Improve consistency of locking for SOG.m_parts in order to avoid race conditions in linking and unlinking

6 files changed, 205 insertions, 136 deletions

diff --git a/OpenSim/Region/Framework/Scenes/Scene.Inventory.cs b/OpenSim/Region/Framework/Scenes/Scene.Inventory.cs
index 9fef8f4..379128a 100644
--- a/OpenSim/Region/Framework/Scenes/Scene.Inventory.cs
+++ b/OpenSim/Region/Framework/Scenes/Scene.Inventory.cs
@@ -1974,7 +1974,7 @@ namespace OpenSim.Region.Framework.Scenes
             if (null == group)
                 return null;
             
-            if (!Permissions.CanRezObject(group.Children.Count, item.OwnerID, pos))
+            if (!Permissions.CanRezObject(group.PrimCount, item.OwnerID, pos))
                 return null;            
 
             if (!Permissions.BypassPermissions())
@@ -2051,8 +2051,11 @@ namespace OpenSim.Region.Framework.Scenes
                     sog.SetGroup(groupID, remoteClient);
                     sog.ScheduleGroupForFullUpdate();
 
-                    foreach (SceneObjectPart child in sog.Children.Values)
-                        child.Inventory.ChangeInventoryOwner(ownerID);
+                    lock (sog.Children)
+                    {
+                        foreach (SceneObjectPart child in sog.Children.Values)
+                            child.Inventory.ChangeInventoryOwner(ownerID);
+                    }
                 }
                 else
                 {
@@ -2062,16 +2065,18 @@ namespace OpenSim.Region.Framework.Scenes
                     if (sog.GroupID != groupID)
                         continue;
 
-                    foreach (SceneObjectPart child in sog.Children.Values)
+                    lock (sog.Children)
                     {
-                        child.LastOwnerID = child.OwnerID;
-                        child.Inventory.ChangeInventoryOwner(groupID);
+                        foreach (SceneObjectPart child in sog.Children.Values)
+                        {
+                            child.LastOwnerID = child.OwnerID;
+                            child.Inventory.ChangeInventoryOwner(groupID);
+                        }
                     }
 
                     sog.SetOwnerId(groupID);
                     sog.ApplyNextOwnerPermissions();
-                }
-                
+                }                
             }
 
             foreach (uint localID in localIDs)
diff --git a/OpenSim/Region/Framework/Scenes/Scene.PacketHandlers.cs b/OpenSim/Region/Framework/Scenes/Scene.PacketHandlers.cs
index e25b1f1..9f1575d 100644
--- a/OpenSim/Region/Framework/Scenes/Scene.PacketHandlers.cs
+++ b/OpenSim/Region/Framework/Scenes/Scene.PacketHandlers.cs
@@ -156,21 +156,29 @@ namespace OpenSim.Region.Framework.Scenes
                         }
                         break;
                     }
-                   else 
-                   {
-                       // We also need to check the children of this prim as they
-                       // can be selected as well and send property information
-                       bool foundPrim = false;
-                       foreach (KeyValuePair<UUID, SceneObjectPart> child in ((SceneObjectGroup) ent).Children)
-                       {
-                           if (child.Value.LocalId == primLocalID) 
-                           {
-                               child.Value.GetProperties(remoteClient);
-                               foundPrim = true;
-                               break;
-                           }
-                       }
-                       if (foundPrim) break;
+                    else 
+                    {
+                        // We also need to check the children of this prim as they
+                        // can be selected as well and send property information
+                        bool foundPrim = false;
+                        
+                        SceneObjectGroup sog = ent as SceneObjectGroup;
+                        
+                        lock (sog.Children)
+                        {
+                            foreach (KeyValuePair<UUID, SceneObjectPart> child in (sog.Children))
+                            {
+                                if (child.Value.LocalId == primLocalID) 
+                                {
+                                    child.Value.GetProperties(remoteClient);
+                                    foundPrim = true;
+                                    break;
+                                }
+                            }
+                        }
+                            
+                        if (foundPrim) 
+                            break;
                    }
                 }
             }
diff --git a/OpenSim/Region/Framework/Scenes/Scene.cs b/OpenSim/Region/Framework/Scenes/Scene.cs
index b808c6d..8556105 100644
--- a/OpenSim/Region/Framework/Scenes/Scene.cs
+++ b/OpenSim/Region/Framework/Scenes/Scene.cs
@@ -1756,8 +1756,9 @@ namespace OpenSim.Region.Framework.Scenes
                 
                 if (group.RootPart == null)
                 {
-                    m_log.ErrorFormat("[SCENE]: Found a SceneObjectGroup with m_rootPart == null and {0} children",
-                                      group.Children == null ? 0 : group.Children.Count);
+                    m_log.ErrorFormat(
+                        "[SCENE]: Found a SceneObjectGroup with m_rootPart == null and {0} children",
+                        group.Children == null ? 0 : group.PrimCount);
                 }
 
