safe_async

This is a wrapper around async functions so that the callback won't be called until all started tasks have called their callbacks. Standard async usually calls the callback right away on an error, but that can mean that there are orphaned tasks that don't know an error happened, so they are still trying to do their work. Usually this is fine, but if in the callback you have to remove some resources that the other task expects to be there, then you can get into a bad position. safe_async wraps the tasks and callback functions to monitor their progress, and will only call the final callback when all the tasks have completed, even if there was an error.