GTK hardware scancodes for wayland / detecting XWayland

[Daniel P. Berrange]

I'm trying to port GTK-VNC to work correctly under Wayland, both as
a native client and as an Xwayland client. The only place we have
any code that is specific to particular display servers is the keyboard
handling. We can't use the GdkEventKey keyval directly because that
results in having todo lossy keymap conversions in QEMU, so we need to
translate the key event to get the values in terms of XT scancodes
that we can send straight to QEMU. This involves a lossless translation
of the values that GTK reports in the hardware_keycode field of
GdkEventKey.

Sadly there is no standard for what scancode values are reported in
the hardware_keycode field, so we need to add a custom remapping for
each display server.

In traditional Xorg, the hardware_keycode is either reporting custom
set of scancodes defined by xfree86 X11 keyboard driver, or with modern
deployments using the Linux evdev scancodes with a simple offset-by-8
applied.

First I wanted to confirm what will be reported in hardware_keycode
for both Wayland native and XWayland ? In my limited testing it
appears that in both cases GTK is getting evdev scancodes with the
same offset-by-8 applied as Xorg+evdev uses. Is this correct and is
that guaranteed to stay with this mapping long term ?

Detecting that we're a native Wayland client is easily done using
GDK_IS_WAYLAND_DISPLAY(), but I'm less sure on the best way to
detect that we're under XWayland. In fact ideally we'd not need
to detect Xwayland, but rather directly query the keymap used.

We have some existing code that uses the XkbGetKeyboard() call
to query the keymap name[1], but this does not return any data when
run under Xwayland.  The ServerVendor simply returns "Fedora"
and not "XWayland", so that's out as a detection mechanism, and
there's no obvious wayland specific extension loaded that I can
use as a detection mechanism. I'm out of ideas of what aspect
of the connected display I can use to detect presence of Xwayland.

If there are any suggestions that'd be great, otherwise I'll go
the somewhat sucky approach of seeing if the WAYLAND_DISPLAY env
variable exists and infer that we're using Xwayland based on
"GDK_IS_X11_DISPLAY && getenv(WAYLAND_DISPLAY) != NULL"

Regards,
Daniel

[1] https://git.gnome.org/browse/gtk-vnc/tree/src/vncdisplaykeymap.c#n150
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