This protocol adds a xwayland_surface role which allows an Xwayland server to associate an X11 window to a wl_surface.
Before this protocol, this would be done via the Xwayland server providing the wl_surface's resource id via the WL_SURFACE_ID atom on the X window. This was problematic as a race could occur if the wl_surface associated with a WL_SURFACE_ID for a window was destroyed before the update of the atom was processed by the compositor and another surface (or other object) had taken its id due to recycling.
This protocol solves the problem by moving the X11 window to wl_surface association step to the Wayland side, which means that the association cannot happen out-of-sync with the resource lifetime of the wl_surface.
This protocol avoids duplicating the race on the other side by adding a
non-zero monotonic serial number which is entirely unique that is set on
both the wl_surface (via. xwayland_surface_v1's associate method) and
the X11 window (via. the
WL_SURFACE_SERIAL atom) that can be used to
associate them, and synchronize the two timelines.
Relevant XWayland issue this is meant to fix: xorg/xserver#1157 (closed)