Fix root directory listing on S3 when empty (#646)

lib/livebook/file_system/s3.ex

@@ -70,7 +70,7 @@ defimpl Livebook.FileSystem, for: Livebook.FileSystem.S3 do
     delimiter = if recursive, do: nil, else: "/"
 
     with {:ok, %{keys: keys}} <- list_objects(file_system, prefix: dir_key, delimiter: delimiter) do
-      if keys == [] do
+      if keys == [] and dir_key != "" do
         FileSystem.Utils.posix_error(:enoent)
       else
         paths = keys |> List.delete(dir_key) |> Enum.map(&("/" <> &1))

test/livebook/file_system/s3_test.exs

@@ -99,6 +99,25 @@ defmodule Livebook.FileSystem.S3Test do
       assert {:error, "no such file or directory"} = FileSystem.list(file_system, dir_path, false)
     end
 
+    test "does not return an error when the root directory is empty", %{bypass: bypass} do
+      Bypass.expect_once(bypass, "GET", "/mybucket", fn conn ->
+        assert %{"delimiter" => "/"} = conn.params
+
+        conn
+        |> Plug.Conn.put_resp_content_type("application/xml")
+        |> Plug.Conn.resp(200, """
+        <ListBucketResult>
+          <Name>mybucket</Name>
+        </ListBucketResult>
+        """)
+      end)
+
+      file_system = S3.new(bucket_url(bypass.port), "key", "secret")
+      dir_path = "/"
+
+      assert {:ok, []} = FileSystem.list(file_system, dir_path, false)
+    end
+
     test "returns a list of absolute child object paths", %{bypass: bypass} do
       Bypass.expect_once(bypass, "GET", "/mybucket", fn conn ->
         assert %{"delimiter" => "/"} = conn.params