[gegl] graph: do not permit constructing cyclig graphs

[Ãyvind KolÃs <ok src gnome org>]

commit 751e564de0cee9c82ec0a3ee134f531ab931c284
Author: Ãyvind KolÃs <pippin gimp org>
Date:   Fri Apr 6 14:34:38 2012 +0200

    graph: do not permit constructing cyclig graphs
    
    Fixing bug #660257

 gegl/graph/gegl-node.c |   34 ++++++++++++++++++++++++++++++++++
 1 files changed, 34 insertions(+), 0 deletions(-)
---
diff --git a/gegl/graph/gegl-node.c b/gegl/graph/gegl-node.c
index bb864b3..4c31a50 100644
--- a/gegl/graph/gegl-node.c
+++ b/gegl/graph/gegl-node.c
@@ -578,6 +578,34 @@ gegl_node_source_invalidated (GeglNode            *source,
   gegl_node_invalidated (destination, &dirty_rect, FALSE);
 }
 
+static GSList *
+gegl_node_get_depends_on (GeglNode *self);
+
+static gboolean
+gegl_node_has_source (GeglNode *self,
+                      GeglNode *potential_source)
+{
+  GSList *producers, *p;
+  gboolean found = FALSE;
+
+  if (self == potential_source)
+    return TRUE;
+
+  producers = gegl_node_get_depends_on (self);
+  for (p = producers; p; p = p->next)
+    {
+      if (p->data == potential_source)
+        found = TRUE;
+      else
+        found = gegl_node_has_source (p->data, potential_source);
+      if (found)
+        break;
+    }
+  g_slist_free (producers);
+
+  return found;
+}
+
 gboolean
 gegl_node_connect_from (GeglNode    *sink,
                         const gchar *sink_pad_name,
@@ -594,6 +622,12 @@ gegl_node_connect_from (GeglNode    *sink,
   g_return_val_if_fail (GEGL_IS_NODE (source), FALSE);
   g_return_val_if_fail (source_pad_name != NULL, FALSE);
 
+  if (gegl_node_has_source (source, sink))
+    {
+      g_warning ("Construction of loop requested, bailing\n");
+      return FALSE;
+    }
+
   /* For graph nodes we implicitly use the proxy nodes */
   if (sink->is_graph)
     {