RFE: Lockfile option for use in cronjobs

[Matt McCutchen]

On Mon, 2008-03-31 at 11:43 +0200, Dag Wieers wrote:
> Looking for an easy way to prevent a repetitive rsync to be running 
> multiple times, I was wondering if it could be useful to have an option 
> like:
> 
>  	--pidfile /some/path/rsync-mirror-org.pid
> 
> So that rsync can be run directly from cron without requiring a wrapper 
> script to do pidfile handling.
> 
> This way rsync on startup could check the pid-file, see if another rsync 
> is using this pid, and bail out with an error if it is. Otherwise clean up 
> the stale pidfile and continue.
> 
> I think this would be very useful to instruct mirrors how to configure it, 
> rather than providing some script that needs local customizations.

I'm not convinced that a pidfile is better implemented in rsync than in
a wrapper script, which could be distributed in support/ of the source
tree.  If you don't care about actually having the pid in the file, you
could use the flock(1) utility, which executes a command while holding a
flock(2) lock on a specified file:

flock --nonblock /some/path/rsync-mirror-org.lock rsync ...

If the process goes away, the lock will too, so no manual cleanup of
stale locks is needed.

Matt