xwayland: Delay wl_surface destruction

X11 and Wayland requests are unordered, causing a race in the X11 window
and wl_surface association.

To mitigate that race, delay the wl_surface destruction by 1 second,
so that the compositor has time to establish the association before the
wl_surface is destroyed: to see both the wl_surface created and the
WL_SURFACE_ID X11 property set.

This is only a mitigation though, a more robust solution requires a
future dedicated Wayland protocol.

v2: Clean up pending wl_surface destroy on exit as well.

Closes: https://gitlab.freedesktop.org/xorg/xserver/-/issues/1157
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Tested-by: Joshua Ashton <joshua@froggi.es>
Tested-by: Sterophonick <sterophonick@gmail.com>
See-also: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/163

hw/xwayland/xwayland-screen.c

@@ -182,6 +182,7 @@ xwl_close_screen(ScreenPtr screen)
     struct xwl_screen *xwl_screen = xwl_screen_get(screen);
     struct xwl_output *xwl_output, *next_xwl_output;
     struct xwl_seat *xwl_seat, *next_xwl_seat;
+    struct xwl_wl_surface *xwl_wl_surface, *xwl_wl_surface_next;
 
     DeleteCallback(&PropertyStateCallback, xwl_property_callback, screen);
 
@@ -204,6 +205,10 @@ xwl_close_screen(ScreenPtr screen)
         xwl_screen_destroy_drm_lease_device(xwl_screen,
                                             device_data->drm_lease_device);
 
+    xorg_list_for_each_entry_safe(xwl_wl_surface, xwl_wl_surface_next,
+                                  &xwl_screen->pending_wl_surface_destroy, link)
+        xwl_window_surface_do_destroy(xwl_wl_surface);
+
     RemoveNotifyFd(xwl_screen->wayland_fd);
 
     wl_display_disconnect(xwl_screen->display);
@@ -818,6 +823,7 @@ xwl_screen_init(ScreenPtr pScreen, int argc, char **argv)
     xorg_list_init(&xwl_screen->drm_lease_devices);
     xorg_list_init(&xwl_screen->queued_drm_lease_devices);
     xorg_list_init(&xwl_screen->drm_leases);
+    xorg_list_init(&xwl_screen->pending_wl_surface_destroy);
     xwl_screen->depth = 24;
 
     if (!monitorResolution)

hw/xwayland/xwayland-screen.h

@@ -107,6 +107,7 @@ struct xwl_screen {
     struct xorg_list queued_drm_lease_devices;
     struct xorg_list drm_leases;
     struct xwl_output *fixed_output;
+    struct xorg_list pending_wl_surface_destroy;
     uint32_t serial;
 
 #define XWL_FORMAT_ARGB8888 (1 << 0)

hw/xwayland/xwayland-window.c

@@ -46,6 +46,8 @@
 #include "viewporter-client-protocol.h"
 #include "xdg-shell-client-protocol.h"
 
+#define DELAYED_WL_SURFACE_DESTROY 1000 /* ms */
+
 static DevPrivateKeyRec xwl_window_private_key;
 static DevPrivateKeyRec xwl_damage_private_key;
 static const char *xwl_surface_tag = "xwl-surface";
@@ -852,6 +854,58 @@ xwl_realize_window(WindowPtr window)
     return ensure_surface_for_window(window);
 }
 
+static void
+xwl_surface_destroy_free_timer(struct xwl_wl_surface *xwl_wl_surface)
+{
+    if (xwl_wl_surface->wl_surface_destroy_timer) {
+        TimerFree(xwl_wl_surface->wl_surface_destroy_timer);
+        xwl_wl_surface->wl_surface_destroy_timer = NULL;
+    }
+}
+
+void
+xwl_window_surface_do_destroy(struct xwl_wl_surface *xwl_wl_surface)
+{
+    wl_surface_destroy(xwl_wl_surface->wl_surface);
+    xorg_list_del(&xwl_wl_surface->link);
+    xwl_surface_destroy_free_timer(xwl_wl_surface);
+    free(xwl_wl_surface);
+}
+
+static CARD32
+xwl_surface_destroy_callback(OsTimerPtr timer, CARD32 now, void *arg)
+{
+    struct xwl_wl_surface *xwl_wl_surface = arg;
+
+    xwl_window_surface_do_destroy(xwl_wl_surface);
+
+    return 0;
+}
+
+static void
+release_wl_surface_for_window(struct xwl_window *xwl_window)
+{
+    struct xwl_wl_surface *xwl_wl_surface;
+
+    /* If the Xserver is terminating, destroy the surface immediately */
+    if ((dispatchException & DE_TERMINATE) == DE_TERMINATE) {
+        wl_surface_destroy(xwl_window->surface);
+        return;
+    }
+
+    /* Otherwise, schedule the destruction later, to mitigate the race
+     * between X11 and Wayland processing so that the compositor has the
+     * time to establish the association before the wl_surface is destroyed.
+     */
+    xwl_wl_surface = xnfcalloc(1, sizeof *xwl_wl_surface);
+    xwl_wl_surface->wl_surface = xwl_window->surface;
+    xorg_list_add(&xwl_wl_surface->link,
+                  &xwl_window->xwl_screen->pending_wl_surface_destroy);
+    xwl_wl_surface->wl_surface_destroy_timer =
+        TimerSet(NULL, 0, DELAYED_WL_SURFACE_DESTROY,
+                 xwl_surface_destroy_callback, xwl_wl_surface);
+}
+
 Bool
 xwl_unrealize_window(WindowPtr window)
 {
@@ -906,7 +960,7 @@ xwl_unrealize_window(WindowPtr window)
     }
 #endif
 
-    wl_surface_destroy(xwl_window->surface);
+    release_wl_surface_for_window(xwl_window);
     xorg_list_del(&xwl_window->link_damage);
     xorg_list_del(&xwl_window->link_window);
     unregister_damage(window);

hw/xwayland/xwayland-window.h

@@ -37,6 +37,12 @@
 
 #include "xwayland-types.h"
 
+struct xwl_wl_surface {
+    OsTimerPtr wl_surface_destroy_timer;
+    struct wl_surface *wl_surface;
+    struct xorg_list link;
+};
+
 struct xwl_window {
     struct xwl_screen *xwl_screen;
     struct wl_surface *surface;
@@ -91,6 +97,8 @@ void xwl_move_window(WindowPtr window,
 Bool xwl_destroy_window(WindowPtr window);
 void xwl_window_post_damage(struct xwl_window *xwl_window);
 void xwl_window_create_frame_callback(struct xwl_window *xwl_window);
+void xwl_window_surface_do_destroy(struct xwl_wl_surface *xwl_wl_surface);
+
 Bool xwl_window_init(void);
 
 #endif /* XWAYLAND_WINDOW_H */