From: khaines@...
Date: 2006-08-30T05:55:05+09:00
Subject: Re: process based locks for files

On Wed, 30 Aug 2006, Mike Fletcher wrote:

> Radu Spineanu wrote:
>> Hi,
>>
>> I have two programs accessing the same file. Both programs open and
>> close the file multiple times while running (for example reading it
>> first, and then writing it back using global functions).
>>
>> In order to avoid race conditions i was wondering if it's possible to
>> lock the file at the start of the running process and then unlock it at
>> the end. During that time the running process should be able to open and
>> close the file without problems.
>
> There's File#flock, but keep in mind that flock is only an advisory lock
> (i.e. it only notifies cooperating processes that try and get a lock
> themselves) and is notoriously flakey on NFS mounts.

I don't see a mention of your platform.  However, you might consider Ara 
Howard's lockfile.rb.  If is an NFS safe file lock library that works 
well.  It just doesn't work on Windows.

If you need something that can work on both Windows and not-windows, email 
me.  I have a hacked and a bit simplified version that will autodetect 
whether the primary locking scheme will work or not, and will fall back to 
flock based locking.  It remembers the result of the autodetection so 
subsequent locks opened in the same process later don't incur the cost of 
detection again.

Ara's library works well, and I believe he uses it extensively.

My hacked version probably works less well because it is an undertested 
ugly hack, but in my (less extensive) use of it, I haven't encountered any 
problems so far.

Find his lockfile.rb here:

http://codeforpeople.com/lib/ruby/lockfile/



Kirk Haines