waylandsink: Avoid race condition on multi-threaded client

When waylandsink is used on some other thread than the main wayland
client thread, the waylandsink implementation is vulnerable to a

condition related to registry and surface events which handled in
seperated event queue.

The race that may happen is that after a proxy is created, but
before the queue is set, events meant to be emitted via the yet to

set queue may already have been queued on the wrong queue.

Wayland 1.11 introduced new API that allows creating a proxy
wrappper which can help to avoid this race condition.

configure.ac

@@ -1334,7 +1334,7 @@ dnl **** Wayland ****
 translit(dnm, m, l) AM_CONDITIONAL(USE_WAYLAND, true)
 AC_PATH_PROG([wayland_scanner], [wayland-scanner])
 AG_GST_CHECK_FEATURE(WAYLAND, [wayland sink], wayland , [
-  PKG_CHECK_MODULES(WAYLAND, wayland-client >= 1.4.0 libdrm >= 2.4.55 wayland-protocols >= 1.4, [
+  PKG_CHECK_MODULES(WAYLAND, wayland-client >= 1.11.0 libdrm >= 2.4.55 wayland-protocols >= 1.4, [
     if test "x$wayland_scanner" != "x"; then
       HAVE_WAYLAND="yes"
       AC_SUBST(WAYLAND_PROTOCOLS_DATADIR, `$PKG_CONFIG --variable=pkgdatadir wayland-protocols`)

ext/wayland/wldisplay.c

@@ -102,6 +102,9 @@ gst_wl_display_finalize (GObject * gobject)
   if (self->registry)
     wl_registry_destroy (self->registry);
 
+  if (self->display_wrapper)
+    wl_proxy_wrapper_destroy (self->display_wrapper);
+
   if (self->queue)
     wl_event_queue_destroy (self->queue);
 
@@ -113,37 +116,6 @@ gst_wl_display_finalize (GObject * gobject)
   G_OBJECT_CLASS (gst_wl_display_parent_class)->finalize (gobject);
 }
 
-static void
-sync_callback (void *data, struct wl_callback *callback, uint32_t serial)
-{
-  gboolean *done = data;
-  *done = TRUE;
-}
-
-static const struct wl_callback_listener sync_listener = {
-  sync_callback
-};
-
-static gint
-gst_wl_display_roundtrip (GstWlDisplay * self)
-{
-  struct wl_callback *callback;
-  gint ret = 0;
-  gboolean done = FALSE;
-
-  g_return_val_if_fail (self != NULL, -1);
-
-  /* We don't own the display, process only our queue */
-  callback = wl_display_sync (self->display);
-  wl_callback_add_listener (callback, &sync_listener, &done);
-  wl_proxy_set_queue ((struct wl_proxy *) callback, self->queue);
-  while (ret != -1 && !done)
-    ret = wl_display_dispatch_queue (self->display, self->queue);
-  wl_callback_destroy (callback);
-
-  return ret;
-}
-
 static void
 shm_format (void *data, struct wl_shm *wl_shm, uint32_t format)
 {
@@ -271,10 +243,10 @@ gst_wl_display_thread_run (gpointer data)
         break;
       else
         goto error;
-    } else {
-      wl_display_read_events (self->display);
-      wl_display_dispatch_queue_pending (self->display, self->queue);
     }
+    if (wl_display_read_events (self->display) == -1)
+      goto error;
+    wl_display_dispatch_queue_pending (self->display, self->queue);
   }
 
   return NULL;
@@ -313,16 +285,17 @@ gst_wl_display_new_existing (struct wl_display * display,
 
   self = g_object_new (GST_TYPE_WL_DISPLAY, NULL);
   self->display = display;
+  self->display_wrapper = wl_proxy_create_wrapper (display);
   self->own_display = take_ownership;
 
   self->queue = wl_display_create_queue (self->display);
-  self->registry = wl_display_get_registry (self->display);
-  wl_proxy_set_queue ((struct wl_proxy *) self->registry, self->queue);
+  wl_proxy_set_queue ((struct wl_proxy *) self->display_wrapper, self->queue);
+  self->registry = wl_display_get_registry (self->display_wrapper);
   wl_registry_add_listener (self->registry, &registry_listener, self);
 
   /* we need exactly 2 roundtrips to discover global objects and their state */
   for (i = 0; i < 2; i++) {
-    if (gst_wl_display_roundtrip (self) < 0) {
+    if (wl_display_roundtrip_queue (self->display, self->queue) < 0) {
       *error = g_error_new (g_quark_from_static_string ("GstWlDisplay"), 0,
           "Error communicating with the wayland display");
       g_object_unref (self);

ext/wayland/wldisplay.h

@@ -46,6 +46,7 @@ struct _GstWlDisplay
 
   /* public objects */
   struct wl_display *display;
+  struct wl_display *display_wrapper;
   struct wl_event_queue *queue;
 
   /* globals */

ext/wayland/wlwindow.c

@@ -92,6 +92,7 @@ gst_wl_window_finalize (GObject * gobject)
   if (self->video_viewport)
     wp_viewport_destroy (self->video_viewport);
 
+  wl_proxy_wrapper_destroy (self->video_surface_wrapper);
   wl_subsurface_destroy (self->video_subsurface);
   wl_surface_destroy (self->video_surface);
 
