Get watchdog interval #6

I was helping a friend implement watchdog notifications, and noticed that, unlike sd_watchdog_enabled(3), this gem doesn’t have a way to get the configured watchdog interval. Instead, pretty much every Ruby gem or app I’ve seen use this has to implement the same pattern:

def do_watchdog
  return unless SdNotify.watchdog?

  # Divide by 1,000,000 since 99% of Ruby tooling uses seconds, not microseconds.
  interval = Integer(ENV.fetch('WATCHDOG_USEC')) / 1_000_000.0
  loop do
    SdNotify.watchdog
    sleep interval
  end
end

It would be much more convenient (and require less digging around to learn the environment variables) if this gem exposed a method to get the watchdog interval:

def do_watchdog
  return unless SdNotify.watchdog?

  loop do
    SdNotify.watchdog
    sleep SdNotify.watchdog_interval
  end
end

Ideally, I think this would:

1. Return 0 (or maybe -1?) if the watchdog is not expecting notifications (that is, if SdNotify.watchdog? would have returned false).

2. Return the number of seconds as a float instead of the number of microseconds as an integer, since almost all use cases will involve doing this anyway.