x11: Fix the _NET_SUPPORTING_WM_CHECK window fetch to be spec compliant

"The child window MUST also have the _NET_SUPPORTING_WM_CHECK property set to
the ID of the child window. […] If the _NET_SUPPORTING_WM_CHECK window on the
client window is missing or not properly set, clients SHOULD assume that no
conforming Window Manager is present."

This commit implements that, which allows us to not have to do a
XGetWindowProperty() every N seconds when running under a compliant WM.

This is also a more correct fix for the bug handled in commit
daf29bffeddc02a7ac55905a79d040c299a37eee.

https://bugzilla.gnome.org/show_bug.cgi?id=666921

1 files changed, 53 insertions, 36 deletions

diff --git a/gdk/x11/gdkscreen-x11.c b/gdk/x11/gdkscreen-x11.c
index 5574f639e0..58bcd92ad0 100644
--- a/gdk/x11/gdkscreen-x11.c
+++ b/gdk/x11/gdkscreen-x11.c
@@ -1311,17 +1311,45 @@ cleanup_atoms(gpointer data)
   g_free (supported_atoms);
 }
 
-static void
-fetch_net_wm_check_window (GdkScreen *screen)
+static Window
+get_net_supporting_wm_check (GdkX11Screen *screen,
+                             Window        window)
 {
-  GdkX11Screen *x11_screen;
   GdkDisplay *display;
   Atom type;
   gint format;
   gulong n_items;
   gulong bytes_after;
   guchar *data;
-  Window *xwindow;
+  Window value;
+
+  display = screen->display;
+  type = None;
+  data = NULL;
+  value = None;
+
+  gdk_x11_display_error_trap_push (display);
+  XGetWindowProperty (screen->xdisplay, window,
+		      gdk_x11_get_xatom_by_name_for_display (display, "_NET_SUPPORTING_WM_CHECK"),
+		      0, G_MAXLONG, False, XA_WINDOW, &type, &format,
+		      &n_items, &bytes_after, &data);
+  gdk_x11_display_error_trap_pop_ignored (display);
+
+  if (type == XA_WINDOW)
+    value = *(Window *)data;
+
+  if (data)
+    XFree (data);
+
+  return value;
+}
+
+static void
+fetch_net_wm_check_window (GdkScreen *screen)
+{
+  GdkX11Screen *x11_screen;
+  GdkDisplay *display;
+  Window window;
   GTimeVal tv;
   gint error;
 
@@ -1329,57 +1357,46 @@ fetch_net_wm_check_window (GdkScreen *screen)
   display = x11_screen->display;
 
   g_return_if_fail (GDK_X11_DISPLAY (display)->trusted_client);
-  
+
+  if (x11_screen->wmspec_check_window != None)
+    return; /* already have it */
+
   g_get_current_time (&tv);
 
   if (ABS  (tv.tv_sec - x11_screen->last_wmspec_check_time) < 15)
     return; /* we've checked recently */
 
-  x11_screen->last_wmspec_check_time = tv.tv_sec;
-
-  data = NULL;
-  XGetWindowProperty (x11_screen->xdisplay, x11_screen->xroot_window,
-		      gdk_x11_get_xatom_by_name_for_display (display, "_NET_SUPPORTING_WM_CHECK"),
-		      0, G_MAXLONG, False, XA_WINDOW, &type, &format,
-		      &n_items, &bytes_after, &data);
-  
-  if (type != XA_WINDOW)
-    {
-      if (data)
-        XFree (data);
-      return;
-    }
-
-  xwindow = (Window *)data;
+  window = get_net_supporting_wm_check (x11_screen, x11_screen->xroot_window);
+  if (window == None)
+    return;
 
-  if (x11_screen->wmspec_check_window == *xwindow)
-    {
-      XFree (xwindow);
-      return;
-    }
+  if (window != get_net_supporting_wm_check (x11_screen, window))
+    return;
 
   gdk_x11_display_error_trap_push (display);
 
   /* Find out if this WM goes away, so we can reset everything. */
-  XSelectInput (x11_screen->xdisplay, *xwindow, StructureNotifyMask);
+  XSelectInput (x11_screen->xdisplay, window, StructureNotifyMask);
 
   error = gdk_x11_display_error_trap_pop (display);
   if (!error)
     {
-      x11_screen->wmspec_check_window = *xwindow;
+      /* We check the window property again because after XGetWindowProperty()
+       * and before XSelectInput() the window may have been recycled in such a
+       * way that XSelectInput() doesn't fail but the window is no longer what
+       * we want.
+       */
+      if (window != get_net_supporting_wm_check (x11_screen, window))
+        return;
+
+      x11_screen->wmspec_check_window = window;
+      x11_screen->last_wmspec_check_time = tv.tv_sec;
       x11_screen->need_refetch_net_supported = TRUE;
       x11_screen->need_refetch_wm_name = TRUE;
 
       /* Careful, reentrancy */
-      _gdk_x11_screen_window_manager_changed (GDK_SCREEN (x11_screen));
+      _gdk_x11_screen_window_manager_changed (screen);
     }
-  else if (error == BadWindow)
-    {
-      /* Leftover property, try again immediately, new wm may be starting up */
-      x11_screen->last_wmspec_check_time = 0;
-    }
-
-  XFree (xwindow);
 }
 
 /**