mem_leak.exs

defmodule Thinger do
  
end

defmodule Thinger.Fubar do
  defstruct :queue, :some_module, :some_other_thing

  use GenServer

  def init(:ok) do
    {
      :ok,
      %__MODULE__{
        queue: :queue.new(),
        some_module: Thinger,
        some_other_thing: %{}
      }
    }
  end

  # bad
  def do_fubar(state) do
    # The closure is not released by the supervisor (parent process)
    # until the child process terminates.
    Task.Supervisor.async_nolink(state.task_supervisor, fn -> 
      # The entire state object is being captured, not just the specific values
      # in this case `queue` is also quietly being copied into the closure as well
      # causing each batch process to balloon in size.
      {:performed, state.some_module.run(batch, retries, state.some_other_thing)} 
    end)
  end

  # without copying entire state object into closure
  def dont_fubar(%{some_module: some_module, some_other_thing: some_other_thing} = state) do
    Task.Supervisor.async_nolink(state.task_supervisor, fn -> 
      {:performed, some_module.run(batch, retries, some_other_thing)} 
    end)
  end
end