code: func executeShutdownCallbacks(signame string) (exitCode int) {
  	shutdownCallbacksOnce.Do(func() {
  		errs := allShutdownCallbacks()
  		if len(errs) > 0 {
  			for _, err := range errs {
  				log.Printf("[ERROR] %s shutdown: %v", signame, err)
  			}
  			exitCode = 1
  		}
  	})
  	return
  }
docs: // executeShutdownCallbacks executes the shutdown callbacks as initiated
  // by signame. It logs any errors and returns the recommended exit status.
  // This function is idempotent; subsequent invocations always return 0.