Gracefully shut down the process tree with name registration

gracefully_shutdown_process_tree.exs

defmodule ParentServer do
  use GenServer
  import Supervisor.Spec

  def start_link() do
    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
  end

  def init(_) do
    Process.flag(:trap_exit, true)

    children = [
      worker(ChildServer, [])
    ]

    {:ok, child_supervisor_pid} = Supervisor.start_link(children, strategy: :one_for_one, name: ChildSupervisor)

    IO.puts("Started Supervisor")

    IO.puts("Started #{__MODULE__}:")

    {:ok, nil}
  end

  def terminate(reason, state) do
    IO.puts("Terminating #{__MODULE__}")

    # This works! Uncomment this line and run the script with no error!
    # GenServer.stop(ChildSupervisor)

    :normal
  end
end

defmodule ChildServer do
  use GenServer

  def start_link() do
    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
  end

  def init(_) do
    Process.flag(:trap_exit, true)

    IO.puts("Started #{__MODULE__}:")
    {:ok, nil}
  end

  def terminate(reason, state) do
    IO.puts("Terminating #{__MODULE__}")
    :normal
  end
end

for i <- 1..1000 do
  {:ok, parent_pid} = ParentServer.start_link()
  GenServer.stop(parent_pid)

  # Uncommenting this out sorts out registration problem
  # :timer.sleep(10)
end

output

$ elixir gracefully_shutdown_process_tree.exs
...
Started Elixir.ChildServer:
Started Supervisor
Started Elixir.ParentServer:
Terminating Elixir.ParentServer
Terminating Elixir.ChildServer
** (EXIT from #PID<0.71.0>) an exception was raised:
    ** (MatchError) no match of right hand side value: {:error, {:already_started, #PID<0.81.0>}}
        gracefully_shutdown_process_tree.exs:16: ParentServer.init/1
        (stdlib) gen_server.erl:328: :gen_server.init_it/6
        (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3