Comment 4 for bug 990661

Robert, can I pass this off to you? I did some investigation (below), but I'm at a point where I don't understand the lightdm code nearly as well as you.

So there are two ways I can see to create zombies:
1) Press Escape in the greeter, which behind the scenes asks LightDM to start another authentication session for the user
2) Try to log in as a user that already has a session running

The same thread for these two ways to trigger zombie processes are authentication workflows that didn't come to fruition.

Looking in the log, the zombie matches the bits that say (for example, if zombie is pid 3704 after I tried to log in as an already-logged in user):

[+301.12s] DEBUG: Greeter start authentication for mike
[+301.12s] DEBUG: Started session 3704 with service 'lightdm', username 'mike'
[+301.15s] DEBUG: Session 3704 got 1 message(s) from PAM
[+301.15s] DEBUG: Prompt greeter with 1 message(s)
[+304.25s] DEBUG: Continue authentication
[+304.39s] DEBUG: Session 3704 authentication complete with return value 0: Success

However, looking at session lifecycles in lightdm has always made my eyes bleed. Robert, does the above make you think of anything? I'm guessing some class isn't cleaning up correctly on "early exits" from an authentication session.