Race condition between assign_async and temporary_assigns

[kelvinst]

Environment

• Elixir version (elixir -v):

Erlang/OTP 27 [erts-15.1.2] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit]

Elixir 1.17.3 (compiled with Erlang/OTP 27)

• Phoenix version (mix deps):

* phoenix 1.7.14 (Hex package) (mix)
  locked at 1.7.14 (phoenix) c7859bc5
  ok

• Phoenix LiveView version (mix deps):

* phoenix_live_view 1.0.0-rc.7 (Hex package) (mix)
  locked at 1.0.0-rc.7 (phoenix_live_view) b82a4575
  ok

• Operating system: MacOS Sonoma 14.4.1
• Browsers you attempted to reproduce this bug on (the more the merrier): It's a problem with tests
• Does the problem persist after removing "assets/node_modules" and trying again? Yes/no: Yes (well, it should, didn't test, but you will get it)

Actual behavior

I have a LiveView like this:

defmodule TwelvexWeb.StocksLive do
  def mount(params, _session, socket) do
    {:ok, socket, temporary_assigns: [time_series: AsyncResult.loading()]}
  end

  # ...
  
  defp apply_action(socket, :time_series, params) do
    assign_async(socket, :time_series, fn ->
      {:ok, %{time_series: get_time_series(params["symbol"])}}
    end)
  end

  # ...
  
  def render(assigns) do
    ~H"""
    <%!-- ... --%>

    <.async_result :let={time_series} assign={@time_series}>
      <:loading> Loading time series </:loading>
      <:failed> Failed to load time series </:failed>
      <.chart
        id="time-series-chart"
        series={time_series}
      />
    </.async_result>


    <%!-- ... --%>
    """
  end
end

Then a test for it, like this:

test "loads the time series chart asynchronously", %{conn, conn} do
  {:ok, live, html} = live(conn, ~p"/time_series?symbol=AAPL")
  assert html =~ "Loading time series"
  html = render_async(live)
  refute html =~ "Loading time series"
  # ...
end

And this test gets flaky, failing randomly in the refute html =~ "Loading time series". During my investigations I figured the flakiness went away when I added a pid |> Process.info() |> dbg() inside the function I pass assign_async(:time_series, <here>). Then I realized the same assign was being set by temporary_assigns too.

My guess is that the error happens when the temporary_assigns logic runs right after assign_async task finishes but before the render. Not even sure if that's fixable on LiveView side, but at least worth mentioning in the docs.

Expected behavior

I would expect to not have problems with assign_async and temporary_assigns being combined.

[kelvinst]

I think I can put up a simple example project for it, let me know if you need that.

[drtheuns-enreach]

I think I've run into the same (or similar) issue, where I have a start_async that, if it finishes too soon, causes the loading state to be rendered even though the state has already updated. In my case this is without temporary assigns. Inside of a live component if that makes a difference. I don't have a small reproduction; I just added a Process.sleep for now, which is less than ideal.