class ActionDispatch::ShowExceptions

catches the exceptions and returns a failsafe response.
code. If any exception happens inside the exceptions app, this middleware
middleware will send an empty response as a result with the correct status
If the application returns an "X-Cascade" => "pass" response, this
config.exceptions_app, which defaults to ActionDispatch::PublicExceptions.
In Rails applications, the exceptions app can be configured with
the PATH_INFO to the exception status code and call the Rack app.
store the exception in env["action_dispatch.exception"], rewrite
ShowExceptions. Every time there is an exception, ShowExceptions will
The exceptions app should be passed as a parameter on initialization of
and calls an exceptions app that will wrap it in a format for the end user.
This middleware rescues any exception returned by the application

def call(env) 

def call(env)
  request = ActionDispatch::Request.new env
  @app.call(env)
rescue Exception => exception
  if request.show_exceptions?
    render_exception(request, exception)
  else
    raise exception
  end
end

def fallback_to_html_format_if_invalid_mime_type(request) 

def fallback_to_html_format_if_invalid_mime_type(request)
  # If the MIME type for the request is invalid then the
  # @exceptions_app may not be able to handle it. To make it
  # easier to handle, we switch to HTML.
  request.formats
rescue ActionDispatch::Http::MimeNegotiation::InvalidType
  request.set_header "HTTP_ACCEPT", "text/html"
end

def initialize(app, exceptions_app) 

def initialize(app, exceptions_app)
  @app = app
  @exceptions_app = exceptions_app
end

def pass_response(status) 

def pass_response(status)
  [status, { "Content-Type" => "text/html; charset=#{Response.default_charset}", "Content-Length" => "0" }, []]
end

def render_exception(request, exception) 

def render_exception(request, exception)
  backtrace_cleaner = request.get_header "action_dispatch.backtrace_cleaner"
  wrapper = ExceptionWrapper.new(backtrace_cleaner, exception)
  status  = wrapper.status_code
  request.set_header "action_dispatch.exception", wrapper.unwrapped_exception
  request.set_header "action_dispatch.original_path", request.path_info
  request.set_header "action_dispatch.original_request_method", request.raw_request_method
  fallback_to_html_format_if_invalid_mime_type(request)
  request.path_info = "/#{status}"
  request.request_method = "GET"
  response = @exceptions_app.call(request.env)
  response[1]["X-Cascade"] == "pass" ? pass_response(status) : response
rescue Exception => failsafe_error
  $stderr.puts "Error during failsafe response: #{failsafe_error}\n  #{failsafe_error.backtrace * "\n  "}"
  [500, { "Content-Type" => "text/plain" },
    ["500 Internal Server Error\n" \
    "If you are the administrator of this website, then please read this web " \
    "application's log file and/or the web server's log file to find out what " \
    "went wrong."]]
end