CGIs and file exclusion

[darkhorse]

Hi all,

While doing a quite big "set of programs" for a university subject I've
found myself in the middle of a problem someone surely has had before, so
I'm looking for some help.

At one point, I call a python cgi that pickle.load's a file, adds or deletes
a registry and dumps the result again in the file.
I'm worried that the cgi could be called simultaneously from two or more
different computers, thus most probably corrupting the files. I don't think
I can use a mutex as it's two different instances of the program and not
different threads, so I was thinking about locking the files between
programs, but I really don't know how to do that. It's not a problem if
there's no portable way of doing this, it's only going to be run on a linux
based computer.
Another solution I would accept is that the second called cgi detects that
other instance is running and displays a message saying to try again later.
Yes, not quite professional, but it'd do the job, after all this is just a
little detail I want to settle for a quite picky professor and not a "real
life" thing.
I think that's all the background you need, if someone can answer with
references on what should I look for or even better example code that would
be simply great.
Many thanks in advance.
DH