Singleton process

[Andrew Bennetts]

On Mon, Dec 22, 2003 at 02:13:46AM -0500, Benjamin Han wrote:
> On Mon, 22 Dec 2003, Andrew Bennetts wrote:
> > On Mon, Dec 22, 2003 at 01:38:27AM -0500, Benjamin Han wrote:
> > >
> > > But this solution creates a file race condition?
> >
> > How?  mkdir is atomic.  Either it creates the directory and succeeds, or it
> > fails.  I don't see any problems here.
> 
> ok I see. The reason I'm thinking of using a lock file through fcntl is that
> even if a lock file exists (maybe due to a crash of some sort before), using
> the locking mechanism would still work. The "test if it exists" approach
> would fail because of some leftover from some accidental 'death' of the process
> before.

This is a potential problem, but Python is much less prone to unhandled
crashes than C code.  Use of a try/finally block, e.g.:

    if shouldRun():
        try:
            main()
        finally:
            os.rmdir(MAGIC_PATH)

You could possibly use an atexit handler instead.

-Andrew.