Reinstate upgradeable read locks

Based on a review comment, this reinstates the upgradeable read locks
that were originally taken before entering write locks.

The intent of this PR is to deprecate `Lock`, not to make functional
changes. While forgoing the upgradeable read locks in favor of enter-
ing write locks right away might *possibly* be an optimization, this
is a different concern and would need to be properly established using
careful benchmarking. Let's do this another time.

src/Castle.Core/Components.DictionaryAdapter/Xml/Internal/Utilities/SingletonDispenser.cs

@@ -65,7 +65,7 @@ private bool TryGetExistingItem(TKey key, out object item)
 				locker.ExitReadLock();
 			}
 
-			locker.EnterWriteLock();
+			locker.EnterUpgradeableReadLock();
 			try
 			{
 				if (items.TryGetValue(key, out item))
@@ -74,13 +74,21 @@ private bool TryGetExistingItem(TKey key, out object item)
 				}
 				else
 				{
-					items[key] = item = new ManualResetEvent(false);
-					return false;
+					locker.EnterWriteLock();
+					try
+					{
+						items[key] = item = new ManualResetEvent(false);
+						return false;
+					}
+					finally
+					{
+						locker.ExitWriteLock();
+					}
 				}
 			}
 			finally
 			{
-				locker.ExitWriteLock();
+				locker.ExitUpgradeableReadLock();
 			}
 		}
 

src/Castle.Core/Core/Internal/SynchronizedDictionary.cs

@@ -59,14 +59,31 @@ public TValue GetOrAdd(TKey key, Func<TKey, TValue> valueFactory)
 				itemsLock.ExitReadLock();
 			}
 
-			itemsLock.EnterWriteLock();
+			itemsLock.EnterUpgradeableReadLock();
 			try
 			{
-				return GetOrAddWithoutTakingLock(key, valueFactory);
+				if (items.TryGetValue(key, out value))
+				{
+					return value;
+				}
+				else
+				{
+					itemsLock.EnterWriteLock();
+					try
+					{
+						value = valueFactory.Invoke(key);
+						items.Add(key, value);
+						return value;
+					}
+					finally
+					{
+						itemsLock.ExitWriteLock();
+					}
+				}
 			}
 			finally
 			{
-				itemsLock.ExitWriteLock();
+				itemsLock.ExitUpgradeableReadLock();
 			}
 		}