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[glib/gobject-performance: 9/10] Implement lock free interface lookup

commit e13f1ee6f3f433f2a19baec7ae2ac506a893584b
Author: Alexander Larsson <alexl redhat com>
Date:   Wed Sep 9 16:51:28 2009 +0200

    Implement lock free interface lookup
    
    We implement lock free interface lookup by moving the n_ifaces
    counter into memory pointed to by TypeNode->iface_entries, and
    then updating this in RCU-style by always copying it, modifying
    the copy and then when the modification is done replace the old
    pointer with g_atomic_pointer_set.
    
    There is one additional complexity when freeing the old memory,
    since the old memory region can be in use. To handle this we
    don't free such memory, but put it on a free list and reuse it
    later. This means that lock-free lookups must be able to
    handle the memory being modified in random ways without crashing,
    and at the end we verify that the memory didn't change and the
    transaction is ok.
    
    With this infrastructure the patch then implements a lock-free
    version of type_lookup_iface_entry_L called type_lookup_iface_vtable_I
    and use it in: g_type_interface_peek, g_type_interface_peek_parent
    and type_node_check_conformities_UorL.
    
    Using the performance tests from bug 557100 shows that the general
    performance difference is negligible, but the lack of a lock for each
    type check and interface vfunc call should greatly enhance threaded
    scalability.

 gobject/gtype.c |  329 ++++++++++++++++++++++++++++++++++---------------------
 1 files changed, 204 insertions(+), 125 deletions(-)
---
diff --git a/gobject/gtype.c b/gobject/gtype.c
index c6dff7f..efd3455 100644
--- a/gobject/gtype.c
+++ b/gobject/gtype.c
@@ -30,6 +30,7 @@
 #include "gvaluecollector.h"
 #include "gbsearcharray.h"
 #include "gobjectalias.h"
+#include "gatomicarray.h"
 
 
 /**
@@ -171,6 +172,7 @@ typedef struct _IFaceData       IFaceData;
 typedef struct _ClassData       ClassData;
 typedef struct _InstanceData    InstanceData;
 typedef union  _TypeData        TypeData;
+typedef struct _IFaceEntries    IFaceEntries;
 typedef struct _IFaceEntry      IFaceEntry;
 typedef struct _IFaceHolder	IFaceHolder;
 
@@ -225,7 +227,7 @@ struct _TypeNode
   GTypePlugin *plugin;
   guint        n_children : 12;
   guint        n_supers : 8;
-  guint        _prot_n_ifaces_prerequisites : 9;
+  guint        _prot_n_prerequisites : 9;
   guint        is_classed : 1;
   guint        is_instantiatable : 1;
   guint        mutatable_check_cache : 1;	/* combines some common path checks */
@@ -234,7 +236,7 @@ struct _TypeNode
   GQuark       qname;
   GData       *global_gdata;
   union {
-    IFaceEntry  *iface_entries;		/* for !iface types */
+    GAtomicArray iface_entries;		/* for !iface types */
     GType       *prerequisistes;
   } _prot;
   GType        supers[1]; /* flexible array */
@@ -243,16 +245,15 @@ struct _TypeNode
 #define SIZEOF_BASE_TYPE_NODE()			(G_STRUCT_OFFSET (TypeNode, supers))
 #define MAX_N_SUPERS				(255)
 #define MAX_N_CHILDREN				(4095)
-#define MAX_N_IFACES				(511)
-#define	MAX_N_PREREQUISITES			(MAX_N_IFACES)
+#define	MAX_N_PREREQUISITES			(511)
 #define NODE_TYPE(node)				(node->supers[0])
 #define NODE_PARENT_TYPE(node)			(node->supers[1])
 #define NODE_FUNDAMENTAL_TYPE(node)		(node->supers[node->n_supers])
 #define NODE_NAME(node)				(g_quark_to_string (node->qname))
 #define	NODE_IS_IFACE(node)			(NODE_FUNDAMENTAL_TYPE (node) == G_TYPE_INTERFACE)
-#define	CLASSED_NODE_N_IFACES(node)		((node)->_prot_n_ifaces_prerequisites)
-#define	CLASSED_NODE_IFACES_ENTRIES(node)	((node)->_prot.iface_entries)
-#define	IFACE_NODE_N_PREREQUISITES(node)	((node)->_prot_n_ifaces_prerequisites)
+#define	CLASSED_NODE_IFACES_ENTRIES(node)	(&(node)->_prot.iface_entries)
+#define	CLASSED_NODE_IFACES_ENTRIES_LOCKED(node)(G_ATOMIC_ARRAY_GET_LOCKED(CLASSED_NODE_IFACES_ENTRIES((node)), IFaceEntries))
+#define	IFACE_NODE_N_PREREQUISITES(node)	((node)->_prot_n_prerequisites)
 #define	IFACE_NODE_PREREQUISITES(node)		((node)->_prot.prerequisistes)
 #define	iface_node_get_holders_L(node)		((IFaceHolder*) type_get_qdata_L ((node), static_quark_iface_holder))
 #define	iface_node_set_holders_W(node, holders)	(type_set_qdata_W ((node), static_quark_iface_holder, (holders)))
@@ -264,7 +265,6 @@ struct _TypeNode
         ((ancestor)->n_supers <= (node)->n_supers &&                                        \
 	 (node)->supers[(node)->n_supers - (ancestor)->n_supers] == NODE_TYPE (ancestor))
 
