Make it possible to configure custom plug for all requests (#576)

* Make it possible to configure custom plug for all requests

* Apply review comments

* Use fetch_env!

config/config.exs

@@ -26,6 +26,13 @@ config :livebook, :authentication_mode, :token
 # configured arguments.
 config :livebook, :default_runtime, {Livebook.Runtime.ElixirStandalone, []}
 
+# A list of custom plugs in the following format:
+#
+#    [{plug_module :: module(), opts :: keyword()}]
+#
+# The plugs are called directly before the Livebook router.
+config :livebook, :plugs, []
+
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.
 import_config "#{Mix.env()}.exs"

lib/livebook_web/endpoint.ex

@@ -71,6 +71,10 @@ defmodule LivebookWeb.Endpoint do
   plug Plug.MethodOverride
   plug Plug.Head
   plug Plug.Session, @session_options
+
+  # Run custom plugs from the app configuration
+  plug LivebookWeb.ConfiguredPlug
+
   plug LivebookWeb.Router
 
   def access_url() do

lib/livebook_web/plugs/configured_plug.ex

@@ -0,0 +1,18 @@
+defmodule LivebookWeb.ConfiguredPlug do
+  @moduledoc false
+
+  # Runs plugs configured for the :livebook application
+
+  @behaviour Plug
+
+  @impl true
+  def init(opts), do: opts
+
+  @impl true
+  def call(conn, _opts) do
+    case Application.fetch_env!(:livebook, :plugs) do
+      [] -> conn
+      plugs -> Plug.run(conn, plugs)
+    end
+  end
+end