacon_variant_roll, Enum.random(1..100))
+  def call(conn, _opts) do
+    if Enum.any?(@private_routes, &(&1 in conn.path_info)) do
+      conn
+    else
+      put_roll(conn)
+    end
+  end
+
+  defp put_roll(conn) do
+    path_list = conn.path_params["path"]
+
+    with %{private: %{phoenix_live_view: {_, _, %{extra: %{session: %{"beacon_site" => site}}}}}} <- conn,
+         {_, _} <- Beacon.RouterServer.lookup_path(site, path_list, 1) do
+      Plug.Conn.put_session(conn, "beacon_variant_roll", Enum.random(1..100))
+    else
+      _ -> conn
+    end
+  end
 end