                 AddRestoredSceneObject(group, true, true);
@@ -2064,18 +2065,22 @@ namespace OpenSim.Region.Framework.Scenes
                 group.RemoveScriptInstances(true);
             }
 
-            foreach (SceneObjectPart part in group.Children.Values)
+            lock (group.Children)
             {
-                if (part.IsJoint() && ((part.Flags & PrimFlags.Physics) != 0))
+                foreach (SceneObjectPart part in group.Children.Values)
                 {
-                    PhysicsScene.RequestJointDeletion(part.Name); // FIXME: what if the name changed?
-                }
-                else if (part.PhysActor != null)
-                {
-                    PhysicsScene.RemovePrim(part.PhysActor);
-                    part.PhysActor = null;
+                    if (part.IsJoint() && ((part.Flags & PrimFlags.Physics) != 0))
+                    {
+                        PhysicsScene.RequestJointDeletion(part.Name); // FIXME: what if the name changed?
+                    }
+                    else if (part.PhysActor != null)
+                    {
+                        PhysicsScene.RemovePrim(part.PhysActor);
+                        part.PhysActor = null;
+                    }
                 }
             }
+            
 //            if (rootPart.PhysActor != null)
 //            {
 //                PhysicsScene.RemovePrim(rootPart.PhysActor);
@@ -2426,14 +2431,16 @@ namespace OpenSim.Region.Framework.Scenes
 
             // Force allocation of new LocalId
             //
-            foreach (SceneObjectPart p in sceneObject.Children.Values)
-                p.LocalId = 0;
+            lock (sceneObject.Children)
+            {
+                foreach (SceneObjectPart p in sceneObject.Children.Values)
+                    p.LocalId = 0;
+            }
 
             if (sceneObject.IsAttachmentCheckFull()) // Attachment
             {
                 sceneObject.RootPart.AddFlag(PrimFlags.TemporaryOnRez);
                 sceneObject.RootPart.AddFlag(PrimFlags.Phantom);
-
                       
                 // Don't sent a full update here because this will cause full updates to be sent twice for 
                 // attachments on region crossings, resulting in viewer glitches.                
@@ -2447,7 +2454,6 @@ namespace OpenSim.Region.Framework.Scenes
 
                 if (sp != null)
                 {
-
                     SceneObjectGroup grp = sceneObject;
 
                     m_log.DebugFormat(
@@ -2459,7 +2465,6 @@ namespace OpenSim.Region.Framework.Scenes
                     
                     if (AttachmentsModule != null)
                         AttachmentsModule.AttachObject(sp.ControllingClient, grp, 0, false);
-
                 }
                 else
                 {
diff --git a/OpenSim/Region/Framework/Scenes/SceneGraph.cs b/OpenSim/Region/Framework/Scenes/SceneGraph.cs
index 31faeec..2b24706 100644
--- a/OpenSim/Region/Framework/Scenes/SceneGraph.cs
+++ b/OpenSim/Region/Framework/Scenes/SceneGraph.cs
@@ -364,45 +364,48 @@ namespace OpenSim.Region.Framework.Scenes
 //                        "[SCENE GRAPH]: Adding object {0} {1} to region {2}", 
 //                        sceneObject.Name, sceneObject.UUID, m_parentScene.RegionInfo.RegionName);                
             
