3 files changed, 41 insertions, 17 deletions

diff --git a/OpenSim/Region/Framework/Scenes/Scene.cs b/OpenSim/Region/Framework/Scenes/Scene.cs
index d16b73b..08a2301 100644
--- a/OpenSim/Region/Framework/Scenes/Scene.cs
+++ b/OpenSim/Region/Framework/Scenes/Scene.cs
@@ -1757,6 +1757,7 @@ namespace OpenSim.Region.Framework.Scenes
         {
             if (group != null)
             {
+                group.HasGroupChanged = true;
                 group.ProcessBackup(SimulationDataService, true);
             }
         }
@@ -2345,13 +2346,12 @@ namespace OpenSim.Region.Framework.Scenes
             {
                 if (!softDelete)
                 {
-                    // Force a database update so that the scene object group ID is accurate.  It's possible that the
-                    // group has recently been delinked from another group but that this change has not been persisted
-                    // to the DB.
+                    // If the group contains prims whose SceneGroupID is incorrect then force a
+                    // database update, because RemoveObject() works by searching on the SceneGroupID.
                     // This is an expensive thing to do so only do it if absolutely necessary.
-                    if (so.HasGroupChangedDueToDelink)
-                        ForceSceneObjectBackup(so);                
-                    
+                    if (so.GroupContainsForeignPrims)
+                        ForceSceneObjectBackup(so);
+
                     so.DetachFromBackup();
                     SimulationDataService.RemoveObject(so.UUID, RegionInfo.RegionID);
                 }
diff --git a/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs b/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
index 6f26176..a78a0cb 100644
--- a/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
+++ b/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
@@ -150,12 +150,27 @@ namespace OpenSim.Region.Framework.Scenes
 
             get { return m_hasGroupChanged; }
         }
+
+        private bool m_groupContainsForeignPrims = false;
         
         /// <summary>
-        /// Has the group changed due to an unlink operation?  We record this in order to optimize deletion, since
-        /// an unlinked group currently has to be persisted to the database before we can perform an unlink operation.
+        /// Whether the group contains prims that came from a different group. This happens when
+        /// linking or delinking groups. The implication is that until the group is persisted,
+        /// the prims in the database still use the old SceneGroupID. That's a problem if the group
+        /// is deleted, because we delete groups by searching for prims by their SceneGroupID.
         /// </summary>
-        public bool HasGroupChangedDueToDelink { get; private set; }
+        public bool GroupContainsForeignPrims
+        {
+            private set
+            {
+                m_groupContainsForeignPrims = value;
+                if (m_groupContainsForeignPrims)
+                    HasGroupChanged = true;
+            }
+
+            get { return m_groupContainsForeignPrims; }
+        }
+
 
         private bool isTimeToPersist()
         {
@@ -1624,7 +1639,7 @@ namespace OpenSim.Region.Framework.Scenes
                         backup_group.RootPart.AngularVelocity = RootPart.AngularVelocity;
                         backup_group.RootPart.ParticleSystem = RootPart.ParticleSystem;
                         HasGroupChanged = false;
-                        HasGroupChangedDueToDelink = false;
+                        GroupContainsForeignPrims = false;
 
                         m_scene.EventManager.TriggerOnSceneObjectPreSave(backup_group, this);
                         datastore.StoreObject(backup_group, m_scene.RegionInfo.RegionID);
@@ -2388,6 +2403,8 @@ namespace OpenSim.Region.Framework.Scenes
             // If linking prims with different permissions, fix them
             AdjustChildPrimPermissions();
 
+            GroupContainsForeignPrims = true;
+
             AttachToBackup();
 
             // Here's the deal, this is ABSOLUTELY CRITICAL so the physics scene gets the update about the 
@@ -2531,9 +2548,16 @@ namespace OpenSim.Region.Framework.Scenes
 
             linkPart.Rezzed = RootPart.Rezzed;
 
-            // When we delete a group, we currently have to force persist to the database if the object id has changed
-            // (since delete works by deleting all rows which have a given object id)
-            objectGroup.HasGroupChangedDueToDelink = true;
+            // We must persist the delinked group to the database immediately, for safety. The problem
+            // is that although in memory the new group has a new SceneGroupID, in the database it
+            // still has the parent group's SceneGroupID (until the next backup). This means that if the
+            // parent group is deleted then the delinked group will also be deleted from the database.
+            // This problem will disappear if the region remains alive long enough for another backup,
+            // since at that time the delinked group's new SceneGroupID will be written to the database.
+            // But if the region crashes before that then the prims will be permanently gone, and this must
+            // not happen. (We can't use a just-in-time trick like GroupContainsForeignPrims in this case
+            // because the delinked group doesn't know when the source group is deleted.)
+            m_scene.ForceSceneObjectBackup(objectGroup);
 
             return objectGroup;
         }
diff --git a/OpenSim/Region/Framework/Scenes/Tests/SceneObjectLinkingTests.cs b/OpenSim/Region/Framework/Scenes/Tests/SceneObjectLinkingTests.cs
index 9378e20..fa8277c 100644
--- a/OpenSim/Region/Framework/Scenes/Tests/SceneObjectLinkingTests.cs
+++ b/OpenSim/Region/Framework/Scenes/Tests/SceneObjectLinkingTests.cs
@@ -143,7 +143,7 @@ namespace OpenSim.Region.Framework.Scenes.Tests
 
             Assert.That(grp1.Parts.Length, Is.EqualTo(1), "Group 1 still contained part2 after delink.");
             Assert.That(part2.AbsolutePosition == Vector3.Zero, "The absolute position should be zero");
-            Assert.That(grp3.HasGroupChangedDueToDelink, Is.True);
+            Assert.That(grp3.GroupContainsForeignPrims, Is.True);
         }
 
         [Test]
@@ -349,10 +349,10 @@ namespace OpenSim.Region.Framework.Scenes.Tests
                         
             // These changes should occur immediately without waiting for a backup pass
             SceneObjectGroup groupToDelete = sog.DelinkFromGroup(linkPart, false);
-            
-            Assert.That(groupToDelete.HasGroupChangedDueToDelink, Is.True);
+
+            Assert.That(groupToDelete.GroupContainsForeignPrims, Is.True);
             scene.DeleteSceneObject(groupToDelete, false);
-            Assert.That(groupToDelete.HasGroupChangedDueToDelink, Is.False);
+            Assert.That(groupToDelete.GroupContainsForeignPrims, Is.False);
             
             List<SceneObjectGroup> storedObjects = scene.SimulationDataService.LoadObjects(scene.RegionInfo.RegionID);