defmodule Handler do
  @moduledoc """
  A helper for running functions that might take too long, or use too much memory.

  Handler will run these functions in their own process and "take care of" problematic processes.
  """

  alias Handler.{OOM, ProcessExit, Timeout}
  @type opts :: keyword()
  @type exception :: OOM.t() | ProcessExit.t() | Timeout.t()

  @doc """
  Run a potentially dangerous function in a safe way.

  ## Examples

      iex> Handler.run(fn -> 1 + 1 end)
      2

      iex> Handler.run(fn -> :timer.sleep(200) end, max_ms: 10)
      {:error, %Handler.Timeout{message: "Took more than 10ms to complete"}}

      iex> Handler.run(fn -> Enum.map(1..10_000, & &1*100) end, max_heap_bytes: 4096)
      {:error, %Handler.OOM{message: "Process tried to use more than 4096 bytes of memory"}}

      iex> Handler.run(fn -> Process.exit(self(), :i_am_ded) end)
      {:error, %Handler.ProcessExit{message: "Process exited with :i_am_ded", reason: :i_am_ded}}

  """
  def run(fun, opts \\ []) do
    max_ms = max_ms(opts)
    max_heap_bytes = max_heap_bytes(opts)

    old_trap_exit = Process.flag(:trap_exit, true)

    result =
      fun
      |> kickoff_fun(max_heap_bytes)
      |> await_results(max_ms, max_heap_bytes)

    Process.flag(:trap_exit, old_trap_exit)

    result
  end

  @doc false
  def bytes_to_words(max_bytes) when is_integer(max_bytes) do
    div(max_bytes, :erlang.system_info(:wordsize))
  end

  @doc false
  def max_heap_bytes(opts) do
    Keyword.get(opts, :max_heap_bytes, 10 * 1024 * 1024)
  end

  @doc false
  def max_ms(opts) do
    Keyword.get(opts, :max_ms, 60_000)
  end

  defp await_results(%Task{ref: ref, pid: pid} = task, max_ms, max_heap_bytes) do
    receive do
      {^ref, result} ->
        Process.demonitor(ref, [:flush])
        result

      {:DOWN, ^ref, :process, ^pid, :killed} ->
        Process.demonitor(ref, [:flush])
        message = "Process tried to use more than #{max_heap_bytes} bytes of memory"
        {:error, OOM.exception(message: message)}

      {:DOWN, ^ref, :process, ^pid, {header, _stacktrace} = reason} ->
        Process.demonitor(ref, [:flush])
        message = "Process exited with #{inspect(header)}"
        {:error, ProcessExit.exception(message: message, reason: reason)}

      {:DOWN, ^ref, :process, ^pid, reason} ->
        Process.demonitor(ref, [:flush])
        message = "Process exited with #{inspect(reason)}"
        {:error, ProcessExit.exception(message: message, reason: reason)}
    after
      max_ms ->
        Process.demonitor(ref, [:flush])
        Task.shutdown(task, :brutal_kill)
        message = "Took more than #{max_ms}ms to complete"
        {:error, Timeout.exception(message: message)}
    end
  end

  defp kickoff_fun(fun, max_heap_bytes) do
    Task.async(fn ->
      Process.flag(:max_heap_size, bytes_to_words(max_heap_bytes))
      fun.()
    end)
  end
end