-                if (m_parentScene.m_clampPrimSize)
+                lock (sceneObject.Children)
                 {
-                    foreach (SceneObjectPart part in sceneObject.Children.Values)
+                    if (m_parentScene.m_clampPrimSize)
                     {
-                        Vector3 scale = part.Shape.Scale;
-    
-                        if (scale.X > m_parentScene.m_maxNonphys)
-                            scale.X = m_parentScene.m_maxNonphys;
-                        if (scale.Y > m_parentScene.m_maxNonphys)
-                            scale.Y = m_parentScene.m_maxNonphys;
-                        if (scale.Z > m_parentScene.m_maxNonphys)
-                            scale.Z = m_parentScene.m_maxNonphys;
-    
-                        part.Shape.Scale = scale;
+                        foreach (SceneObjectPart part in sceneObject.Children.Values)
+                        {
+                            Vector3 scale = part.Shape.Scale;
+        
+                            if (scale.X > m_parentScene.m_maxNonphys)
+                                scale.X = m_parentScene.m_maxNonphys;
+                            if (scale.Y > m_parentScene.m_maxNonphys)
+                                scale.Y = m_parentScene.m_maxNonphys;
+                            if (scale.Z > m_parentScene.m_maxNonphys)
+                                scale.Z = m_parentScene.m_maxNonphys;
+        
+                            part.Shape.Scale = scale;
+                        }
                     }
-                }
+        
+                    sceneObject.AttachToScene(m_parentScene);
+        
+                    if (sendClientUpdates)
+                        sceneObject.ScheduleGroupForFullUpdate();
+                                         
+                    Entities.Add(sceneObject);
+                    m_numPrim += sceneObject.Children.Count;
     
-                sceneObject.AttachToScene(m_parentScene);
+                    if (attachToBackup)
+                        sceneObject.AttachToBackup();
     
-                if (sendClientUpdates)
-                    sceneObject.ScheduleGroupForFullUpdate();
-                                     
-                Entities.Add(sceneObject);
-                m_numPrim += sceneObject.Children.Count;
-
-                if (attachToBackup)
-                    sceneObject.AttachToBackup();
-
-                if (OnObjectCreate != null)
-                    OnObjectCreate(sceneObject);
-                
-                lock (m_dictionary_lock)
-                {
-                    SceneObjectGroupsByFullID[sceneObject.UUID] = sceneObject;
-                    SceneObjectGroupsByLocalID[sceneObject.LocalId] = sceneObject;
-                    foreach (SceneObjectPart part in sceneObject.Children.Values)
+                    if (OnObjectCreate != null)
+                        OnObjectCreate(sceneObject);
+                    
+                    lock (m_dictionary_lock)
                     {
-                        SceneObjectGroupsByFullID[part.UUID] = sceneObject;
-                        SceneObjectGroupsByLocalID[part.LocalId] = sceneObject;
+                        SceneObjectGroupsByFullID[sceneObject.UUID] = sceneObject;
+                        SceneObjectGroupsByLocalID[sceneObject.LocalId] = sceneObject;
+                        foreach (SceneObjectPart part in sceneObject.Children.Values)
+                        {
+                            SceneObjectGroupsByFullID[part.UUID] = sceneObject;
+                            SceneObjectGroupsByLocalID[part.LocalId] = sceneObject;
+                        }
                     }
                 }
             }
@@ -420,11 +423,16 @@ namespace OpenSim.Region.Framework.Scenes
             {
                 if (!resultOfObjectLinked)
                 {
-                    m_numPrim -= ((SceneObjectGroup) Entities[uuid]).Children.Count;
-
-                    if ((((SceneObjectGroup)Entities[uuid]).RootPart.Flags & PrimFlags.Physics) == PrimFlags.Physics)
+                    SceneObjectGroup sog = Entities[uuid] as SceneObjectGroup;
+                    
+                    lock (sog.Children)
                     {
-                        RemovePhysicalPrim(((SceneObjectGroup)Entities[uuid]).Children.Count);
+                        m_numPrim -= sog.PrimCount;
+    
+                        if ((((SceneObjectGroup)Entities[uuid]).RootPart.Flags & PrimFlags.Physics) == PrimFlags.Physics)
+                        {
+                            RemovePhysicalPrim(sog.PrimCount);
+                        }
                     }
                 }
 
