Add configuration to serve public routes from a different base url path (#2704)

Co-authored-by: Jonatan Kłosko <jonatanklosko@gmail.com>

README.md

@@ -196,6 +196,11 @@ The following environment variables can be used to configure Livebook on boot:
   * `LIVEBOOK_BASE_URL_PATH` - sets the base url path the web application is
     served on. Useful when deploying behind a reverse proxy.
 
+  * `LIVEBOOK_PUBLIC_BASE_URL_PATH` - sets the base url path the `/public/*` routes
+    are served on. Note that this takes precedence over `LIVEBOOK_BASE_URL_PATH`,
+    if both are set. Setting this may be useful to create exceptions when deploying
+    behind a reverse proxy that requires au1thentication.
+
   * `LIVEBOOK_CACERTFILE` - path to a local file containing CA certificates.
     Those certificates are used during for server authentication when Livebook
     accesses files from external sources.

assets/js/hooks/js_view.js

@@ -507,7 +507,7 @@ const JSView = {
  * once and the response is cached.
  */
 function cachedPublicEndpointCheck() {
-  const healthUrl = window.LIVEBOOK_BASE_URL_PATH + "/public/health";
+  const healthUrl = window.LIVEBOOK_PUBLIC_BASE_URL_PATH + "/public/health";
 
   cachedPublicEndpointCheck.promise =
     cachedPublicEndpointCheck.promise ||

lib/livebook.ex

@@ -114,6 +114,11 @@ defmodule Livebook do
       config :livebook, LivebookWeb.Endpoint, url: [path: base_url_path]
     end
 
+    if public_base_url_path =
+         Livebook.Config.base_url_path!("LIVEBOOK_PUBLIC_BASE_URL_PATH") do
+      config :livebook, :public_base_url_path, public_base_url_path
+    end
+
     if password = Livebook.Config.password!("LIVEBOOK_PASSWORD") do
       config :livebook, :authentication, {:password, password}
     else

lib/livebook/config.ex

@@ -226,6 +226,17 @@ defmodule Livebook.Config do
     String.trim_trailing(path, "/")
   end
 
+  @doc """
+  Returns the base url path for Livebook public endpoints (health & assets)
+  """
+  @spec public_base_url_path() :: String.t()
+  def public_base_url_path() do
+    case Application.get_env(:livebook, :public_base_url_path) do
+      nil -> base_url_path()
+      path -> String.trim_trailing(path, "/")
+    end
+  end
+
   @doc """
   Returns the configured port for the iframe endpoint.
   """

lib/livebook_web.ex

@@ -78,8 +78,10 @@ defmodule LivebookWeb do
         statics: LivebookWeb.static_paths()
 
       # We don't know the hostname Livebook runs on, so we don't use
-      # absolute URL helpers
-      import Phoenix.VerifiedRoutes, only: :sigils
+      # absolute URL helpers. We don't import sigil_p either, because
+      # we override it.
+      import Phoenix.VerifiedRoutes, only: []
+      import LivebookWeb, only: [sigil_p: 2]
     end
   end
 
@@ -89,4 +91,31 @@ defmodule LivebookWeb do
   defmacro __using__(which) when is_atom(which) do
     apply(__MODULE__, which, [])
   end
+
+  # Overrides
+  require Phoenix.VerifiedRoutes
+
+  defmacro sigil_p({:<<>>, _meta, ["/public/" <> _ | _]} = route, extra) do
+    # We allow configuring a base path for all routes and we configure
+    # Phoenix to use it. However, we have an additional configuration
+    # for base path applying only to /public. We use a custom sigil_p
+    # to insert this base path if needed.
+    quote do
+      path = Phoenix.VerifiedRoutes.sigil_p(unquote(route), unquote(extra))
+      LivebookWeb.__rewrite_public_base_path__(path)
+    end
+  end
+
+  defmacro sigil_p(route, extra) do
+    quote do
+      Phoenix.VerifiedRoutes.sigil_p(unquote(route), unquote(extra))
+    end
+  end
+
+  def __rewrite_public_base_path__(path) do
+    base_url_path = Livebook.Config.base_url_path()
+    public_base_url_path = Livebook.Config.public_base_url_path()
+    ^base_url_path <> rest = path
+    public_base_url_path <> rest
+  end
 end

lib/livebook_web/components/layouts/root.html.heex

@@ -13,6 +13,7 @@
     <link phx-track-static rel="stylesheet" href={~p"/assets/app.css"} />
     <script>
       window.LIVEBOOK_BASE_URL_PATH = "<%= Livebook.Config.base_url_path() %>";
+      window.LIVEBOOK_PUBLIC_BASE_URL_PATH = "<%= Livebook.Config.public_base_url_path() %>"
     </script>
     <LivebookWeb.LayoutComponents.dev_script />
     <%!-- This prevents the script to be loaded twice in Chrome --%>