display: Initialize MetaCompositor in two steps

MetaCompositor is the place in mutter that manages the higher level
state of compositing, such as handling what happens before and after
paint. In order for other units that depend on having a compositor
instance active, but should be initialized before the X11 implementation
of MetaCompositor registers as a X11 compositing manager, split the
initialization of compositing into two steps:

 1) Instantiate the object - only construct the instance, making it
    possible for users to start listening to signals etc
 2) Manage - this e.g. establishes the compositor as the X11 compositing
    manager and similar things.

This will enable us to put compositing dependent scattered global
variables into a MetaCompositor owned object.

For now, compositor management is internally done by calling a new
`meta_compositor_do_manage()`, as right now we can't change the API of
`meta_compositor_manage()` as it is public. For the next version, manual
management of compositing will removed from the public API, and only
managed internally.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/798

src/compositor/compositor-private.h

@@ -18,7 +18,8 @@ struct _MetaCompositorClass
 {
   GObjectClass parent_class;
 
-  void (* manage) (MetaCompositor *compositor);
+  gboolean (* manage) (MetaCompositor  *compositor,
+                       GError         **error);
   void (* unmanage) (MetaCompositor *compositor);
   void (* pre_paint) (MetaCompositor *compositor);
   void (* post_paint) (MetaCompositor *compositor);
@@ -26,6 +27,9 @@ struct _MetaCompositorClass
                           MetaWindow     *window);
 };
 
+gboolean meta_compositor_do_manage (MetaCompositor  *compositor,
+                                    GError         **error);
+
 void meta_compositor_remove_window_actor (MetaCompositor  *compositor,
                                           MetaWindowActor *window_actor);
 

src/compositor/compositor.c

@@ -550,8 +550,9 @@ meta_compositor_redirect_x11_windows (MetaCompositor *compositor)
     redirect_windows (display->x11_display);
 }
 
-void
-meta_compositor_manage (MetaCompositor *compositor)
+gboolean
+meta_compositor_do_manage (MetaCompositor  *compositor,
+                           GError         **error)
 {
   MetaCompositorPrivate *priv =
     meta_compositor_get_instance_private (compositor);
@@ -587,9 +588,21 @@ meta_compositor_manage (MetaCompositor *compositor)
   clutter_actor_add_child (priv->stage, priv->top_window_group);
   clutter_actor_add_child (priv->stage, priv->feedback_group);
 
-  META_COMPOSITOR_GET_CLASS (compositor)->manage (compositor);
+  if (!META_COMPOSITOR_GET_CLASS (compositor)->manage (compositor, error))
+    return FALSE;
 
   priv->plugin_mgr = meta_plugin_manager_new (compositor);
+
+  return TRUE;
+}
+
+void
+meta_compositor_manage (MetaCompositor *compositor)
+{
+  GError *error = NULL;
+
+  if (!meta_compositor_do_manage (compositor, &error))
+    g_error ("Compositor failed to manage display: %s", error->message);
 }
 
 void

src/compositor/meta-compositor-server.c

@@ -29,9 +29,11 @@ struct _MetaCompositorServer
 
 G_DEFINE_TYPE (MetaCompositorServer, meta_compositor_server, META_TYPE_COMPOSITOR)
 
-static void
-meta_compositor_server_manage (MetaCompositor *compositor)
+static gboolean
+meta_compositor_server_manage (MetaCompositor  *compositor,
+                               GError         **error)
 {
+  return TRUE;
 }
 
 static void

src/compositor/meta-compositor-x11.c

@@ -100,15 +100,39 @@ meta_compositor_x11_process_xevent (MetaCompositorX11 *compositor_x11,
     meta_x11_handle_event (xevent);
 }
 