-
 struct _IFaceHolder
 {
   GType           instance_type;
@@ -280,6 +280,14 @@ struct _IFaceEntry
   InitState       init_state;
 };
 
+struct _IFaceEntries {
+  guint offset_index; /* not used yet */
+  IFaceEntry entry[1];
+};
+
+#define IFACE_ENTRIES_HEADER_SIZE (sizeof(IFaceEntries) - sizeof(IFaceEntry))
+#define IFACE_ENTRIES_N_ENTRIES(_entries) ( (ATOMIC_ARRAY_DATA_SIZE((_entries)) - IFACE_ENTRIES_HEADER_SIZE) / sizeof(IFaceEntry) )
+
 struct _CommonData
 {
   guint             ref_count;
@@ -359,7 +367,6 @@ static GQuark          static_quark_iface_holder = 0;
 static GQuark          static_quark_dependants_array = 0;
 GTypeDebugFlags	       _g_type_debug_flags = 0;
 
-
 /* --- type nodes --- */
 static GHashTable       *static_type_nodes_ht = NULL;
 static TypeNode		*static_fundamental_type_nodes[(G_TYPE_FUNDAMENTAL_MAX >> G_TYPE_FUNDAMENTAL_SHIFT) + 1] = { NULL, };
@@ -385,7 +392,7 @@ type_node_any_new_W (TypeNode             *pnode,
   GType type;
   TypeNode *node;
   guint i, node_size = 0;
-  
+
   n_supers = pnode ? pnode->n_supers + 1 : 0;
   
   if (!pnode)
@@ -419,10 +426,7 @@ type_node_any_new_W (TypeNode             *pnode,
 	  IFACE_NODE_PREREQUISITES (node) = NULL;
 	}
       else
-	{
-	  CLASSED_NODE_N_IFACES (node) = 0;
-	  CLASSED_NODE_IFACES_ENTRIES (node) = NULL;
-	}
+	g_atomic_array_init (CLASSED_NODE_IFACES_ENTRIES (node));
     }
   else
     {
@@ -440,18 +444,23 @@ type_node_any_new_W (TypeNode             *pnode,
       else
 	{
 	  guint j;
-	  
-	  CLASSED_NODE_N_IFACES (node) = CLASSED_NODE_N_IFACES (pnode);
-	  CLASSED_NODE_IFACES_ENTRIES (node) = g_memdup (CLASSED_NODE_IFACES_ENTRIES (pnode),
-							 sizeof (CLASSED_NODE_IFACES_ENTRIES (pnode)[0]) *
-							 CLASSED_NODE_N_IFACES (node));
-	  for (j = 0; j < CLASSED_NODE_N_IFACES (node); j++)
+	  IFaceEntries *entries;
+
+	  entries = g_atomic_array_copy (CLASSED_NODE_IFACES_ENTRIES (pnode),
+					 IFACE_ENTRIES_HEADER_SIZE,
+					 0);
+	  if (entries)
 	    {
-	      CLASSED_NODE_IFACES_ENTRIES (node)[j].vtable = NULL;
-	      CLASSED_NODE_IFACES_ENTRIES (node)[j].init_state = UNINITIALIZED;
+	      for (j = 0; j < IFACE_ENTRIES_N_ENTRIES (entries); j++)
+		{
+		  entries->entry[j].vtable = NULL;
+		  entries->entry[j].init_state = UNINITIALIZED;
+		}
+	      g_atomic_array_update (CLASSED_NODE_IFACES_ENTRIES (node),
+				     entries);
 	    }
 	}
-      
+
       i = pnode->n_children++;
       pnode->children = g_renew (GType, pnode->children, pnode->n_children);
       pnode->children[i] = type;
@@ -519,15 +528,15 @@ type_node_new_W (TypeNode    *pnode,
 }
 
