window/x11: Add freeze_commits()/thaw_commits()

Xwayland may post damages for an X11 window as soon as the frame
callback is triggered, while the X11 window manager/compositor has not
yet finished updating the windows.

If Xwayland becomes compliant enough to not permit updates after the
buffer has been committed (see [1]), then the partial redraw of the X11
window at the time it was posted will show on screen.

To avoid that issue, the X11 window manager can use the X11 property
`_XWAYLAND_ALLOW_COMMITS` to control when Xwayland should be allowed to
post the pending damages.

Add `freeze_commits()` and `thaw_commits()` methods to `MetaWindowX11`
which are a no-op on plain X11, but sets `_XWAYLAND_ALLOW_COMMITS` on
the toplevel X11 windows running on Xwayland.

[1] https://gitlab.freedesktop.org/xorg/xserver/merge_requests/316
See-also: https://gitlab.gnome.org/GNOME/mutter/merge_requests/855

https://gitlab.gnome.org/GNOME/mutter/merge_requests/942

src/wayland/meta-window-xwayland.c

@@ -18,6 +18,8 @@
 
 #include "config.h"
 
+#include "core/frame.h"
+#include "meta/meta-x11-errors.h"
 #include "x11/window-x11.h"
 #include "x11/window-x11-private.h"
 #include "x11/xprops.h"
@@ -40,6 +42,7 @@ struct _MetaWindowXwayland
   MetaWindowX11 parent;
 
   gboolean xwayland_may_grab_keyboard;
+  int freeze_count;
 };
 
 struct _MetaWindowXwaylandClass
@@ -159,6 +162,63 @@ meta_window_xwayland_shortcuts_inhibited (MetaWindow         *window,
   return meta_wayland_compositor_is_shortcuts_inhibited (compositor, source);
 }
 
+static void
+apply_allow_commits_x11_property (MetaWindowXwayland *xwayland_window,
+                                  gboolean            allow_commits)
+{
+  MetaWindow *window = META_WINDOW (xwayland_window);
+  MetaDisplay *display = window->display;
+  MetaX11Display *x11_display = display->x11_display;
+  Display *xdisplay = x11_display->xdisplay;
+  MetaFrame *frame;
+  Window xwin;
+  guint32 property[1];
+
+  frame = meta_window_get_frame (window);
+  if (!frame)
+    xwin = window->xwindow;
+  else
+    xwin = meta_frame_get_xwindow (frame);
+
+  if (!xwin)
+    return;
+
+  property[0] = !!allow_commits;
+
+  meta_x11_error_trap_push (x11_display);
+  XChangeProperty (xdisplay, xwin,
+                   x11_display->atom__XWAYLAND_ALLOW_COMMITS,
+                   XA_CARDINAL, 32, PropModeReplace,
+                   (guchar*) &property, 1);
+  meta_x11_error_trap_pop (x11_display);
+  XFlush (xdisplay);
+}
+
+static void
+meta_window_xwayland_freeze_commits (MetaWindow *window)
+{
+  MetaWindowXwayland *xwayland_window = META_WINDOW_XWAYLAND (window);
+
+  if (xwayland_window->freeze_count == 0)
+    apply_allow_commits_x11_property (xwayland_window, FALSE);
+
+  xwayland_window->freeze_count++;
+}
+
+static void
+meta_window_xwayland_thaw_commits (MetaWindow *window)
+{
+  MetaWindowXwayland *xwayland_window = META_WINDOW_XWAYLAND (window);
+
+  g_return_if_fail (xwayland_window->freeze_count > 0);
+
+  xwayland_window->freeze_count--;
+  if (xwayland_window->freeze_count > 0)
+    return;
+
+  apply_allow_commits_x11_property (xwayland_window, TRUE);
+}
+
 static void
 meta_window_xwayland_get_property (GObject    *object,
                                    guint       prop_id,
@@ -201,12 +261,16 @@ static void
 meta_window_xwayland_class_init (MetaWindowXwaylandClass *klass)
 {
   MetaWindowClass *window_class = META_WINDOW_CLASS (klass);
+  MetaWindowX11Class *window_x11_class = META_WINDOW_X11_CLASS (klass);
   GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
 
