Elixir test PID capture

pid_snitch.ex

defmodule PidSnitchFormatter do
  @moduledoc """
  Report test PIDs?

  ```bash
  mix test --formatter PidSnitchFormatter
  ```

  See `ExUnit.CLIFormatter` for protocol details.
  """
  use GenServer

  @impl GenServer
  def init(opts), do: {:ok, opts}

  def handle_trace({:trace, _pid, :call, {ExUnit.Runner, :exec_test, [_test]}}, fmt_pid) do
    # ignore
    fmt_pid
  end

  def handle_trace({:trace, pid, :return_from, {ExUnit.Runner, :exec_test, 1}, test}, fmt_pid) do
    IO.puts("#{inspect(pid)} at #{test.tags[:file]}:#{test.tags[:line]}")
    fmt_pid
  end

  @impl GenServer
  def handle_cast(message, opts)

  # test protocol; see ExUnit.CLIFormatter
  def handle_cast({:suite_started, _opts}, opts) do
    :dbg.stop_clear() |> IO.inspect(label: "#{__ENV__.file}:#{__ENV__.line}")

    :dbg.tracer(:process, {&handle_trace/2, self()})
    :dbg.tpl(ExUnit.Runner, :exec_test, [{:_, [], [{:return_trace}]}])
    :dbg.p(:all, :call)

    {:noreply, opts}
  end

  def handle_cast({:suite_finished, _, _}, opts) do
    :dbg.stop_clear()
    {:noreply, opts}
  end

  def handle_cast(_message, opts) do
    {:noreply, opts}
  end
end