Discussing DM-20763 with John Parejko leads me to think that datastore should guarantee that Formatter instances are not reused. This would allow formatter implementations to cache information with the knowledge that they don't have to worry about the cache being wrong if the file descriptor changes. To make this more obvious I'm going to investigate changing Formatter so that FileDescriptor is a mandatory constructor argument becoming a read-only property. This will make it impossible for the formatter to be used for any other file and will mean that the argument is removed from the read and write methods.