compositor-wayland: handle wl_keyboard.enter(NULL)

Destroying an output (wl_surface) can race against the parent compositor
sending wl_keyboard.enter. When this race is lost, wayland-backend
receives wl_keyboard.enter with a NULL wl_surface for the surface it
just destroyed.

Handle this case by ignoring such enter events. Since it is
theoretically possible to follow enter with key events, drop those too.

The modifiers event is sent before enter, so we cannot drop that on the
same condition.

wl_keyboard.leave handler seems to already handle the NULL focus case,
but there is a question if the notify_keyboard_focus_out() call should
be avoided.

This patch fixes a hard to reproduce crash. I was running weston/x11
with two outputs, and weston/wayland --sprawl inside that, then closing
the parent compositor windows one by one. Sometimes it would trigger
this crash.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>

libweston/compositor-wayland.c

@@ -1858,6 +1858,11 @@ input_handle_keyboard_enter(void *data,
 			weston_output_schedule_repaint(&focus->base);
 	}
 
+	if (!surface) {
+		input->keyboard_focus = NULL;
+		return;
+	}
+
 	input->keyboard_focus = wl_surface_get_user_data(surface);
 	input->keyboard_focus->keyboard_count++;
 
@@ -1907,6 +1912,9 @@ input_handle_key(void *data, struct wl_keyboard *keyboard,
 	struct wayland_input *input = data;
 	struct timespec ts;
 
+	if (!input->keyboard_focus)
+		return;
+
 	timespec_from_msec(&ts, time);
 
 	input->key_serial = serial;