 static inline IFaceEntry*
-type_lookup_iface_entry_L (TypeNode *node,
-			   TypeNode *iface_node)
+lookup_iface_entry_I (IFaceEntries *entries,
+		      TypeNode *iface_node)
 {
-  if (NODE_IS_IFACE (iface_node) && CLASSED_NODE_N_IFACES (node))
+  if (entries != NULL)
     {
-      IFaceEntry *ifaces = CLASSED_NODE_IFACES_ENTRIES (node) - 1;
-      guint n_ifaces = CLASSED_NODE_N_IFACES (node);
+      IFaceEntry *ifaces = &entries->entry[-1];
+      guint n_ifaces = IFACE_ENTRIES_N_ENTRIES (entries);
       GType iface_type = NODE_TYPE (iface_node);
-      
+
       do
 	{
 	  guint i;
@@ -551,6 +560,50 @@ type_lookup_iface_entry_L (TypeNode *node,
   return NULL;
 }
 
+static inline IFaceEntry*
+type_lookup_iface_entry_L (TypeNode *node,
+			   TypeNode *iface_node)
+{
+  if (!NODE_IS_IFACE (iface_node))
+    return NULL;
+
+  return lookup_iface_entry_I (CLASSED_NODE_IFACES_ENTRIES_LOCKED (node),
+			       iface_node);
+}
+
+
+static inline gboolean
+type_lookup_iface_vtable_I (TypeNode *node,
+			    TypeNode *iface_node,
+			    gpointer *vtable_ptr)
+{
+  IFaceEntry *entry;
+  gboolean res;
+
+  if (!NODE_IS_IFACE (iface_node))
+    {
+      if (vtable_ptr)
+	*vtable_ptr = NULL;
+      return FALSE;
+    }
+
+  G_ATOMIC_ARRAY_DO_TRANSACTION
+    (CLASSED_NODE_IFACES_ENTRIES (node), IFaceEntries,
+
+     entry = lookup_iface_entry_I (transaction_data, iface_node);
+     res = entry != NULL;
+     if (vtable_ptr)
+       {
+	 if (entry)
+	   *vtable_ptr = entry->vtable;
+	 else
+	   *vtable_ptr = NULL;
+       }
+     );
+
+  return res;
+}
+
 static inline gboolean
 type_lookup_prerequisite_L (TypeNode *iface,
 			    GType     prerequisite_type)
@@ -1167,54 +1220,72 @@ type_node_add_iface_entry_W (TypeNode   *node,
 			     GType       iface_type,
                              IFaceEntry *parent_entry)
 {
-  IFaceEntry *entries;
+  IFaceEntries *entries;
+  IFaceEntry *entry;
   guint i;
-  
-  g_assert (node->is_instantiatable && CLASSED_NODE_N_IFACES (node) < MAX_N_IFACES);
-  
-  entries = CLASSED_NODE_IFACES_ENTRIES (node);
-  for (i = 0; i < CLASSED_NODE_N_IFACES (node); i++)
-    if (entries[i].iface_type == iface_type)
-      {
-	/* this can happen in two cases:
-         * - our parent type already conformed to iface_type and node
-         *   got its own holder info. here, our children already have
-         *   entries and NULL vtables, since this will only work for
-         *   uninitialized classes.
-	 * - an interface type is added to an ancestor after it was
-         *   added to a child type.
-	 */
-        if (!parent_entry)
-          g_assert (entries[i].vtable == NULL && entries[i].init_state == UNINITIALIZED);
-        else
-          {
-            /* sick, interface is added to ancestor *after* child type;
-             * nothing todo, the entry and our children were already setup correctly
-             */
-          }
-        return;
-      }
-    else if (entries[i].iface_type > iface_type)
-      break;
-  CLASSED_NODE_N_IFACES (node) += 1;
-  CLASSED_NODE_IFACES_ENTRIES (node) = g_renew (IFaceEntry,
-						CLASSED_NODE_IFACES_ENTRIES (node),
-						CLASSED_NODE_N_IFACES (node));
-  entries = CLASSED_NODE_IFACES_ENTRIES (node);
-  g_memmove (entries + i + 1, entries + i, sizeof (entries[0]) * (CLASSED_NODE_N_IFACES (node) - i - 1));
-  entries[i].iface_type = iface_type;
-  entries[i].vtable = NULL;
-  entries[i].init_state = UNINITIALIZED;
+  int num_entries;
+
+  g_assert (node->is_instantiatable);
+
+  entries = CLASSED_NODE_IFACES_ENTRIES_LOCKED (node);
+  i = 0;
+  if (entries != NULL)
+    {
+      num_entries = IFACE_ENTRIES_N_ENTRIES (entries);
+
+      for (i = 0; i < num_entries; i++)
+	{
+	  entry = &entries->entry[i];
+	  if (entry->iface_type == iface_type)
+	    {
+	      /* this can happen in two cases:
+	       * - our parent type already conformed to iface_type and node
+	       *   got its own holder info. here, our children already have
+	       *   entries and NULL vtables, since this will only work for
+	       *   uninitialized classes.
+	       * - an interface type is added to an ancestor after it was
+	       *   added to a child type.
+	       */
+	      if (!parent_entry)
+		g_assert (entry->vtable == NULL && entry->init_state == UNINITIALIZED);
+	      else
+		{
+		  /* sick, interface is added to ancestor *after* child type;
+		   * nothing todo, the entry and our children were already setup correctly
+		   */
+		}
+	      return;
+	    }
+	  else if (entry->iface_type > iface_type)
+	    break;
+	}
+    }
+
+  entries = g_atomic_array_copy (CLASSED_NODE_IFACES_ENTRIES (node),
+				 IFACE_ENTRIES_HEADER_SIZE,
+				 sizeof (IFaceEntry));
+  num_entries = IFACE_ENTRIES_N_ENTRIES (entries);
+  g_memmove (&entries->entry[i + 1], &entries->entry[i],
+	     sizeof (IFaceEntry) * (num_entries - i - 1));
+  entries->entry[i].iface_type = iface_type;
+  entries->entry[i].vtable = NULL;
+  entries->entry[i].init_state = UNINITIALIZED;
 
