antikythera

# Copyright(c) 2015-2018 ACCESS CO., LTD. All rights reserved.

use Croma

defmodule Antikythera.GearApplication.ErrorHandler do
  @moduledoc """
  Helper module for gear's custom error handler.

  To customize HTTP responses returned on errors, gear must implement an error handler module which

  - is named `YourGear.Controller.Error`, and
  - defines the following functions:
      - Mandatory error handlers
          - `error/2`
          - `no_route/1`
          - `bad_request/1`
      - Optional error handlers (when your gear uses websocket)
          - `ws_too_many_connections/1`

  This module generates `YourGear.error_handler_module/0` function, which is called by antikythera when handling errors.
  """

  alias Antikythera.GearName
  alias AntikytheraCore.GearModule

  @all_handlers [
    error:       2,
    no_route:    1,
    bad_request: 1,
  ]

  @doc false
  defun find_error_handler_module(gear_name :: v[GearName.t]) :: nil | module do
    mod = GearModule.error_handler_unsafe(gear_name) # during compilation of gear, allowed to call unsafe function
    try do
      exported_funs = mod.module_info(:exports)
      not_defined_handlers = Enum.reject(@all_handlers, &(&1 in exported_funs))
      if Enum.empty?(not_defined_handlers) do
        mod
      else
        IO.puts("""
        [antikythera] warning: #{mod} exists but some of the error handlers are not defined: #{inspect(not_defined_handlers)};
        [antikythera]   #{mod} is not eligible for a custom error handler module.
        """)
        nil
      end
    rescue
      UndefinedFunctionError -> nil # module not defined
    end
  end

  defmacro __using__(_) do
    quote do
      # Assuming that module attribute `@gear_name` is defined in the __CALLER__'s context
      @error_handler_module Antikythera.GearApplication.ErrorHandler.find_error_handler_module(@gear_name)
      defun error_handler_module() :: nil | module do
        @error_handler_module
      end
    end
  end
end