Tighten up OpenSim.Framework.Cache locking to avoid race conditions.

This is to resolve a reported issue in http://opensimulator.org/mantis/view.php?id=6232
Here, the land management module is using OpenSim.Framework.Cache (the only code to currently do so apart from the non-default CoreAssetCache).

OpenSim/Framework/Cache.cs

@@ -199,7 +199,14 @@ namespace OpenSim.Framework
     //
     public class Cache
     {
+        /// <summary>
+        /// Must only be accessed under lock.
+        /// </summary>
         private List<CacheItemBase> m_Index = new List<CacheItemBase>();
+
+        /// <summary>
+        /// Must only be accessed under m_Index lock.
+        /// </summary>
         private Dictionary<string, CacheItemBase> m_Lookup =
             new Dictionary<string, CacheItemBase>();
 
@@ -320,7 +327,6 @@ namespace OpenSim.Framework
             {
                 if (m_Lookup.ContainsKey(index))
                     item = m_Lookup[index];
-            }
 
                 if (item == null)
                 {
@@ -332,6 +338,7 @@ namespace OpenSim.Framework
                 item.lastUsed = DateTime.Now;
     
                 Expire(true);
+            }
 
             return item;
         }
@@ -385,7 +392,10 @@ namespace OpenSim.Framework
         //
         public Object Find(Predicate<CacheItemBase> d)
         {
-            CacheItemBase item = m_Index.Find(d);
+            CacheItemBase item;
+
+            lock (m_Index)
+                item = m_Index.Find(d);
 
             if (item == null)
                 return null;
@@ -419,12 +429,12 @@ namespace OpenSim.Framework
         public virtual void Store(string index, Object data, Type container,
                 Object[] parameters)
         {
-            Expire(false);
-
             CacheItemBase item;
 
             lock (m_Index)
             {
+                Expire(false);
+
                 if (m_Index.Contains(new CacheItemBase(index)))
                 {
                     if ((m_Flags & CacheFlags.AllowUpdate) != 0)
@@ -450,9 +460,17 @@ namespace OpenSim.Framework
                 m_Index.Add(item);
                 m_Lookup[index] = item;
             }
+
             item.Store(data);
         }
 
+        /// <summary>
+        /// Expire items as appropriate.
+        /// </summary>
+        /// <remarks>
+        /// Callers must lock m_Index.
+        /// </remarks>
+        /// <param name='getting'></param>
         protected virtual void Expire(bool getting)
         {
             if (getting && (m_Strategy == CacheStrategy.Aggressive))
@@ -479,8 +497,6 @@ namespace OpenSim.Framework
                     if (Count < Size)
                         return;
 
-                lock (m_Index)
-                {
                     m_Index.Sort(new SortLRU());
                     m_Index.Reverse();
 
@@ -513,14 +529,17 @@ namespace OpenSim.Framework
                         foreach (CacheItemBase item in m_Index)
                             m_Lookup[item.uuid] = item;
                     }
-                }
+
                     break;
+
                     default:
                         break;
             }
         }
 
         public void Invalidate(string uuid)
+        {
+            lock (m_Index)
             {
                 if (!m_Lookup.ContainsKey(uuid))
                     return;
@@ -529,11 +548,15 @@ namespace OpenSim.Framework
                 m_Lookup.Remove(uuid);
                 m_Index.Remove(item);
             }
+        }
 
         public void Clear()
+        {
+            lock (m_Index)
             {
                 m_Index.Clear();
                 m_Lookup.Clear();
             }
         }
+    }
 }