   if (parent_entry)
     {
       if (node->data && node->data->class.init_state >= BASE_IFACE_INIT)
         {
-          entries[i].init_state = INITIALIZED;
-          entries[i].vtable = parent_entry->vtable;
+          entries->entry[i].init_state = INITIALIZED;
+          entries->entry[i].vtable = parent_entry->vtable;
         }
+    }
+
+  g_atomic_array_update (CLASSED_NODE_IFACES_ENTRIES (node), entries);
+
+  if (parent_entry)
+    {
       for (i = 0; i < node->n_children; i++)
-        type_node_add_iface_entry_W (lookup_type_node_I (node->children[i]), iface_type, &entries[i]);
+        type_node_add_iface_entry_W (lookup_type_node_I (node->children[i]), iface_type, &entries->entry[i]);
     }
 }
 
@@ -1227,7 +1298,7 @@ type_add_interface_Wm (TypeNode             *node,
   IFaceHolder *iholder = g_new0 (IFaceHolder, 1);
   IFaceEntry *entry;
   guint i;
-  
+
   g_assert (node->is_instantiatable && NODE_IS_IFACE (iface) && ((info && !plugin) || (!info && plugin)));
   
   iholder->next = iface_node_get_holders_L (iface);
@@ -1880,6 +1951,7 @@ type_class_init_Wm (TypeNode   *node,
 {
   GSList *slist, *init_slist = NULL;
   GTypeClass *class;
+  IFaceEntries *entries;
   IFaceEntry *entry;
   TypeNode *bnode, *pnode;
   guint i;
@@ -1936,39 +2008,42 @@ type_class_init_Wm (TypeNode   *node,
   pnode = lookup_type_node_I (NODE_PARENT_TYPE (node));
 
   i = 0;
-  while (i < CLASSED_NODE_N_IFACES (node))
+  while ((entries = CLASSED_NODE_IFACES_ENTRIES_LOCKED (node)) != NULL &&
+	  i < IFACE_ENTRIES_N_ENTRIES (entries))
     {
-      entry = &CLASSED_NODE_IFACES_ENTRIES (node)[i];
-      while (i < CLASSED_NODE_N_IFACES (node) &&
+      entry = &entries->entry[i];
+      while (i < IFACE_ENTRIES_N_ENTRIES (entries) &&
 	     entry->init_state == IFACE_INIT)
 	{
 	  entry++;
 	  i++;
 	}
 
