Idea for joystick support in sandbox/SDL

[Bastien Nocera]

Hey,

I've been looking at how we can support joysticks in flatpak, inside
the sandbox[1]. As udev's API is a private/system API, and not one
stable enough for sandboxed applications to use, and the fact that even
though they're input devices their events don't go through the
compositor itself, we need to find a way to enumerate joysticks, signal
additions and removals, and proxy opening them.

My idea was for the compositor offering this D-Bus API:

- Method:
ao ListJoysticks()

- Signals
JoystickAdded(o)
JoystickRemoved(o)

And on the object path for each joystick:

- Method:
fd OpenJoystick()

Signal:
JoystickRemoved()

Properties:
s HapticsFeatures [2]

To avoid one application taking the joystick and doing things with it
(rumble to death?), we'd revoke the fd given to an application when the
application loses focus.

Is this doable? Is this good API?

I'd be happy working on the client-side implementation for SDL2 at
least. I'll try to see whether support for SDL1 is necessary (might
only be necessary for *ancient* proprietary games, like the old Loki
games).

Cheers

[1]: https://github.com/flatpak/flatpak/issues/7
[2]: As used in src/haptic/linux/SDL_syshaptic.c in SDL. Maybe a string
list might be better, or a bitmask?