Hi,

I'm been looking at using redis to synchronize work between multiple
processes and I was surprised to see there wasn't a compare-and-swap
http://en.wikipedia.org/wiki/Compare-and-swap method available.

It's a well understood mechanism for synchronization and would make the
locking process described in http://redis.io/commands/setnx a bit better as
taking an expired lock would be done via CAS rather than GETSET which may
replace a lock someone else has put in place (and avoid two people taking
the same lock if they took it in the same millisecond). Also, it would make
expiring a lock you hold safer as you wouldn't delete a newly created lock
(had your lock expired), instead you could CAS the expiry date you set with
one in the past.

Does that make sense? Is this a desirable feature? Is there something I'm
overlooking?

I can look into creating a PR for this if needed but I don't know C so it
might take me a while.

Cheers,

Garry Shutler

www.robustsoftware.co.uk | @gshutler <http://twitter.com/gshutler>