[PATCH weston] xwm: Check whether the seat is NULL sometimes in weston_wm_handle_button

[Pekka Paalanen]

On Fri, 29 Aug 2014 22:10:32 +0800
Boyan Ding <stu_dby at 126.com> wrote:

> Under some certain circumstances, pointer button may have been released
> when frame is still being resized/moved. When this happens, the picked
> seat is NULL and it will segfault when moving/resizing surfaces. Check
> whether the seat is NULL and ignore move/resize in that case.
> 
> Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=82827
> Signed-off-by: Boyan Ding <stu_dby at 126.com>
> ---
>  xwayland/window-manager.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/xwayland/window-manager.c b/xwayland/window-manager.c
> index a216b76..f633324 100644
> --- a/xwayland/window-manager.c
> +++ b/xwayland/window-manager.c
> @@ -1648,12 +1648,14 @@ weston_wm_handle_button(struct weston_wm *wm, xcb_generic_event_t *event)
>  		weston_wm_window_schedule_repaint(window);
>  
>  	if (frame_status(window->frame) & FRAME_STATUS_MOVE) {
> -		shell_interface->move(window->shsurf, seat);
> +		if (seat != NULL)
> +			shell_interface->move(window->shsurf, seat);
>  		frame_status_clear(window->frame, FRAME_STATUS_MOVE);
>  	}
>  
>  	if (frame_status(window->frame) & FRAME_STATUS_RESIZE) {
> -		shell_interface->resize(window->shsurf, seat, location);
> +		if (seat != NULL)
> +			shell_interface->resize(window->shsurf, seat, location);
>  		frame_status_clear(window->frame, FRAME_STATUS_RESIZE);
>  	}
>  

Hi,

do you know if this condition is something that should be silently
ignored like in your patch, or should we at least print an error that
something unexpected is happening and being papered over?

Looking at how the seat is found:

static struct weston_seat *
weston_wm_pick_seat_for_window(struct weston_wm_window *window)
{
	struct weston_wm *wm = window->wm;
	struct weston_seat *seat, *s;

	seat = NULL;
	wl_list_for_each(s, &wm->server->compositor->seat_list, link) {
		if (s->pointer != NULL &&
		    s->pointer->focus == window->view &&
		    s->pointer->button_count > 0 &&
		    (seat == NULL ||
		     s->pointer->grab_serial -
		     seat->pointer->grab_serial < (1 << 30)))
			seat = s;
	}

	return seat;
}

and that gets called as a response to an XCB input event via
weston_wm_handle_button...

The function will return NULL if there are no buttons pressed, even if
the pointer is focused on the window. Does that make sense in general?
Does it not cause every last-button-up event to hit seat==NULL?
So why don't we see this problem more often?

Could there be a problem in the shared frame code, maybe it makes
assumptions that won't work with X11?

If no-one knows (i.e. no-one replies assuring one way or the other), I
can merge this patch if someone at least confirms it fixes an issue.


Thanks,
pq