Util: add an accessor for the root background pixmap

The background of the root window is accessible as the _XROOTPMAP_ID
property on the root window. Given that the root window is always
fully covered by the Composite Overlay Window, we use that as temporary
storage while the COW is not mapped, that is, before the GDM greeter
is started and between killing the greeter and starting GDM from the
session.

https://bugzilla.gnome.org/show_bug.cgi?id=682429

src/Makefile.am

@@ -292,7 +292,7 @@ libgnome_shell_la_LIBADD =		\
 libgnome_shell_la_CPPFLAGS = $(gnome_shell_cflags)
 
 Shell-0.1.gir: libgnome-shell.la St-1.0.gir
-Shell_0_1_gir_INCLUDES = Clutter-1.0 ClutterX11-1.0 Meta-3.0 TelepathyGLib-0.12 TelepathyLogger-0.2 Soup-2.4 GMenu-3.0 NetworkManager-1.0 NMClient-1.0
+Shell_0_1_gir_INCLUDES = Clutter-1.0 ClutterX11-1.0 Meta-3.0 TelepathyGLib-0.12 TelepathyLogger-0.2 Soup-2.4 GMenu-3.0 NetworkManager-1.0 NMClient-1.0 xlib-2.0
 Shell_0_1_gir_CFLAGS = $(libgnome_shell_la_CPPFLAGS) -I $(srcdir)
 Shell_0_1_gir_LIBS = libgnome-shell.la
 Shell_0_1_gir_FILES = $(libgnome_shell_la_gir_sources)

src/shell-util.c

@@ -9,6 +9,8 @@
 #include <glib/gi18n-lib.h>
 #include <gtk/gtk.h>
 #include <gdk-pixbuf/gdk-pixbuf.h>
+#include <gdk/gdkx.h>
+#include <X11/Xatom.h>
 
 #ifdef HAVE__NL_TIME_FIRST_WEEKDAY
 #include <langinfo.h>
@@ -430,3 +432,56 @@ shell_util_create_pixbuf_from_data (const guchar      *data,
                                    bits_per_sample, width, height, rowstride,
                                    (GdkPixbufDestroyNotify) g_free, NULL);
 }
+
+Pixmap
+shell_util_get_root_background (void)
+{
+  Display *display;
+  Pixmap pixmap;
+  Window rootwin;
+  Atom xrootpmap;
+  Atom actual_type;
+  int actual_format;
+  unsigned long n_items;
+  unsigned long bytes_after;
+  unsigned char *buffer;
+
+  display = gdk_x11_get_default_xdisplay ();
+
+  xrootpmap = gdk_x11_atom_to_xatom (gdk_atom_intern_static_string ("_XROOTPMAP_ID"));
+  rootwin = gdk_x11_get_default_root_xwindow ();
+
+  gdk_error_trap_push ();
+  actual_type = None;
+  buffer = NULL;
+  if (XGetWindowProperty (display, rootwin, xrootpmap,
+                          0, G_MAXLONG,
+                          False, XA_PIXMAP, &actual_type, &actual_format,
+                          &n_items,
+                          &bytes_after,
+                          &buffer) != Success ||
+      actual_type == None)
+    {
+      if (buffer)
+        XFree (buffer);
+      gdk_error_trap_pop_ignored ();
+      return FALSE;
+    }
+
+  if (gdk_error_trap_pop () != Success ||
+      n_items == 0 ||
+      actual_type != XA_PIXMAP ||
+      actual_format != 32)
+    {
+      if (buffer)
+        XFree (buffer);
+      return FALSE;
+    }
+
+  pixmap = *((Pixmap*) buffer);
+
+  XFree (buffer);
+  return pixmap;
+}
+
+

src/shell-util.h

@@ -7,6 +7,7 @@
 #include <clutter/clutter.h>
 #include <libsoup/soup.h>
 #include <gdk-pixbuf/gdk-pixbuf.h>
+#include <gdk/gdkx.h>
 
 G_BEGIN_DECLS
 
@@ -46,6 +47,8 @@ GdkPixbuf *shell_util_create_pixbuf_from_data (const guchar      *data,
                                                int                height,
                                                int                rowstride);
 
+Pixmap  shell_util_get_root_background        (void);
+
 G_END_DECLS
 
 #endif /* __SHELL_UTIL_H__ */