xl-nopw: A simple transparent X11 lock with no password handling

This is a simple, transparent screen locker for X11, based on Danny Dulai's xl.

The goal here is to have very simple, very small, very robust program to prevent passers-by from interacting with the screen. All this program does is lock your keyboard and mouse. The screen is not blanked, which is useful if you want to leave a display running for public viewing. The only way to unlock the screen is by killing the program somehow, which cannot be done through the locked screen.

The main differences with respect to the original xl are:

• It avoids the need for any password management by simply not having any way to unlock, by password or otherwise. You must arrange some other way to kill the program.

• A countdown has been added so that user has time to hide the terminal window if this is invoked from the terminal.

• Added some checks for failure to obtain the pointer and keyboard grabs.

Display manager hot buttons and keys

Some display managers intercept key presses and mouse movements in ways that bypass pointer and keyboard grabs. In particular, a hidden Unity launcher bar will still appear when the mouse pointer is moved to the edge of the window, and the "Meta" and "Left-Alt" keys might still cause HUD's to appear. This is probably harmless as the user still can't type or click on anything. But to achieve complete non-interactivity, you'd have to disable these features or simply use a different windows manager. (twm, anyone?)

Alternative approaches

• xinput can be used to completely disable input devices, as described here. An advantage of that approach is that the devices are totally disabled, such that not even the display managers can receive them. A disadvantage is that a visitor could plug in another device and it might start working by default, unless you take special actions to configure X differently; or simply unplugging and reconnecting a device could do this; or a mouse might even keep re-enabling itself, as described here. The basic xinput approach also needs a little manual attention to find the correct device number(s).

• If you have physical control of the computer and a way of stopping people from plugging their own keyboards or pointers into it, you could simply lock away or remove the keyboard and mouse.

• xtrlock was an old favorite, but last time I checked it only supported passwords in /etc/passwd or /etc/shadow. Using /etc/shadow required xtrlock to run with elevated privileges. A version supporting PAM that can be run unprivileged is available on github as xtrlock-pam. These both suffer from the feature noted above under "Display manager hot buttons and keys", but they have the ability to unlock the screen with a password, which might be nice depending on your application.