@@ -1603,7 +1611,7 @@ namespace OpenSim.Region.Framework.Scenes
                 {
                     if (part != null)
                     {
-                        if (part.ParentGroup.Children.Count != 1) // Skip single
+                        if (part.ParentGroup.PrimCount != 1) // Skip single
                         {
                             if (part.LinkNum < 2) // Root
                                 rootParts.Add(part);
@@ -1631,8 +1639,15 @@ namespace OpenSim.Region.Framework.Scenes
                     // However, editing linked parts and unlinking may be different
                     //
                     SceneObjectGroup group = root.ParentGroup;
-                    List<SceneObjectPart> newSet = new List<SceneObjectPart>(group.Children.Values);
-                    int numChildren = group.Children.Count;
+                    
+                    List<SceneObjectPart> newSet = null;
+                    int numChildren = -1;
+                    
+                    lock (group.Children)
+                    {
+                        newSet = new List<SceneObjectPart>(group.Children.Values);
+                        numChildren = group.PrimCount;
+                    }
 
                     // If there are prims left in a link set, but the root is
                     // slated for unlink, we need to do this
@@ -1711,12 +1726,17 @@ namespace OpenSim.Region.Framework.Scenes
             {
                 if (ent is SceneObjectGroup)
                 {
-                    foreach (KeyValuePair<UUID, SceneObjectPart> subent in ((SceneObjectGroup)ent).Children)
+                    SceneObjectGroup sog = ent as SceneObjectGroup;
+                    
+                    lock (sog.Children)
                     {
-                        if (subent.Value.LocalId == localID)
+                        foreach (KeyValuePair<UUID, SceneObjectPart> subent in sog.Children)
                         {
-                            objid = subent.Key;
-                            obj = subent.Value;
+                            if (subent.Value.LocalId == localID)
+                            {
+                                objid = subent.Key;
+                                obj = subent.Value;
+                            }
                         }
                     }
                 }
@@ -1781,7 +1801,8 @@ namespace OpenSim.Region.Framework.Scenes
             SceneObjectGroup original = GetGroupByPrim(originalPrimID);
             if (original != null)
             {
-                if (m_parentScene.Permissions.CanDuplicateObject(original.Children.Count, original.UUID, AgentID, original.AbsolutePosition))
+                if (m_parentScene.Permissions.CanDuplicateObject(
+                    original.PrimCount, original.UUID, AgentID, original.AbsolutePosition))
                 {
                     SceneObjectGroup copy = original.Copy(true);
                     copy.AbsolutePosition = copy.AbsolutePosition + offset;
diff --git a/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs b/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
index 952d280..5ee8d73 100644
--- a/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
+++ b/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
@@ -213,7 +213,7 @@ namespace OpenSim.Region.Framework.Scenes
         /// </summary>
         public int PrimCount
         {
-            get { return m_parts.Count; }
+            get { lock (m_parts) { return m_parts.Count; } }
         }
 
         protected Quaternion m_rotation = Quaternion.Identity;
@@ -237,6 +237,7 @@ namespace OpenSim.Region.Framework.Scenes
 
         /// <value>
         /// The parts of this scene object group.  You must lock this property before using it.
+        /// If you want to know the number of children, consider using the PrimCount property instead
         /// </value>
         public Dictionary<UUID, SceneObjectPart> Children
         {
@@ -298,6 +299,7 @@ namespace OpenSim.Region.Framework.Scenes
                 {
                     m_scene.CrossPrimGroupIntoNewRegion(val, this, true);
                 }
+                
                 if (RootPart.GetStatusSandbox())
                 {
                     if (Util.GetDistanceTo(RootPart.StatusSandboxPos, value) > 10)
@@ -308,6 +310,7 @@ namespace OpenSim.Region.Framework.Scenes
                         return;
                     }
                 }
+                
                 lock (m_parts)
                 {
                     foreach (SceneObjectPart part in m_parts.Values)
@@ -558,21 +561,23 @@ namespace OpenSim.Region.Framework.Scenes
             if (m_rootPart.LocalId == 0)
                 m_rootPart.LocalId = m_scene.AllocateLocalId();
 
