[launchd-dev] Starting launch agents

[Quinn]

At 14:58 -0400 29/10/07, Jeremy Reichman wrote:
>So now, with Leopard, LaunchAgents in /Library/LaunchAgents will run at
>login for each user, as the user [...], and without having to restart
>the computer between each user login session?

Yes.  The only significant gotcha that I can think of relates to 
installing and upgrading.  Except in the simple case (where you have 
one user logged in and your installer/upgrader is running in that 
user's context), you run into problems with the install/upgrade 
process.  There's no built in way for you to tell all per-user 
launchd's on the system that a new property list has been installed 
(or that an old one has been upgraded).  Thus, you can get into 
situations where one user is running version X of your agent and 
another is running version X+1.  Or one user doesn't have the agent 
installed and another does.  There's no really good way around this 
right now (although there are plenty of hackish ways).

Just for the record, we're tracking this as <rdar://problem/5476420>.

>as the user (by default)

The "(by default)" indicates some confusion about how agents work. 
An agent is always launched by a per-user launchd, and that per-user 
launchd is always running with its EUID and RUID set to that of the 
user on whose behalf it's working.  Thus, there's no "by default" 
about it.  An agent will always run as that user and there's no way 
around that (other than by doing something silly like making your 
agent setuid root).  The UserName and GroupName properties in an 
agent's plist are ignored (as they were on 10.4.x).

I've rolled all of this info into a new Technote 2083 "Daemons and 
Agents" that scheduled to be published in the next couple of weeks. 
Watch the skies! (actually, <http://developer.apple.com> :-)

S+E
-- 
Quinn "The Eskimo!"                    <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware