Do not allow a script to attach a prim if its being sat upon.

This prevents a stack overflow where a get position on the avatar will refer to the attachment which will in turn refer back to the avatar.
This required recording of all sitting avatars on a prim which is done separately from recording the sit target avatar.
Recording HashSet is null if there are no sitting avatars in order to save memory.

OpenSim/Region/Framework/Scenes/SceneObjectPart.cs

@@ -374,7 +374,6 @@ namespace OpenSim.Region.Framework.Scenes
         private uint _category;
         private Int32 _creationDate;
         private uint _parentID = 0;
-        private UUID m_sitTargetAvatar = UUID.Zero;
         private uint _baseMask = (uint)PermissionMask.All;
         private uint _ownerMask = (uint)PermissionMask.All;
         private uint _groupMask = (uint)PermissionMask.None;
@@ -1233,13 +1232,20 @@ namespace OpenSim.Region.Framework.Scenes
         }
 
         /// <summary>
-        /// ID of the avatar that is sat on us.  If there is no such avatar then is UUID.Zero
+        /// ID of the avatar that is sat on us if we have a sit target.  If there is no such avatar then is UUID.Zero
         /// </summary>
-        public UUID SitTargetAvatar
-        {
-            get { return m_sitTargetAvatar; }
-            set { m_sitTargetAvatar = value; }
-        }
+        public UUID SitTargetAvatar { get; set; }
+
+        /// <summary>
+        /// IDs of all avatars start on this object part.
+        /// </summary>
+        /// <remarks>
+        /// We need to track this so that we can stop sat upon prims from being attached.
+        /// </remarks>
+        /// <value>
+        /// null if there are no sitting avatars.  This is to save us create a hashset for every prim in a scene.
+        /// </value>
+        private HashSet<UUID> m_sittingAvatars;
 
         public virtual UUID RegionID
         {
@@ -4493,5 +4499,83 @@ namespace OpenSim.Region.Framework.Scenes
             Color color = Color;
             return new Color4(color.R, color.G, color.B, (byte)(0xFF - color.A));
         }
+
+        /// <summary>
+        /// Record an avatar sitting on this part.
+        /// </summary>
+        /// <remarks>This is called for all the sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns>
+        /// true if the avatar was not already recorded, false otherwise.
+        /// </returns>
+        /// <param name='avatarId'></param>
+        protected internal bool AddSittingAvatar(UUID avatarId)
+        {
+            HashSet<UUID> sittingAvatars = m_sittingAvatars;
+
+            if (sittingAvatars == null)
+                sittingAvatars = new HashSet<UUID>();
+
+            lock (sittingAvatars)
+            {
+                m_sittingAvatars = sittingAvatars;
+                return m_sittingAvatars.Add(avatarId);
+            }
+        }
+
+        /// <summary>
+        /// Remove an avatar recorded as sitting on this part.
+        /// </summary>
+        /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns>
+        /// true if the avatar was present and removed, false if it was not present.
+        /// </returns>
+        /// <param name='avatarId'></param>
+        protected internal bool RemoveSittingAvatar(UUID avatarId)
+        {
+            HashSet<UUID> sittingAvatars = m_sittingAvatars;
+
+            // This can occur under a race condition where another thread
+            if (sittingAvatars == null)
+                return false;
+
+            lock (sittingAvatars)
+            {
+                if (sittingAvatars.Remove(avatarId))
+                {
+                    if (sittingAvatars.Count == 0)
+                        m_sittingAvatars = null;
+
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        /// <summary>
+        /// Get a copy of the list of sitting avatars.
+        /// </summary>
+        /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns></returns>
+        public HashSet<UUID> GetSittingAvatars()
+        {
+            return new HashSet<UUID>(m_sittingAvatars);
+        }
+
+        /// <summary>
+        /// Gets the number of sitting avatars.
+        /// </summary>
+        /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns></returns>
+        public int GetSittingAvatarsCount()
+        {
+            HashSet<UUID> sittingAvatars = m_sittingAvatars;
+
+            if (sittingAvatars == null)
+                return 0;
+
+            lock (sittingAvatars)
+                return sittingAvatars.Count;
+        }
     }
 }