   window_class->adjust_fullscreen_monitor_rect = meta_window_xwayland_adjust_fullscreen_monitor_rect;
   window_class->force_restore_shortcuts = meta_window_xwayland_force_restore_shortcuts;
   window_class->shortcuts_inhibited = meta_window_xwayland_shortcuts_inhibited;
 
+  window_x11_class->freeze_commits = meta_window_xwayland_freeze_commits;
+  window_x11_class->thaw_commits = meta_window_xwayland_thaw_commits;
+
   gobject_class->get_property = meta_window_xwayland_get_property;
   gobject_class->set_property = meta_window_xwayland_set_property;
 

src/x11/atomnames.h

@@ -82,6 +82,7 @@ item(_XKB_RULES_NAMES)
 item(WL_SURFACE_ID)
 item(_XWAYLAND_MAY_GRAB_KEYBOARD)
 item(_XWAYLAND_RANDR_EMU_MONITOR_RECTS)
+item(_XWAYLAND_ALLOW_COMMITS)
 
 /* Oddities: These are used, and we need atoms for them,
  * but when we need all _NET_WM hints (i.e. when we're making

src/x11/window-x11-private.h

@@ -33,6 +33,9 @@ typedef struct _MetaWindowX11Private MetaWindowX11Private;
 struct _MetaWindowX11Class
 {
   MetaWindowClass parent_class;
+
+  void (*freeze_commits) (MetaWindow *window);
+  void (*thaw_commits)   (MetaWindow *window);
 };
 
 struct _MetaWindowX11

src/x11/window-x11.c

@@ -1973,6 +1973,16 @@ meta_window_x11_calculate_layer (MetaWindow *window)
   return layer;
 }
 
+static void
+meta_window_x11_impl_freeze_commits (MetaWindow *window)
+{
+}
+
+static void
+meta_window_x11_impl_thaw_commits (MetaWindow *window)
+{
+}
+
 static void
 meta_window_x11_map (MetaWindow *window)
 {
@@ -2024,6 +2034,9 @@ meta_window_x11_class_init (MetaWindowX11Class *klass)
   window_class->calculate_layer = meta_window_x11_calculate_layer;
   window_class->map = meta_window_x11_map;
   window_class->unmap = meta_window_x11_unmap;
+
+  klass->freeze_commits = meta_window_x11_impl_freeze_commits;
+  klass->thaw_commits = meta_window_x11_impl_thaw_commits;
 }
 
 void
@@ -3995,3 +4008,17 @@ meta_window_x11_get_toplevel_xwindow (MetaWindow *window)
 {
   return window->frame ? window->frame->xwindow : window->xwindow;
 }
+
+void
+meta_window_x11_freeze_commits (MetaWindow *window)
+{
+  MetaWindowX11 *window_x11 = META_WINDOW_X11 (window);
+  META_WINDOW_X11_GET_CLASS (window_x11)->freeze_commits (window);
+}
+
+void
+meta_window_x11_thaw_commits (MetaWindow *window)
+{
+  MetaWindowX11 *window_x11 = META_WINDOW_X11 (window);
+  META_WINDOW_X11_GET_CLASS (window_x11)->thaw_commits (window);
+}

src/x11/window-x11.h

@@ -81,4 +81,7 @@ void     meta_window_x11_configure_notify        (MetaWindow      *window,
 
 Window   meta_window_x11_get_toplevel_xwindow    (MetaWindow *window);
 
+void     meta_window_x11_freeze_commits          (MetaWindow *window);
+void     meta_window_x11_thaw_commits            (MetaWindow *window);
+
 #endif