deliveroo/routemaster

require 'logger'
require 'delegate'
require 'singleton'

module Routemaster
  module Services
    class Logger < SimpleDelegator
      include Singleton

      def initialize
        file_path = ENV['ROUTEMASTER_LOG_FILE']
        file = (file_path && File.exist?(file_path)) ? File.open(file_path, 'a') : $stderr

        instance = ::Logger.new(file).tap do |logger|
          logger.level     = _log_level_constant
          logger.formatter = method(:_formatter)
        end

        unless _log_level_valid?
          instance.warn("log level #{_log_level} is invalid, defaulting to INFO")
        end

        super(instance)
      end

      def context=(string)
        ::Thread.current[:_log_context] = string
      end

      private

      TIMESTAMP_FORMAT = '%F %T.%L'

      def _formatter(severity, datetime, _progname, message)
        _format % {
          timestamp: datetime.utc.strftime(TIMESTAMP_FORMAT),
          level:     severity,
          message:   message,
          context:   ::Thread.current[:_log_context] || 'main',
        }
      end

      def _format
        @@_format ||= begin
          # In "deployed" environments (normally running Foreman), timestamps are
          # already added by the wrapper.
          _show_timestamp? ?
            "[%<timestamp>s] %<level>s: [%<context>s] %<message>s\n" :
            "%<level>s: [%<context>s] %<message>s\n"
        end
      end

      def _show_timestamp?
        ENV.fetch('RACK_ENV', 'development') !~ /staging|production/
      end

      def _log_level_constant
        if _log_level_valid?
          ::Logger.const_get(_log_level)
        else
          ::Logger::INFO
        end
      end

      def _log_level_valid?
        _log_levels.include?(_log_level)
      end

      def _log_level
        ENV.fetch('ROUTEMASTER_LOG_LEVEL', 'INFO')
      end

      def _log_levels
        ::Logger::Severity.constants.map(&:to_s)
      end
    end
  end
end