Question

In linux, a process opens a file and doesn’t close it. Another process over a network opens the file and deletes it by file name. The first process continues to use the file as it was when it was opened, the delete is delayed on disk until the first process closes it. What manages the different versions of the same file? What type of protocols could be useful? What issues come up in unexpected situations – like a server crashes and when it comes back, the state of the remote server has been flushed – what do you do when the either process can’t keep the promise to maintain state?

Answer 1

If the file is replaced by a new file, it depends exactly how.
If the file's contents are overwritten, the file handle will still be
valid and access the new content. If the existing file is unlinked and
a new one created with the same name or,
if a new file is moved onto the existing file using rename(), it's the same as deletion (see above) -
that is, the file handle will continue to refer to the original version of the file.

In general, once the file is open, the file is open, and nobody changing
the directory structure can change that - they can move, rename the file
, or put something else in its place, it simply remains open.

In Unix there is no delete, only unlink(), which makes sense as
it doesn't necessarily delete the file - just removes the link from the directory.

If on the other hand the underlying device disappears (e.g. USB unplug)
then the file handle won't be valid any more and is likely to give IO/error
on any operation. You still have to close it though.
This is going to be true even if the device is plugged back in,
as it's not sensible to keep a file open in this case.