Launchd - can it run a script when users log in OR switch?

Question

I'm hoping to find someone who knows all the gory details of launchd. Here's the why and what: I'm trying to coerce Aperture into letting two users on the same computer share the same Aperture Library, both able to import photos and see and use what the other has done. Initially it is no problem, change the permissions so everyone has read/write/execute, put the Library in a common area, and point both person's Aperture at it. The problem is that Aperture changes permissions of things inside the Library as you use it, and especially when one person imports, the other person can't see that stuff.
So I want to run a script whenever someone logs in or becomes the active user by fast user switching; the script will simply change ownership and permissions of the library and everything in it to restore full access.
After days of research and testing, I made a test launchd plist file (this is the configuration file or agent that launchd looks at and runs your script when criteria are met). It sits in /Library/LaunchAgents/, here it is:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>org.jim.test</string>
<key>LimitLoadToSessionType</key>
<string>LoginWindow</string>
<key>ProgramArguments</key>
<array>
<string>/Library/Scripts/testscript.sh</string>
</array>
<key>RunAtLoad</key>
<true/>
<key>StandardErrorPath</key>
<string>/Users/jim/Desktop/test.err</string>
<key>StandardOutPath</key>
<string>/Users/jim/Desktop/test.out</string>
</dict>
</plist>
The actual test script is sitting in /Library/Scripts/:
#!/bin/bash
echo "org.jim.test.plist was run at `date`"
So if the plist is loaded and the script run, the date and time are written in the test.out file on my desktop. This thing runs great when a user logs in. The problem is, it doesn't run when users switch by fast user switching. In reading the documentation, it was vague about this, whether that could be done; I thought it could.
Can anyone help with a way to get the script to run when any user takes control by any means? As an added complication, which I'll worry about later, the script has to run as root because it's the only way to get the ownership and permissions changed.
Thanks,
Jim

Answer 1

BobHarris is the MAN. ACLs seem to have worked. I put the Aperture Library in /Users/Shared/ApertureLibrary, restored the regular permissions as best I could (me as owner, staff as group, 755), pointed each person's Aperture to the library, then proceeded with the ACL magic. First I created a group for my wife and I, called 'aperture', in the Accounts preference pane. That was to simplify the ACLs (one entry instead of 2) and so that other users (son) aren't able to muck it up.
It took one stinking Terminal command (note that 'aperture' at the beginning of the quote is the group, not the program, folder or library):
chmod -R +a "aperture allow list,addfile,search,delete,add_subdirectory,delete_child,file_inherit,directoryinherit" ApertureLibrary.aplibrary
I was unsure whether this list of permissions was necessary or sufficient, but it seems to have worked so far. Both users can access the library. I imported a different picture by each user, and the other user could then see and delete the picture imported by the other. This was not possible before.
Bob, thanks for the great idea. After a little more testing I'll post a complete how-to in the latest "how do we share Aperture" thread, and give you due credit. The bad news is all the launchd fun is over
Jim