Lock out a file

[Guest]

I've created webform gui where I can chose a file, and move it a another
folder. If the user clicks on "OK", the file should be moved to the specified
folder.

The problem is, the files are public, that means, they are downloadable and
it can be that some one is downloading a file at the same time it should be
moved.


So, my question is, how do I lock a file in way that blocks all write and
read access to it immediately? If a file is marked to be moved, and the
process starts, all downloads of the file needs to be aborted.