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diff --git a/OpenSim/Region/Framework/Scenes/ScenePresenceStateMachine.cs b/OpenSim/Region/Framework/Scenes/ScenePresenceStateMachine.cs
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+/*
+ * Copyright (c) Contributors, http://opensimulator.org/
+ * See CONTRIBUTORS.TXT for a full list of copyright holders.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the OpenSimulator Project nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE CONTRIBUTORS BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+using System;
+
+namespace OpenSim.Region.Framework.Scenes
+{
+    /// <summary>
+    /// The possible states that a scene presence can be in.  This is currently orthagonal to whether a scene presence
+    /// is root or child.
+    /// </summary>
+    /// <remarks>
+    /// This is a state machine.
+    ///
+    /// [Entry]               => Running
+    /// Running               => PreRemove, Removing
+    /// PreRemove             => Running, Removing
+    /// Removing              => Removed
+    /// 
+    /// All other methods should only see the scene presence in running state - this is the normal operational state
+    /// Removed state occurs when the presence has been removed.  This is the end state with no exit.
+    /// </remarks>
+    public enum ScenePresenceState
+    {
+        Running,                // Normal operation state.  The scene presence is available.
+        PreRemove,              // The presence is due to be removed but can still be returning to running.
+        Removing,               // The presence is in the process of being removed from the scene via Scene.RemoveClient.
+        Removed,                // The presence has been removed from the scene and is effectively dead.
+                                // There is no exit from this state.
+    }
+
+    internal class ScenePresenceStateMachine
+    {
+        private ScenePresence m_sp;
+        private ScenePresenceState m_state;
+
+        internal ScenePresenceStateMachine(ScenePresence sp)
+        {
+            m_sp = sp;
+            m_state = ScenePresenceState.Running;
+        }
+
+        internal ScenePresenceState GetState()
+        {
+            return m_state;
+        }
+
+        /// <summary>
+        /// Updates the state of an agent that is already in transit.
+        /// </summary>
+        /// <param name='id'></param>
+        /// <param name='newState'></param>
+        /// <returns></returns>
+        /// <exception cref='Exception'>Illegal transitions will throw an Exception</exception>
+        internal void SetState(ScenePresenceState newState)
+        {
+            bool transitionOkay = false;
+
+            lock (this)
+            {
+                if (newState == m_state)
+                    return;
+                else if (newState == ScenePresenceState.Running && m_state == ScenePresenceState.PreRemove)
+                    transitionOkay = true;
+                else if (newState == ScenePresenceState.PreRemove && m_state == ScenePresenceState.Running)
+                    transitionOkay = true;
+                else if (newState == ScenePresenceState.Removing)
+                {   
+                    if (m_state == ScenePresenceState.Running || m_state == ScenePresenceState.PreRemove)
+                        transitionOkay = true;
+                }
+                else if (newState == ScenePresenceState.Removed && m_state == ScenePresenceState.Removing)
+                    transitionOkay = true;
+            }           
+
+            if (!transitionOkay)
+            {
+                throw new Exception(
+                    string.Format(
+                        "Scene presence {0} is not allowed to move from state {1} to new state {2} in {3}",
+                        m_sp.Name, m_state, newState, m_sp.Scene.Name));
+            }
+            else
+            {
+                m_state = newState;
+            }
+        }
+    }
+}
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