SVN post-commit user permissions

I've a debian webserver with subversion running on it.

I'm trying to use post-commit script to update my staging version of my site.

#!/bin/sh

/usr/bin/svn update /home/sites/example.com/www >> /var/log/svn/example.log 2>&1

if I run that command from the command line logged in as user 'derek' then it all works fine

but when it runs as post-commit I get the following error in my log file:

svn: Can't open file '/home/sites/example.com/www/.svn/lock': Permission denied

Ok, so I realize what is happening here is the user calling the post-commit script isn't 'derek' so it hasn't permission.

So my question is what user is calling the post-commit script.

the svnserve daemon is run as derek ... I thought that would mean the post-commit command would be called as derek too but it seems not.

Any ideas how I can find out what user is calling it

and secondly what would be the best practice method to allow access for that user? I don't think adding it to the group would help because the group doesn't have the write access to the .svn directories by default.

UPDATE: I've just found that www-data is the user that seems to be calling the script. Now naturally www-data can't run the svn command.