[mutter] Fixes for compositor replacement

[Owen Taylor <otaylor src gnome org>]

commit 66a830fd461c56ddc2d97ecd8ae898794180eb99
Author: Owen W. Taylor <otaylor fishsoup net>
Date:   Thu Jun 30 15:23:41 2011 -0400

    Fixes for compositor replacement
    
    * When unmanaging a screen, stop redirecting subwindows explicitly,
      so that we do that before destroying the window manager selection
      window.
    * Improve comment in the retry code
    * When exiting because the previous compositor couldn't be replaced,
      don't g_error() and drop a core file.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=653121

 src/compositor/compositor.c |   20 +++++++++++++++++---
 1 files changed, 17 insertions(+), 3 deletions(-)
---
diff --git a/src/compositor/compositor.c b/src/compositor/compositor.c
index e770ef5..8c03b4a 100644
--- a/src/compositor/compositor.c
+++ b/src/compositor/compositor.c
@@ -489,8 +489,9 @@ meta_compositor_manage_screen (MetaCompositor *compositor,
 
   n_retries = 0;
 
-  /* We can race with an exiting process to claim compositing over the root window;
-   * There's really not a great way to deal with this, so we just sleep and retry.
+  /* Some compositors (like old versions of Mutter) might not properly unredirect
+   * subwindows before destroying the WM selection window; so we wait a while
+   * for such a compositor to exit before giving up.
    */
   while (TRUE)
     {
@@ -502,7 +503,12 @@ meta_compositor_manage_screen (MetaCompositor *compositor,
         break;
 
       if (n_retries == max_retries)
-        g_error ("Another compositing manager is running on screen %i", screen_number);
+        {
+          /* This probably means that a non-WM compositor like xcompmgr is running;
+           * we have no way to get it to exit */
+          meta_fatal (_("Another compositing manager is already running on screen %i on display \"%s\"."),
+                      screen_number, display->name);
+        }
 
       n_retries++;
       g_usleep (G_USEC_PER_SEC);
@@ -605,6 +611,14 @@ void
 meta_compositor_unmanage_screen (MetaCompositor *compositor,
                                  MetaScreen     *screen)
 {
+  MetaDisplay    *display       = meta_screen_get_display (screen);
+  Display        *xdisplay      = meta_display_get_xdisplay (display);
+  Window          xroot         = meta_screen_get_xroot (screen);
+
+  /* This is the most important part of cleanup - we have to do this
+   * before giving up the window manager selection or the next
+   * window manager won't be able to redirect subwindows */
+  XCompositeUnredirectSubwindows (xdisplay, xroot, CompositeRedirectManual);
 }
 
 void