Failing task in start_async not calling appropriate handle_async({:error}) callback

Phoenix Forum Questions / Help
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1

Hello there,

I inherited a rather complicated LiveComponent from a colleague: due to complex JS there’s no render/1 function, only update/2.
I have a hook which publishes an event to said component, which should then start an async operation on the backend.

  @impl true
  def handle_event("classify-users", _params, socket) do
    socket =
      socket
      |> start_async(:perform_classification, fn ->
        raise "Some error"
      end)

    {:noreply, socket}
  end

I expect this code to eventually call the handle_async callback:

 @impl true
  def handle_async(:perform_classification, {:exit, reason}, socket) do
    Logger.error("Could not perform classification: #{reason}")
    {:noreply,
     push_event(socket, "classification-finished", %{class: "Error #{reason}"})}
  end

However, the component simply crashes and I don’t know why it never executes said handle_async function.
Any idea?

2

The following is just a guess, I’m on my phone so can’t check it out.

A live component lives in the same process of the liveview. Maybe the error can be caught on the liveview itself?

As I said this is just a guess, it could be something completely different.

3

thanks, but after careful investigation I came to the conclusion that it’s a layer 8 problem:

The :exit-matching function was indeed called but not logged as the interpolation of #{reason} raises an Exception if reason is a tuple…

[error] Task #PID<0.12550.0> started from #PID<0.12511.0> terminating
** (RuntimeError) Some error
    handle_event/3
    (phoenix_live_view 0.20.17) lib/phoenix_live_view/async.ex:220: Phoenix.LiveView.Async.do_async/5
    (elixir 1.18.2) lib/task/supervised.ex:101: Task.Supervised.invoke_mfa/2
Function: #Function<7.25247813/0 in Phoenix.LiveView.Async.run_async_task/5>
    Args: []
[error] GenServer #PID<0.12511.0> terminating
** (Protocol.UndefinedError) protocol String.Chars not implemented for type Tuple

Got value:

    {%RuntimeError{message: "Some error"},
     [...]

Using "#{inspect(reason)}" instead.

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