-            // No need to lock here since the object isn't yet in a scene
-            foreach (SceneObjectPart part in m_parts.Values)
+            lock (m_parts)
             {
-                if (Object.ReferenceEquals(part, m_rootPart))
-                {
-                    continue;
-                }
-
-                if (part.LocalId == 0)
+                foreach (SceneObjectPart part in m_parts.Values)
                 {
-                    part.LocalId = m_scene.AllocateLocalId();
+                    if (Object.ReferenceEquals(part, m_rootPart))
+                    {
+                        continue;
+                    }
+    
+                    if (part.LocalId == 0)
+                    {
+                        part.LocalId = m_scene.AllocateLocalId();
+                    }
+    
+                    part.ParentID = m_rootPart.LocalId;
+                    //m_log.DebugFormat("[SCENE]: Given local id {0} to part {1}, linknum {2}, parent {3} {4}", part.LocalId, part.UUID, part.LinkNum, part.ParentID, part.ParentUUID);
                 }
-
-                part.ParentID = m_rootPart.LocalId;
-                //m_log.DebugFormat("[SCENE]: Given local id {0} to part {1}, linknum {2}, parent {3} {4}", part.LocalId, part.UUID, part.LinkNum, part.ParentID, part.ParentUUID);
             }
             
             ApplyPhysics(m_scene.m_physicalPrim);
@@ -670,7 +675,7 @@ namespace OpenSim.Region.Framework.Scenes
             minY = 256f;
             minZ = 8192f;
 
-            lock(m_parts);
+            lock(m_parts)
             {
                 foreach (SceneObjectPart part in m_parts.Values)
                 {
@@ -995,9 +1000,12 @@ namespace OpenSim.Region.Framework.Scenes
                 m_rootPart.AttachedAvatar = agentID;
 
                 //Anakin Lohner bug #3839 
-                foreach (SceneObjectPart p in m_parts.Values)
+                lock (m_parts)
                 {
-                    p.AttachedAvatar = agentID;
+                    foreach (SceneObjectPart p in m_parts.Values)
+                    {
+                        p.AttachedAvatar = agentID;
+                    }
                 }
 
                 if (m_rootPart.PhysActor != null)
@@ -1065,10 +1073,14 @@ namespace OpenSim.Region.Framework.Scenes
 
             AbsolutePosition = detachedpos;
             m_rootPart.AttachedAvatar = UUID.Zero;
-            //Anakin Lohner bug #3839 
-            foreach (SceneObjectPart p in m_parts.Values)
+            
+            //Anakin Lohner bug #3839            
+            lock (m_parts)
             {
-                p.AttachedAvatar = UUID.Zero;
+                foreach (SceneObjectPart p in m_parts.Values)
+                {
+                    p.AttachedAvatar = UUID.Zero;
+                }
             }
 
             m_rootPart.SetParentLocalId(0);
@@ -1094,10 +1106,14 @@ namespace OpenSim.Region.Framework.Scenes
             }
 
             m_rootPart.AttachedAvatar = UUID.Zero;
+            
             //Anakin Lohner bug #3839 
-            foreach (SceneObjectPart p in m_parts.Values)
+            lock (m_parts)
             {
-                p.AttachedAvatar = UUID.Zero;
+                foreach (SceneObjectPart p in m_parts.Values)
+                {
+                    p.AttachedAvatar = UUID.Zero;
+                }
             }
 
             m_rootPart.SetParentLocalId(0);
@@ -1160,9 +1176,8 @@ namespace OpenSim.Region.Framework.Scenes
                 part.ParentID = 0;
             part.LinkNum = 0;
             
-            // No locking required since the SOG should not be in the scene yet - one can't change root parts after
-            // the scene object has been attached to the scene
-            m_parts.Add(m_rootPart.UUID, m_rootPart);
+            lock (m_parts)
+                m_parts.Add(m_rootPart.UUID, m_rootPart);
         }
 
         /// <summary>
@@ -1625,7 +1640,7 @@ namespace OpenSim.Region.Framework.Scenes
         }
 
         /// <summary>
-        ///
+        /// Copy the given part as the root part of this scene object.
         /// </summary>
         /// <param name="part"></param>
         /// <param name="cAgentID"></param>
@@ -1882,11 +1897,12 @@ namespace OpenSim.Region.Framework.Scenes
         /// <param name="cGroupID"></param>
         public SceneObjectPart CopyPart(SceneObjectPart part, UUID cAgentID, UUID cGroupID, bool userExposed)
         {
-            SceneObjectPart newPart = part.Copy(m_scene.AllocateLocalId(), OwnerID, GroupID, m_parts.Count, userExposed);
-            newPart.SetParent(this);
-
+            SceneObjectPart newPart = null;
+            
             lock (m_parts)
-            {
+            {            
+                newPart = part.Copy(m_scene.AllocateLocalId(), OwnerID, GroupID, m_parts.Count, userExposed);
+                newPart.SetParent(this);
                 m_parts.Add(newPart.UUID, newPart);
             }
 
