Add multiple buffering to xwl_window

Xwayland takes care of not attaching a new buffer if a frame callback is pending.

Yet, the existing buffer (which was previously attached) may still be updated from the X11 side, causing unexpected visual glitches to the buffer.

Add multiple buffering to the xwl_window and alternate between buffers, to leave the Wayland buffer untouched between frame callbacks and avoid stuttering or tearing issues.

Closes: #835 (closed) Signed-off-by: Olivier Fourdan ofourdan@redhat.com

hw/xwayland/xwayland.c

@@ -1065,6 +1065,17 @@ void xwl_surface_damage(struct xwl_screen *xwl_screen,
         wl_surface_damage(surface, x, y, width, height);
 }
 
+static void
+buffer_release_callback (void *data,
+                         struct wl_buffer *wl_buffer)
+{
+    wl_buffer_destroy(wl_buffer);
+}
+
+static const struct wl_buffer_listener buffer_listener = {
+    .release = buffer_release_callback,
+};
+
 static void
 xwl_window_post_damage(struct xwl_window *xwl_window)
 {
@@ -1073,6 +1084,7 @@ xwl_window_post_damage(struct xwl_window *xwl_window)
     BoxPtr box;
     struct wl_buffer *buffer;
     PixmapPtr pixmap;
+    Bool new_buffer = FALSE;
     int i;
 
     assert(!xwl_window->frame_callback);
@@ -1083,8 +1095,8 @@ xwl_window_post_damage(struct xwl_window *xwl_window)
 #ifdef XWL_HAS_GLAMOR
     if (xwl_screen->glamor)
         buffer = xwl_glamor_pixmap_get_wl_buffer(pixmap,
-                                                 FALSE,
-                                                 NULL);
+                                                 TRUE, /* detached mode */
+                                                 &new_buffer);
     else
 #endif
         buffer = xwl_shm_pixmap_get_wl_buffer(pixmap);
@@ -1116,6 +1128,9 @@ xwl_window_post_damage(struct xwl_window *xwl_window)
     xwl_window->frame_callback = wl_surface_frame(xwl_window->surface);
     wl_callback_add_listener(xwl_window->frame_callback, &frame_listener, xwl_window);
 
+    if (new_buffer)
+        wl_buffer_add_listener(buffer, &buffer_listener, NULL);
+
     wl_surface_commit(xwl_window->surface);
     DamageEmpty(window_get_damage(xwl_window->window));