New subject: ANN: withlock - reliable & easy locking for cron jobs

22 Feb 2010


      Hi,
[Apologies to those of you who received this mail more than once, due to crossposting to different forums in order to reach most relevant people]
This is a one-time post to announce the creation of "withlock" - a locking wrapper script to make sure that some program isn't run more than once. It is ideal to prevent periodic jobs spawned by cron from stacking up. The locks created are valid only while the wrapper is running, and thus will *never* require additional cleanup, even after a reboot. This makes the wrapper safe and easy to use, and much better than implementing half-hearted locking within scripts.
Usage is simple. Instead of your command
    CMD ARGS...
you simply use
    withlock LOCKFILE CMD ARGS...
If you are a mirror admin, there is a 99.9% chance that you might want to use this locking wrapper. Likely you ran into the situation where a script was (unexpectedly...) not finished before it was started another time; and/or you actually hacked some kind of locking into a script to prevent that from happening.
Since I suffered the same situation for some years, I looked around for solutions, and since I couldn't find one, I finally took the time to create one. The resulting wrapper is used in production since summer 2009, and proved to work reliably. Put the wrapper around all your cron jobs and be happy :-)
Use cases are to be found not only in mirroring, but that's where I come from, so I know that this will be useful for you guys.
Home page: http://code.google.com/p/withlock/
Features:
   - locks that never need a cleanup, whatever happens
   - can wait a defined time for a lock to become "free"
   - disallows lock files in unsafe locations (to prevent symlink attacks)
   - easily installed (it's just one file), highly portable
Requirements:
   - Python 2.4 or newer
   - lots of platforms are supported (see list on home page)
For more information, please come to http://code.google.com/p/withlock/ .
Thanks,
Peter