Ignore asset directories if they are empty

macOS removes files from the temporary directory but
keeps the directory. This would cause LiveView to
misbehave as it would assume all assets are checked out.

lib/livebook/session.ex

@@ -209,14 +209,14 @@ defmodule Livebook.Session do
   def fetch_assets(pid, hash) do
     local_assets_path = local_assets_path(hash)
 
-    if File.exists?(local_assets_path) do
+    if non_empty_dir?(local_assets_path) do
       :ok
     else
       with {:ok, runtime, archive_path} <-
              GenServer.call(pid, {:get_runtime_and_archive_path, hash}, @timeout) do
         fun = fn ->
           # Make sure the file hasn't been fetched by this point
-          unless File.exists?(local_assets_path) do
+          unless non_empty_dir?(local_assets_path) do
             {:ok, archive_binary} = Runtime.read_file(runtime, archive_path)
             extract_archive!(archive_binary, local_assets_path)
           end
@@ -232,6 +232,10 @@ defmodule Livebook.Session do
     end
   end
 
+  defp non_empty_dir?(path) do
+    match?({:ok, [_ | _]}, File.ls(path))
+  end
+
   @doc """
   Sends notebook attributes update to the server.
   """
@@ -1067,7 +1071,10 @@ defmodule Livebook.Session do
     FileSystem.File.remove(tmp_dir)
   end
 
-  defp local_assets_path(hash) do
+  @doc """
+  Returns a local path to the directory for all assets for hash.
+  """
+  def local_assets_path(hash) do
     Path.join([livebook_tmp_path(), "assets", encode_path_component(hash)])
   end
 

test/livebook_web/controllers/session_controller_test.exs

@@ -152,6 +152,22 @@ defmodule LivebookWeb.SessionControllerTest do
 
       assert redirected_to(conn, 301) ==
                Routes.session_path(conn, :show_cached_asset, hash, ["main.js"])
+
+      {:ok, asset_path} = Session.local_asset_path(hash, "main.js")
+      assert File.exists?(asset_path)
+    end
+
+    test "fetches assets and redirects even on empty asset directories", %{conn: conn} do
+      %{notebook: notebook, hash: hash} = notebook_with_js_output()
+      assets_path = Session.local_assets_path(hash)
+      File.mkdir_p!(assets_path)
+
+      conn = start_session_and_request_asset(conn, notebook, hash)
+
+      assert redirected_to(conn, 301) ==
+               Routes.session_path(conn, :show_cached_asset, hash, ["main.js"])
+
+      assert File.exists?(Path.join(assets_path, "main.js"))
     end
 
     test "skips the session if assets are in cache", %{conn: conn} do