[mutter] compositor: Handle fences in the frontend X connection

[Rui Matos <rtcm src gnome org>]

commit 299ed424d36921dc2fa6b5d33f9ef32d0532e5d6
Author: Rui Matos <tiagomatos gmail com>
Date:   Wed Aug 12 15:26:34 2015 +0200

    compositor: Handle fences in the frontend X connection
    
    Since mutter has two X connections and does damage handling on the
    frontend while fence triggering is done on the backend, we have a race
    between XDamageSubtract() and XSyncFenceTrigger() causing missed
    redraws in the GL_EXT_X11_sync_object path.
    
    If the fence trigger gets processed first by the server, any client
    drawing that happens between that and the damage subtract being
    processed and is completely contained in the last damage event box
    that mutter got, won't be included in the current frame nor will it
    cause a new damage event.
    
    A simple fix for this would be XSync()ing on the frontend connection
    after doing all the damage subtracts but that would add a round trip
    on every frame again which defeats the asynchronous design of X
    fences.
    
    Instead, if we move fence handling to the frontend we automatically
    get the right ordering between damage subtracts and fence triggers.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=728464

 src/backends/x11/meta-backend-x11.c |    3 ---
 src/compositor/compositor.c         |    6 ++++--
 src/compositor/meta-sync-ring.c     |   15 ++++++++++++++-
 3 files changed, 18 insertions(+), 6 deletions(-)
---
diff --git a/src/backends/x11/meta-backend-x11.c b/src/backends/x11/meta-backend-x11.c
index c576601..a76dbc5 100644
--- a/src/backends/x11/meta-backend-x11.c
+++ b/src/backends/x11/meta-backend-x11.c
@@ -45,7 +45,6 @@
 #include <meta/util.h>
 #include "display-private.h"
 #include "compositor/compositor-private.h"
-#include "compositor/meta-sync-ring.h"
 
 typedef enum {
   /* We're a traditional CM running under the host. */
@@ -267,8 +266,6 @@ handle_host_xevent (MetaBackend *backend,
         MetaCompositor *compositor = display->compositor;
         if (meta_plugin_manager_xevent_filter (compositor->plugin_mgr, event))
           bypass_clutter = TRUE;
-        if (compositor->have_x11_sync_object)
-          meta_sync_ring_handle_event (event);
       }
   }
 
diff --git a/src/compositor/compositor.c b/src/compositor/compositor.c
index 1282279..615ebd8 100644
--- a/src/compositor/compositor.c
+++ b/src/compositor/compositor.c
@@ -529,8 +529,7 @@ meta_compositor_manage (MetaCompositor *compositor)
        */
       XMapWindow (xdisplay, compositor->output);
 
-      compositor->have_x11_sync_object =
-        meta_sync_ring_init (meta_backend_x11_get_xdisplay (META_BACKEND_X11 (backend)));
+      compositor->have_x11_sync_object = meta_sync_ring_init (xdisplay);
     }
 
   redirect_windows (display->screen);
@@ -732,6 +731,9 @@ meta_compositor_process_event (MetaCompositor *compositor,
         process_damage (compositor, (XDamageNotifyEvent *) event, window);
     }
 
+  if (compositor->have_x11_sync_object)
+    meta_sync_ring_handle_event (event);
+
   /* Clutter needs to know about MapNotify events otherwise it will
      think the stage is invisible */
   if (!meta_is_wayland_compositor () && event->type == MapNotify)
diff --git a/src/compositor/meta-sync-ring.c b/src/compositor/meta-sync-ring.c
index 217ebe5..336ccd4 100644
--- a/src/compositor/meta-sync-ring.c
+++ b/src/compositor/meta-sync-ring.c
@@ -322,7 +322,7 @@ meta_sync_new (Display *xdisplay)
   self->xdisplay = xdisplay;
 
   self->xfence = XSyncCreateFence (xdisplay, DefaultRootWindow (xdisplay), FALSE);
-  self->gl_x11_sync = meta_gl_import_sync (GL_SYNC_X11_FENCE_EXT, self->xfence, 0);
+  self->gl_x11_sync = 0;
   self->gpu_fence = 0;
 
   self->xcounter = XSyncCreateCounter (xdisplay, SYNC_VALUE_ZERO);
@@ -347,6 +347,13 @@ meta_sync_new (Display *xdisplay)
   return self;
 }
 
+static void
+meta_sync_import (MetaSync *self)
+{
+  g_return_if_fail (self->gl_x11_sync == 0);
+  self->gl_x11_sync = meta_gl_import_sync (GL_SYNC_X11_FENCE_EXT, self->xfence, 0);
+}
+
 static Bool
 alarm_event_predicate (Display  *dpy,
                        XEvent   *event,
@@ -437,6 +444,12 @@ meta_sync_ring_init (Display *xdisplay)
       ring->syncs_array[i] = sync;
       g_hash_table_replace (ring->alarm_to_sync, (gpointer) sync->xalarm, sync);
     }
+  /* Since the connection we create the X fences on isn't the same as
+   * the one used for the GLX context, we need to XSync() here to
+   * ensure glImportSync() succeeds. */
+  XSync (xdisplay, False);
+  for (i = 0; i < NUM_SYNCS; ++i)
+    meta_sync_import (ring->syncs_array[i]);
 
   ring->current_sync_idx = 0;
   ring->current_sync = ring->syncs_array[0];