-      if (i == CLASSED_NODE_N_IFACES (node))
+      if (i == IFACE_ENTRIES_N_ENTRIES (entries))
 	break;
 
       if (!type_iface_vtable_base_init_Wm (lookup_type_node_I (entry->iface_type), node))
 	{
 	  guint j;
+	  IFaceEntries *pentries = CLASSED_NODE_IFACES_ENTRIES_LOCKED (pnode);
 	  
 	  /* need to get this interface from parent, type_iface_vtable_base_init_Wm()
 	   * doesn't modify write lock upon FALSE, so entry is still valid; 
 	   */
 	  g_assert (pnode != NULL);
-	  
-	  for (j = 0; j < CLASSED_NODE_N_IFACES (pnode); j++)
-	    {
-	      IFaceEntry *pentry = CLASSED_NODE_IFACES_ENTRIES (pnode) + j;
-	      
-	      if (pentry->iface_type == entry->iface_type)
-		{
-		  entry->vtable = pentry->vtable;
-		  entry->init_state = INITIALIZED;
-		  break;
-		}
-	    }
+
+	  if (pentries)
+	    for (j = 0; j < IFACE_ENTRIES_N_ENTRIES (pentries); j++)
+	      {
+		IFaceEntry *pentry = &pentries->entry[j];
+
+		if (pentry->iface_type == entry->iface_type)
+		  {
+		    entry->vtable = pentry->vtable;
+		    entry->init_state = INITIALIZED;
+		    break;
+		  }
+	      }
 	  g_assert (entry->vtable != NULL);
 	}
 
@@ -1999,17 +2074,17 @@ type_class_init_Wm (TypeNode   *node,
    * an anchestor type.
    */
   i = 0;
-  while (TRUE)
+  while ((entries = CLASSED_NODE_IFACES_ENTRIES_LOCKED (node)) != NULL)
     {
-      entry = &CLASSED_NODE_IFACES_ENTRIES (node)[i];
-      while (i < CLASSED_NODE_N_IFACES (node) &&
+      entry = &entries->entry[i];
+      while (i < IFACE_ENTRIES_N_ENTRIES (entries) &&
 	     entry->init_state == INITIALIZED)
 	{
 	  entry++;
 	  i++;
 	}
 
-      if (i == CLASSED_NODE_N_IFACES (node))
+      if (i == IFACE_ENTRIES_N_ENTRIES (entries))
 	break;
 
       type_iface_vtable_iface_init_Wm (lookup_type_node_I (entry->iface_type), node);
@@ -2028,13 +2103,15 @@ static void
 type_data_finalize_class_ifaces_Wm (TypeNode *node)
 {
   guint i;
+  IFaceEntries *entries;
 
   g_assert (node->is_instantiatable && node->data && node->data->class.class && node->data->common.ref_count == 0);
 
  reiterate:
-  for (i = 0; i < CLASSED_NODE_N_IFACES (node); i++)
+  entries = CLASSED_NODE_IFACES_ENTRIES_LOCKED (node);
+  for (i = 0; entries != NULL && i < IFACE_ENTRIES_N_ENTRIES (entries); i++)
     {
-      IFaceEntry *entry = CLASSED_NODE_IFACES_ENTRIES (node) + i;
+      IFaceEntry *entry = &entries->entry[i];
       if (entry->vtable)
 	{
           if (type_iface_vtable_finalize_Wm (lookup_type_node_I (entry->iface_type), node, entry->vtable))
@@ -2134,7 +2211,7 @@ type_data_last_unref_Wm (GType    type,
       tdata = node->data;
       if (node->is_classed && tdata->class.class)
 	{
-	  if (CLASSED_NODE_N_IFACES (node))
+	  if (CLASSED_NODE_IFACES_ENTRIES_LOCKED (node) != NULL)
 	    type_data_finalize_class_ifaces_Wm (node);
 	  node->mutatable_check_cache = FALSE;
 	  node->data = NULL;
@@ -2846,17 +2923,7 @@ g_type_interface_peek (gpointer instance_class,
   node = lookup_type_node_I (class->g_type);
   iface = lookup_type_node_I (iface_type);
   if (node && node->is_instantiatable && iface)
-    {
-      IFaceEntry *entry;
-      
-      G_READ_LOCK (&type_rw_lock);
-      
-      entry = type_lookup_iface_entry_L (node, iface);
-      if (entry && entry->vtable)	/* entry is relocatable */
-	vtable = entry->vtable;
-      
-      G_READ_UNLOCK (&type_rw_lock);
-    }
+    type_lookup_iface_vtable_I (node, iface, &vtable);
   else
     g_warning (G_STRLOC ": invalid class pointer `%p'", class);
   