-static void
-meta_compositor_x11_manage (MetaCompositor *compositor)
+static gboolean
+meta_compositor_x11_manage (MetaCompositor  *compositor,
+                            GError         **error)
 {
   MetaCompositorX11 *compositor_x11 = META_COMPOSITOR_X11 (compositor);
   MetaDisplay *display = meta_compositor_get_display (compositor);
-  Display *xdisplay = meta_x11_display_get_xdisplay (display->x11_display);
+  MetaX11Display *x11_display = display->x11_display;
+  Display *xdisplay = meta_x11_display_get_xdisplay (x11_display);
+  int composite_version;
   MetaBackend *backend = meta_get_backend ();
   Window xwindow;
 
+  if (!META_X11_DISPLAY_HAS_COMPOSITE (x11_display) ||
+      !META_X11_DISPLAY_HAS_DAMAGE (x11_display))
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "Missing required extension %s",
+                   !META_X11_DISPLAY_HAS_COMPOSITE (x11_display) ?
+                   "composite" : "damage");
+      return FALSE;
+    }
+
+  composite_version = ((x11_display->composite_major_version * 10) +
+                       x11_display->composite_minor_version);
+  if (composite_version < 3)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "COMPOSITE extension 3.0 required (found %d.%d)",
+                   x11_display->composite_major_version,
+                   x11_display->composite_minor_version);
+      return FALSE;
+    }
+
   meta_x11_display_set_cm_selection (display->x11_display);
 
   compositor_x11->output = display->x11_display->composite_overlay_window;
@@ -139,6 +163,8 @@ meta_compositor_x11_manage (MetaCompositor *compositor)
   compositor_x11->have_x11_sync_object = meta_sync_ring_init (xdisplay);
 
   meta_compositor_redirect_x11_windows (META_COMPOSITOR (compositor));
+
+  return TRUE;
 }
 
 static void

src/core/display.c

@@ -579,39 +579,6 @@ create_compositor (MetaDisplay *display)
     return META_COMPOSITOR (meta_compositor_x11_new (display));
 }
 
-static void
-enable_compositor (MetaDisplay *display)
-{
-  MetaX11Display *x11_display = display->x11_display;
-
-  if (x11_display)
-    {
-      if (!META_X11_DISPLAY_HAS_COMPOSITE (x11_display) ||
-          !META_X11_DISPLAY_HAS_DAMAGE (x11_display))
-        {
-          meta_fatal ("Missing %s extension required for compositing",
-                      !META_X11_DISPLAY_HAS_COMPOSITE (x11_display) ?
-                      "composite" : "damage");
-          return;
-        }
-
-      int version = (x11_display->composite_major_version * 10) +
-                     x11_display->composite_minor_version;
-      if (version < 3)
-        {
-          meta_fatal ("Your version of COMPOSITE (%d.%d) is too old. Version 3.0 or later required.",
-                      x11_display->composite_major_version,
-                      x11_display->composite_minor_version);
-          return;
-        }
-    }
-
-  if (!display->compositor)
-    display->compositor = create_compositor (display);
-
-  meta_compositor_manage (display->compositor);
-}
-
 static void
 meta_display_init (MetaDisplay *disp)
 {
@@ -885,6 +852,8 @@ meta_display_open (void)
   g_signal_connect (settings, "ui-scaling-factor-changed",
                     G_CALLBACK (on_ui_scaling_factor_changed), display);
 
+  display->compositor = create_compositor (display);
+
   meta_display_set_cursor (display, META_CURSOR_DEFAULT);
 
   display->stack = meta_stack_new (display);
@@ -922,7 +891,6 @@ meta_display_open (void)
 
   display->last_focus_time = timestamp;
   display->last_user_time = timestamp;
-  display->compositor = NULL;
 
   if (!meta_is_wayland_compositor ())
     meta_prop_get_window (display->x11_display,
@@ -930,7 +898,11 @@ meta_display_open (void)
                           display->x11_display->atom__NET_ACTIVE_WINDOW,
                           &old_active_xwindow);
 
-  enable_compositor (display);
+  if (!meta_compositor_do_manage (display->compositor, &error))
+    {
+      g_error ("Compositor failed to manage display: %s",
+               error->message);
+    }
 
   if (display->x11_display)
     {