call

# File actionpack/lib/action_dispatch/middleware/flash.rb, line 240
    def call(env)
      @app.call(env)
    ensure
      session    = Request::Session.find(env) || {}
      flash_hash = env[KEY]

      if flash_hash
        if !flash_hash.empty? || session.key?('flash')
          session["flash"] = flash_hash.to_session_value
          new_hash = flash_hash.dup
        else
          new_hash = flash_hash
        end

        env[KEY] = new_hash
      end

      if (!session.respond_to?(:loaded?) || session.loaded?) && # (reset_session uses {}, which doesn't implement #loaded?)
         session.key?('flash') && session['flash'].nil?
        session.delete('flash')
      end
    end