avoid script running twice

[Jeff McNeil]

Note that in real life, the script exits cleanly if another copy is running...

On 6/18/07, Jeff McNeil <jeff at jmcneil.net> wrote:
> I've got a rather large log processing job here that has the same
> requirement.  I process and sort Apache logs from an 8-way cluster. I
> sort and calculate statistics in 15-minute batch jobs. Only one copy
> should run at once.
>
> I open a file and lock it via something like this:
>
> import fcntl
>
> fhandle = file("ourlockfile.txt", "w")
>
> try:
>     fcntl.lockf(fhandle.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)
> except IOError, e:
>     if e.errno == errno.EAGAIN:
>         print >>sys.stderr, "exiting, another copy currently running"
>     else:
>         raise
>
> I've got it wrapped in a 'FileBasedLock' class that quacks like Lock
> objects in the threading module.
>
> If the system bombs and reboots, the locks are cleared. If the first
> process dies via 'kill -XX', the locks it owns are released.  Of
> course, external influences can screw it all up, but that's not so
> much a concern in our environment.
>
> While I have no idea how portable this is, it works great for what I'm
> doing on RHES 5.
>
> -Jeff
>
> On 6/18/07, Tim Williams <tim at tdw.net> wrote:
> > On 18/06/07, Nick Craig-Wood <nick at craig-wood.com> wrote:
> > > Tim Williams <tim at tdw.net> wrote:
> > > >  You can also do this by holding a file open in write mode until the
> > > >  script has finished.
> > > >
> > > >        try:
> > > >             open('lock.txt','w')
> > > >             my_script()
> > > >       except:
> > > >            #print script is already running
> > >
> > > That only works under windows
> > >
> > >  >>> f=open('lock.txt','w')
> > >  >>> g=open('lock.txt','w')
> > >  >>> f.write('hi')
> > >  >>> g.write('ho')
> > >  >>> f.close()
> > >  >>> g.close()
> > >  >>> open('lock.txt').read()
> > >  'ho'
> > >  >>>
> > >
> > > The best cross platform way to create a lock is creating a directory.
> > > It is atomic on both windows and linux anyway.
> > >
> > >  try:
> > >    os.mkdir("lock")
> > >  except OSError:
> > >    print "locked!"
> > >  else:
> > >    try:
> > >      do_stuff()
> > >    finally:
> > >      os.rmdir("lock")
> > >
> > > (untested)
> > >
> >
> > Being a windows-only person, I didn't know that :)
> >
> > Actually I think I did,  this thread has happened before - a few months ago :)
> >
> > I would be worried with the directory-exists option for the same
> > reason I don't use the file-exists method currently.  It is possible
> > for the directory to exist when the script isn't running, thus
> > preventing the script from running again until someone notices.
> >
> > On Windows the open-a-file-for-writing method works well, but as *nix
> > doesn't work the same way then maybe the socket solution is the best
> > cross-platform option.  The socket can't exist when the script isn't
> > running, and if you try and create a duplicate socket you catch the
> > exception and exit.
> >
> > IMHO of course.
> >
> > :)
> > --
> > http://mail.python.org/mailman/listinfo/python-list
> >
>