Screen locking in Wayland

This is continuation to my Wayland desktop-shell post.

My goal was to implement a simple screen locking feature, a similar idea to what xlockmore does for X. In Wayland it is much simpler and more reliable to implement than in X, because the implementation will be in the display server (compositor). While the "lock" itself is in the compositor, also an unlock dialog is required. The unlock dialog usually asks the user to input his password, but I settled for "click the green ball". Screenshots below...

First a protocol (commit) is needed to drive the compositor locking and unlocking, since the unlock dialog is exported to the desktop-shell client. When the compositor hits the idle timeout, it fades out to black, and then locks itself in shell plugin. The compositor is woken up by input events, and sends prepare_lock_surface event to desktop-shell. The client replies with set_lock_surface request, with the unlock dialog's surface as an argument. Only on getting the surface, the compositor fades in, to have a nice transition to the dialog. The dialog then runs like any other application on screen, and when the user has dismissed it, desktop-shell sends unlock request to the compositor. On unlock, compositor brings all windows (surfaces) back to the desktop.

The shell plugin implements screen locking by stealing all the surfaces from the compositor's rendering list. Only the background surface and pointer cursor surfaces are left. This has the side-effect that none of the stolen (hidden) surfaces can be activated nor receive input. The compositor-side surface objects still continue living as usual. New surfaces can be created and they are automatically hidden. Output assignment of the hidden surfaces is set to NULL, which prevents sending any frame events for them, effectively also stopping any animations that might have been running. On unlock, the surfaces are simply put back into the compositor's list, and assigned to outputs.

After the last commit in the screen locking series, you can enjoy automatic screen locking in the Wayland demo compositor:

Normal desktop.

Locked, with the unlock dialog.

Note, that locking does not imply a fancy animated screensaver. The black screen is the screensaver ;-)

Thanks to Kristian Høgsberg for his reviews, comments and bug fixes.

This feature is sponsored by Collabora, Ltd.