@@ -1903,14 +1919,15 @@ namespace OpenSim.Region.Framework.Scenes
         /// </summary>
         public void ResetIDs()
         {
-            // As this is only ever called for prims which are not currently part of the scene (and hence
-            // not accessible by clients), there should be no need to lock
-            List<SceneObjectPart> partsList = new List<SceneObjectPart>(m_parts.Values);
-            m_parts.Clear();
-            foreach (SceneObjectPart part in partsList)
+            lock (m_parts)
             {
-                part.ResetIDs(part.LinkNum); // Don't change link nums
-                m_parts.Add(part.UUID, part);
+                List<SceneObjectPart> partsList = new List<SceneObjectPart>(m_parts.Values);
+                m_parts.Clear();
+                foreach (SceneObjectPart part in partsList)
+                {
+                    part.ResetIDs(part.LinkNum); // Don't change link nums
+                    m_parts.Add(part.UUID, part);
+                }
             }
         }
 
@@ -2136,10 +2153,15 @@ namespace OpenSim.Region.Framework.Scenes
         public SceneObjectPart GetChildPart(UUID primID)
         {
             SceneObjectPart childPart = null;
-            if (m_parts.ContainsKey(primID))
+            
+            lock (m_parts)
             {
-                childPart = m_parts[primID];
+                if (m_parts.ContainsKey(primID))
+                {
+                    childPart = m_parts[primID];
+                }
             }
+            
             return childPart;
         }
 
@@ -2174,9 +2196,10 @@ namespace OpenSim.Region.Framework.Scenes
         /// <returns></returns>
         public bool HasChildPrim(UUID primID)
         {
-            if (m_parts.ContainsKey(primID))
+            lock (m_parts)
             {
-                return true;
+                if (m_parts.ContainsKey(primID))
+                    return true;
             }
 
             return false;
@@ -2370,17 +2393,19 @@ namespace OpenSim.Region.Framework.Scenes
             lock (m_parts)
             {
                 m_parts.Remove(linkPart.UUID);
-            }
-
-            if (m_parts.Count == 1 && RootPart != null) //Single prim is left
-                RootPart.LinkNum = 0;
-            else
-            {
-                foreach (SceneObjectPart p in m_parts.Values)
+                
+                if (m_parts.Count == 1 && RootPart != null) //Single prim is left
                 {
-                    if (p.LinkNum > linkPart.LinkNum)
-                        p.LinkNum--;
+                    RootPart.LinkNum = 0;
                 }
+                else
+                {
+                    foreach (SceneObjectPart p in m_parts.Values)
+                    {
+                        if (p.LinkNum > linkPart.LinkNum)
+                            p.LinkNum--;
+                    }
+                }                
             }
 
             linkPart.ParentID = 0;
@@ -2762,9 +2787,11 @@ namespace OpenSim.Region.Framework.Scenes
         public void UpdatePermissions(UUID AgentID, byte field, uint localID,
                 uint mask, byte addRemTF)
         {
-            foreach (SceneObjectPart part in m_parts.Values)
-                part.UpdatePermissions(AgentID, field, localID, mask,
-                        addRemTF);
+            lock (m_parts)
+            {
+                foreach (SceneObjectPart part in m_parts.Values)
+                    part.UpdatePermissions(AgentID, field, localID, mask, addRemTF);
+            }
 
             HasGroupChanged = true;
         }
diff --git a/OpenSim/Region/Framework/Scenes/SceneObjectPartInventory.cs b/OpenSim/Region/Framework/Scenes/SceneObjectPartInventory.cs
index 84b7365..e08fa77 100644
--- a/OpenSim/Region/Framework/Scenes/SceneObjectPartInventory.cs
+++ b/OpenSim/Region/Framework/Scenes/SceneObjectPartInventory.cs
@@ -601,7 +601,10 @@ namespace OpenSim.Region.Framework.Scenes
             rootPart.Name = item.Name;
             rootPart.Description = item.Description;
 
-            List<SceneObjectPart> partList = new List<SceneObjectPart>(group.Children.Values);
+            List<SceneObjectPart> partList = null;
+            
+            lock (group.Children)
+                partList = new List<SceneObjectPart>(group.Children.Values);
 
             group.SetGroup(m_part.GroupID, null);