Double locking a seat causes an X server to be launched and killed

If a seat is locked when a (non-resettable) greeter is already executing, a new X server is launched. But anyway, when the X server has finished launching, lightdm decides to switch back to the existing greeter instead of executing a new one, and kills the X server. Therefore the X server launch is useless and a waste of time and resources.

[Test Case]
1. Lock a session (e.g. executing "dm-tool lock").
2. When the greeter is launched, switch back to the locked session using the VT mechanisms (i.e. Ctrl+Alt+F7).
3. Lock the session again.
4. After a short delay, the greeter is activated again. But if you check the lightdm log file, you can observe that a new X server (display :2) has been started then killed once initialized.

[Root cause]
In function seat_lock() in seat.c, an existing greeter is searched using function find_resettable_greeter (seat). If a greeter exists, but is not resettable, then it's not reused (at least at that moment), a new greeter is created, and an X server is launched.

But when the X server launch process has finished and authentication of the new greeter session has succeeded, in the callback function session_authentication_complete_cb() in seat.c, a previous user session is searched, which the previous greeter matches, and therefore that previous session is activated and the new session is stopped (also stopping the associated new X server).