Fix concurrency issue accessing Dictionary instance

--HG--
branch : dev

src/Orchard/Caching/Cache.cs

@@ -1,31 +1,35 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 
 namespace Orchard.Caching {
     public class Cache<TKey, TResult> : ICache<TKey, TResult> {
-        private readonly Dictionary<TKey, CacheEntry> _entries;
+        private readonly ConcurrentDictionary<TKey, CacheEntry> _entries;
 
         public Cache() {
-            _entries = new Dictionary<TKey, CacheEntry>();
+            _entries = new ConcurrentDictionary<TKey, CacheEntry>();
         }
 
         public TResult Get(TKey key, Func<AcquireContext<TKey>, TResult> acquire) {
-            CacheEntry entry;
-            if (!_entries.TryGetValue(key, out entry) || entry.Tokens.Any(t => !t.IsCurrent)) {
-                entry = new CacheEntry { Tokens = new List<IVolatileToken>() };
-
-                var context = new AcquireContext<TKey>(key, volatileItem => entry.Tokens.Add(volatileItem));
-                entry.Result = acquire(context);
-                _entries[key] = entry;
-            }
+            var entry = _entries.AddOrUpdate(key,
+                // "Add" lambda
+                k => CreateEntry(k, acquire),
+                // "Update" lamdba
+                (k, currentEntry) => (currentEntry.Tokens.All(t => t.IsCurrent) ? currentEntry : CreateEntry(k, acquire)));
             return entry.Result;
         }
 
+        private static CacheEntry CreateEntry(TKey k, Func<AcquireContext<TKey>, TResult> acquire) {
+            var entry = new CacheEntry { Tokens = new List<IVolatileToken>() };
+            var context = new AcquireContext<TKey>(k, volatileItem => entry.Tokens.Add(volatileItem));
+            entry.Result = acquire(context);
+            return entry;
+        }
+
         private class CacheEntry {
             public TResult Result { get; set; }
             public IList<IVolatileToken> Tokens { get; set; }
         }
     }
-
 }