@@ -2891,17 +2958,7 @@ g_type_interface_peek_parent (gpointer g_iface)
   if (node)
     node = lookup_type_node_I (NODE_PARENT_TYPE (node));
   if (node && node->is_instantiatable && iface)
-    {
-      IFaceEntry *entry;
-      
-      G_READ_LOCK (&type_rw_lock);
-      
-      entry = type_lookup_iface_entry_L (node, iface);
-      if (entry && entry->vtable)	/* entry is relocatable */
-	vtable = entry->vtable;
-      
-      G_READ_UNLOCK (&type_rw_lock);
-    }
+    type_lookup_iface_vtable_I (node, iface, &vtable);
   else if (node)
     g_warning (G_STRLOC ": invalid interface pointer `%p'", g_iface);
   
@@ -3188,21 +3245,33 @@ type_node_check_conformities_UorL (TypeNode *node,
 				   gboolean  have_lock)
 {
   gboolean match;
-  
+
   if (/* support_inheritance && */
       NODE_IS_ANCESTOR (iface_node, node))
     return TRUE;
-  
+
   support_interfaces = support_interfaces && node->is_instantiatable && NODE_IS_IFACE (iface_node);
   support_prerequisites = support_prerequisites && NODE_IS_IFACE (node);
   match = FALSE;
-  if (support_interfaces || support_prerequisites)
+  if (support_interfaces)
+    {
+      if (have_lock)
+	{
+	  if (type_lookup_iface_entry_L (node, iface_node))
+	    match = TRUE;
+	}
+      else
+	{
+	  if (type_lookup_iface_vtable_I (node, iface_node, NULL))
+	    match = TRUE;
+	}
+    }
+  if (!match &&
+      support_prerequisites)
     {
       if (!have_lock)
 	G_READ_LOCK (&type_rw_lock);
-      if (support_interfaces && type_lookup_iface_entry_L (node, iface_node))
-	match = TRUE;
-      else if (support_prerequisites && type_lookup_prerequisite_L (node, NODE_TYPE (iface_node)))
+      if (support_prerequisites && type_lookup_prerequisite_L (node, NODE_TYPE (iface_node)))
 	match = TRUE;
       if (!have_lock)
 	G_READ_UNLOCK (&type_rw_lock);
@@ -3313,17 +3382,27 @@ g_type_interfaces (GType  type,
   node = lookup_type_node_I (type);
   if (node && node->is_instantiatable)
     {
+      IFaceEntries *entries;
       GType *ifaces;
       guint i;
       
       G_READ_LOCK (&type_rw_lock);
-      ifaces = g_new (GType, CLASSED_NODE_N_IFACES (node) + 1);
-      for (i = 0; i < CLASSED_NODE_N_IFACES (node); i++)
-	ifaces[i] = CLASSED_NODE_IFACES_ENTRIES (node)[i].iface_type;
+      entries = CLASSED_NODE_IFACES_ENTRIES_LOCKED (node);
+      if (entries)
+	{
+	  ifaces = g_new (GType, IFACE_ENTRIES_N_ENTRIES (entries) + 1);
+	  for (i = 0; i < IFACE_ENTRIES_N_ENTRIES (entries); i++)
+	    ifaces[i] = entries->entry[i].iface_type;
+	}
+      else
+	{
+	  ifaces = g_new (GType, 1);
+	  i = 0;
+	}
       ifaces[i] = 0;
       
       if (n_interfaces)
-	*n_interfaces = CLASSED_NODE_N_IFACES (node);
+	*n_interfaces = i;
       G_READ_UNLOCK (&type_rw_lock);
       
       return ifaces;
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