Maxximiliann

Maxximiliann

Need help unblocking a supervised Task in the handle_info() of a GenServer

defmodule SomeMonitor do
  use GenServer

  @timeout 5_000

  def start_link(_) do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  @impl true
  def init(state) do
    Process.flag(:trap_exit, true)
    Process.send(self(), :kickoff_update, state)

    {:ok, state}
  end

  @impl true
  def handle_info(
        :kickoff_update,
        state
      ) do
    DoStuff.launch()

    Process.send(self(), :update_transactions, state)
    {:noreply, state}
  end

  @impl true
  def handle_info(:update_transactions, state) do
    timeout_reference = Process.send_after(self(), :job_timeout, @timeout)

    task =
      Task.Supervisor.async(
        TasksSupervisor,
        fn -> PyOperatorSupervisor.launch({[], "get_fresh_info", "main"}) end
      )

    results = Task.await(task, 10_000)

    Process.cancel_timer(timeout_reference)

    case results do
      {:ok, _} ->
        {:ok, _} = ReviewTransactions.main(results)
        Process.send(self(), :update_transactions, state)
        {:noreply, state}

      {:error, reason} ->
        {:stop, reason, state}
    end

    {:noreply, state}
  end

  @impl true
  def handle_info(:job_timeout, state) do
    LogBook.main(
      "Still waiting for job to finish . . . Elapsed time: #{@timeout / 1000} second(s).",
      __MODULE__,
      70,
      :warning
    )

    {:noreply, state}
  end

  @impl true
  def handle_info({:EXIT, _, :normal}, state), do: {:noreply, state}

  @impl true
  def handle_info(critical_error, state), do: {:stop, critical_error, state}

  @impl true
  def terminate(critical_error, state),
    do: LogBook.main({state, critical_error}, __MODULE__, 77, :error)
end

What’s an efficient approach to avoid Process.send_after/3 from being blocked by Task.await/2?

Most Liked

al2o3cr

al2o3cr

Calling Task.await in GenServer callbacks is not recommended, because it does exactly what you’re seeing (blocks the GenServer receive loop).

In your case, there would be three handle_info heads involved similar to the example in the docs:

  • one for {ref, result} that handles success and cancels the timeout
  • one for {:DOWN, ...} that handles the task shutting down
  • one for a timeout message that kills the task

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