fix: ensure cleanup after shutdown

[zmstone]

• Ensure wolff_client_sup:ensure_absence and wolff_producers_sup:ensure_absence will perform shutdown and cleanup atomically.
  Previously, if the caller process is killed while waiting for shutdown, a terminated child may leak under the supervisor.
• Fixed a race condition which may cause {Exit, Supervisor, shutdown} message get consumed, causing wolff_client_sup:ensure_absence to force kill instead of normal shutdown.

[thalesmg]

This should not be possible in a gen_server, IIRC.

Indeed, a quick check indicates that's the case:

defmodule A do
  use GenServer

  def start_link() do
    GenServer.start_link(__MODULE__, self())
  end

  def init(owner) do
    Process.flag(:trap_exit, true)
    {:ok, %{owner: owner}}
  end

  def handle_info({:EXIT, pid, reason}, %{owner: owner} = st) do
    IO.inspect({owner, pid, reason})
    {:noreply, st}
  end
end

Process.flag(:trap_exit, true)
{:ok, pid0} = A.start_link()

# not the owner
spawn(fn -> Process.exit(pid0, :die_not_owner) end)

# owner tells it to die
Process.exit(pid0, :die_from_owner)

Output (does not print inspect when exit signal comes from owner):

iex(5)> # not the owner
nil
iex(6)> spawn(fn -> Process.exit(pid0, :die_not_owner) end)
{#PID<0.105.0>, #PID<0.112.0>, :die_not_owner}
#PID<0.112.0>
iex(7)> # owner tells it to die
nil
iex(8)> Process.exit(pid0, :die_from_owner)
true

12:00:03.369 [error] GenServer #PID<0.111.0> terminating
** (stop) :die_from_owner
Last message: {:EXIT, #PID<0.105.0>, :die_from_owner}
State: %{owner: #PID<0.105.0>}