Statement output is null even though state is "available"

During some concurrency testing with an interactive session, I got the following response back:

{{

}}

The issue is related to the following code in InteractiveSessionServlet.scala, which assumes the statement result will be available within 100 milliseconds:

  val output = try {
    Await.result(statement.output(), Duration(100, TimeUnit.MILLISECONDS))
  } catch {
    case _: TimeoutException => null
  }

Workaround is simple: retry the statement REST call until the output is not null.