Reduce server grabs during window creation

Remove some obvious server grabs from the window creation codepath,
also ones that are taken at startup.

During startup, there is no need to grab: we install the event handlers
before querying for the already-existing windows, so there is no danger
that we will 'lose' some window. We might try to create a window twice
(if it comes back in the original query and then we get an event for it)
but the code is already protected against such conditions.

When windows are created later, we also do not need grabs, we just need
appropriate error checking as the window may be destroyed at any time
(or it may have already been destroyed).

The stack tracker is unaffected here - as it listens to CreateNotify and
DestroyNotify events and responds directly, the internal stack
representation will always be consistent even if the window goes away while
we are processing MapRequest or similar.

Now that there are no grabs we don't have to worry about explicitly calling
display_notify_window after grabs have been dropped. Fold that into
meta_window_new_shared().

https://bugzilla.gnome.org/show_bug.cgi?id=721345

src/core/display.c

@@ -825,8 +825,6 @@ meta_display_open (void)
 
   enable_compositor (the_display);
    
-  meta_display_grab (the_display);
-  
   /* Now manage all existing windows */
   tmp = the_display->screens;
   while (tmp != NULL)
@@ -894,8 +892,6 @@ meta_display_open (void)
 
   meta_idle_monitor_init_dbus ();
 
-  meta_display_ungrab (the_display);
-
   /* Done opening new display */
   the_display->display_opening = FALSE;
 

src/core/screen.c

@@ -887,8 +887,6 @@ meta_screen_manage_all_windows (MetaScreen *screen)
   MetaStackWindow *children;
   int n_children, i;
 
-  meta_display_grab (screen->display);
-
   if (screen->guard_window == None)
     screen->guard_window =
       meta_screen_create_guard_window (screen->display->xdisplay, screen);
@@ -907,8 +905,6 @@ meta_screen_manage_all_windows (MetaScreen *screen)
 
   g_free (children);
   meta_stack_thaw (screen->stack);
-
-  meta_display_ungrab (screen->display);
 }
 
 /**

src/core/window.c

@@ -1329,12 +1329,6 @@ meta_window_new_shared (MetaDisplay         *display,
 
   window->constructing = FALSE;
 
-  return window;
-}
-
-static void
-display_notify_window (MetaDisplay *display, MetaWindow *window)
-{
   meta_display_notify_window_created (display, window);
 
   if (window->wm_state_demands_attention)
@@ -1342,6 +1336,8 @@ display_notify_window (MetaDisplay *display, MetaWindow *window)
 
   if (window->wm_hints_urgent)
     g_signal_emit_by_name (window->display, "window-marked-urgent", window);
+
+  return window;
 }
 
 MetaWindow *
@@ -1376,15 +1372,12 @@ meta_window_new_for_wayland (MetaDisplay        *display,
   attrs.override_redirect = 0;
   attrs.screen = scr->xscreen;
 
-  /* XXX: Note: In the Wayland case we currently still grab the
+  /* XXX: Note: In the Wayland case we currently still trap X errors while
-   * xserver and trap X errors while creating a MetaWindow because we
+   * creating a MetaWindow because we will still be making various redundant
-   * will still be making various redundant X requests (passing a
+   * X requests (passing a window xid of None) until we thoroughly audit all
-   * window xid of None) until we thoroughly audit all the code to
+   * the code to make sure it knows about non X based clients...
-   * make sure it knows about non X based clients...
    */
 
-  /* Grab server */
-  meta_display_grab (display);
   meta_error_trap_push (display); /* Push a trap over all of window
                                    * creation, to reduce XSync() calls
                                    */
@@ -1400,14 +1393,12 @@ meta_window_new_for_wayland (MetaDisplay        *display,
                                    &attrs);
 
   meta_error_trap_pop (display); /* pop the XSync()-reducing trap */
-  meta_display_ungrab (display);
 
   /* XXX: Maybe this could be called in meta_window_new_shared() but
    * before splitting the X11 specific code out it came after the
-   * meta_display_ungrab() and we wanted to minimize the risk of
+   * meta_error_trap_pop() and we wanted to minimize the risk of
    * breaking something.
    */
-  display_notify_window (window->display, window);
 
   return window;
 }
@@ -1434,11 +1425,14 @@ meta_window_new (MetaDisplay       *display,
       return NULL;
     }
 
-  /* Grab server */
-  meta_display_grab (display);
   meta_error_trap_push (display); /* Push a trap over all of window
                                    * creation, to reduce XSync() calls
                                    */
+  /*
+   * This function executes without any server grabs held. This means that
+   * the window could have already gone away, or could go away at any point,
+   * so we must be careful with X error handling.
+   */
 
   if (!XGetWindowAttributes (display->xdisplay, xwindow, &attrs))
     {
@@ -1581,20 +1575,11 @@ meta_window_new (MetaDisplay       *display,
                                    &attrs);
 
   meta_error_trap_pop (display); /* pop the XSync()-reducing trap */
-  meta_display_ungrab (display);
-
-  /* XXX: Maybe this could be called in meta_window_new_shared() but
-   * before splitting the X11 specific code out it came after the
-   * meta_display_ungrab() and we wanted to minimize the risk of
-   * breaking something.
-   */
-  display_notify_window (display, window);
 
   return window;
 
 error:
   meta_error_trap_pop (display);
-  meta_display_ungrab (display);
   return NULL;
 }