@@ -101,6 +102,7 @@ gst_wl_window_finalize (GObject * gobject)
   if (self->area_viewport)
     wp_viewport_destroy (self->area_viewport);
 
+  wl_proxy_wrapper_destroy (self->area_surface_wrapper);
   wl_surface_destroy (self->area_surface);
 
   g_clear_object (&self->display);
@@ -121,8 +123,13 @@ gst_wl_window_new_internal (GstWlDisplay * display, GMutex * render_lock)
   window->area_surface = wl_compositor_create_surface (display->compositor);
   window->video_surface = wl_compositor_create_surface (display->compositor);
 
-  wl_proxy_set_queue ((struct wl_proxy *) window->area_surface, display->queue);
-  wl_proxy_set_queue ((struct wl_proxy *) window->video_surface,
+  window->area_surface_wrapper = wl_proxy_create_wrapper (window->area_surface);
+  window->video_surface_wrapper =
+      wl_proxy_create_wrapper (window->video_surface);
+
+  wl_proxy_set_queue ((struct wl_proxy *) window->area_surface_wrapper,
+      display->queue);
+  wl_proxy_set_queue ((struct wl_proxy *) window->video_surface_wrapper,
       display->queue);
 
   /* embed video_surface in area_surface */
@@ -233,7 +240,7 @@ gst_wl_window_get_wl_surface (GstWlWindow * window)
 {
   g_return_val_if_fail (window != NULL, NULL);
 
-  return window->video_surface;
+  return window->video_surface_wrapper;
 }
 
 gboolean
@@ -267,8 +274,8 @@ gst_wl_window_resize_video_surface (GstWlWindow * window, gboolean commit)
   wl_subsurface_set_position (window->video_subsurface, res.x, res.y);
 
   if (commit) {
-    wl_surface_damage (window->video_surface, 0, 0, res.w, res.h);
-    wl_surface_commit (window->video_surface);
+    wl_surface_damage (window->video_surface_wrapper, 0, 0, res.w, res.h);
+    wl_surface_commit (window->video_surface_wrapper);
   }
 
   if (gst_wl_window_is_toplevel (window)) {
@@ -322,20 +329,20 @@ gst_wl_window_render (GstWlWindow * window, GstWlBuffer * buffer,
   }
 
   if (G_LIKELY (buffer))
-    gst_wl_buffer_attach (buffer, window->video_surface);
+    gst_wl_buffer_attach (buffer, window->video_surface_wrapper);
   else
-    wl_surface_attach (window->video_surface, NULL, 0, 0);
+    wl_surface_attach (window->video_surface_wrapper, NULL, 0, 0);
 
-  wl_surface_damage (window->video_surface, 0, 0, window->video_rectangle.w,
-      window->video_rectangle.h);
-  wl_surface_commit (window->video_surface);
+  wl_surface_damage (window->video_surface_wrapper, 0, 0,
+      window->video_rectangle.w, window->video_rectangle.h);
+  wl_surface_commit (window->video_surface_wrapper);
 
   if (G_UNLIKELY (info)) {
     /* commit also the parent (area_surface) in order to change
      * the position of the video_subsurface */
-    wl_surface_damage (window->area_surface, 0, 0, window->render_rectangle.w,
-        window->render_rectangle.h);
-    wl_surface_commit (window->area_surface);
+    wl_surface_damage (window->area_surface_wrapper, 0, 0,
+        window->render_rectangle.w, window->render_rectangle.h);
+    wl_surface_commit (window->area_surface_wrapper);
     wl_subsurface_set_desync (window->video_subsurface);
   }
 
@@ -385,7 +392,7 @@ gst_wl_window_update_borders (GstWlWindow * window)
       gst_wl_shm_memory_construct_wl_buffer (gst_buffer_peek_memory (buf, 0),
       window->display, &info);
   gwlbuf = gst_buffer_add_wl_buffer (buf, wlbuf, window->display);
-  gst_wl_buffer_attach (gwlbuf, window->area_surface);
+  gst_wl_buffer_attach (gwlbuf, window->area_surface_wrapper);
 
   /* at this point, the GstWlBuffer keeps the buffer
    * alive and will free it on wl_buffer::release */
@@ -419,8 +426,8 @@ gst_wl_window_set_render_rectangle (GstWlWindow * window, gint x, gint y,
     gst_wl_window_resize_video_surface (window, TRUE);
   }
 
-  wl_surface_damage (window->area_surface, 0, 0, w, h);
-  wl_surface_commit (window->area_surface);
+  wl_surface_damage (window->area_surface_wrapper, 0, 0, w, h);
+  wl_surface_commit (window->area_surface_wrapper);
 
   if (window->video_width != 0)
     wl_subsurface_set_desync (window->video_subsurface);

ext/wayland/wlwindow.h

@@ -45,9 +45,11 @@ struct _GstWlWindow
 
   GstWlDisplay *display;
   struct wl_surface *area_surface;
+  struct wl_surface *area_surface_wrapper;
   struct wl_subsurface *area_subsurface;
   struct wp_viewport *area_viewport;
   struct wl_surface *video_surface;
+  struct wl_surface *video_surface_wrapper;
   struct wl_subsurface *video_subsurface;
   struct wp_viewport *video_viewport;
   struct wl_shell_surface *shell_surface;