Seat identifiers on x11/wayland

[elinorbgr]

Wayland is a multi-seat aware protocol, and I believe x11 is too. From this, I think it could be reasonable to add a platform specific extension trait unix::DeviceIdExt like so:

pub trait DeviceIdExt {
    /// Get the name of the seat this device is associated with
    ///
    /// Some systems can be multi-seat, meaning that there may be more than
    /// one keyboard / pointer, and these should be treated as different users interacting
    /// with the app.
    ///
    /// If this kind of multi-user interaction would be meaningful with your app, this
    /// method allows you to retrieve a seat-unique identifier from this `DeviceId`.
    fn seat(&self) -> &str;
}

While arguably pretty niche, this seat-awareness can be necessary for downstream apps. For example, on wayland, there is one clipboard per seat, and the app should be careful to trigger the copy and paste actions from the appropriate seat.

Thoughts on this design, @francesca64 ?

[pickfire]

I believe this is the same for X11 which is usually called multi-pointer https://wiki.archlinux.org/index.php/Multi-pointer_X (not to be confused with multi-seat). Quick scanning at the docs, seems like not possible to identify which keyboard active using KeyboardEvent alone.

[notgull]

Is there a way to get the name of the seat from multi-pointer mode? xtrace seems to indicate that the seat name isn't sent over the wire (at least by default) and the Xinput2 specification doesn't mention seats at all. It seems that master devices take priority here, but the seat name doesn't match up with the real seat (e.g. "seat0").

[kchibisov]

I'm not sure about X11 here. I know that with Wayland you have seat IDs.