...

(PS: An alternative scheme would be to introduce a reference counter f.refcnt, which is increased in Files.Old and decreased in Files.Close, which would in addition remove a file descriptor from the list if the counter reaches 0. But that would make things more complicated).

I'm afraid the reference counter will not solve the issue, because, firstly, one is not asked to call Files.Close, one can just leave the file as is, and in this case the file descriptor will still need to be collected if there are no references to it. And, secondly, Files.Close does not actually "close" the file, but it just flushes the buffers, and after Files.Close the file can be still read from and written to. More calls to Files.Close can be done later. "Close" means "close the buffers".