[Http]: Fix a potential race condition in the WebConnectionGroup's co…

…nnection list.

Remove the "ConnectionState" from the connection list before closing the connection,
this should avoid ServicePoint.SendRequest() ever crashing with a NullReferenceException
due to getting a null return value from WebConnectionGroup.GetConnection().

mcs/class/System/System.Net/WebConnectionGroup.cs

@@ -70,14 +70,16 @@ public void Close ()
 			//TODO: abort requests or wait for them to finish
 			lock (sPoint) {
 				closing = true;
-				foreach (var cnc in connections) {
-					if (cnc.Connection == null)
-						continue;
-					cnc.Connection.Close (false);
-					cnc.Connection = null;
+				var iter = connections.First;
+				while (iter != null) {
+					var cnc = iter.Value.Connection;
+					var node = iter;
+					iter = iter.Next;
+
+					connections.Remove (node);
+					cnc.Close (false);
 					OnConnectionClosed ();
 				}
-				connections.Clear ();
 			}
 		}
 
@@ -120,7 +122,7 @@ static void PrepareSharingNtlm (WebConnection cnc, HttpWebRequest request)
 		ConnectionState FindIdleConnection ()
 		{
 			foreach (var cnc in connections) {
-				if (cnc.Busy  || cnc.Connection == null)
+				if (cnc.Busy)
 					continue;
 
 				connections.Remove (cnc);
@@ -140,7 +142,7 @@ WebConnection CreateOrReuseConnection (HttpWebRequest request, out bool created)
 				return cnc.Connection;
 			}
 
-			if (sPoint.ConnectionLimit > connections.Count) {
+			if (sPoint.ConnectionLimit > connections.Count || connections.Count == 0) {
 				created = true;
 				cnc = new ConnectionState (this);
 				connections.AddFirst (cnc);
@@ -177,14 +179,11 @@ internal bool TryRecycle (TimeSpan maxIdleTime, ref DateTime idleSince)
 				}
 
 				int count = 0;
-				for (var node = connections.First; node != null; node = node.Next) {
-					var cnc = node.Value;
-
-					if (cnc.Connection == null) {
-						connections.Remove (node);
-						OnConnectionClosed ();
-						continue;
-					}
+				var iter = connections.First;
+				while (iter != null) {
+					var cnc = iter.Value;
+					var node = iter;
+					iter = iter.Next;
 
 					++count;
 					if (cnc.Busy)
@@ -205,7 +204,7 @@ internal bool TryRecycle (TimeSpan maxIdleTime, ref DateTime idleSince)
 					if (connectionsToClose == null)
 						connectionsToClose = new List<WebConnection> ();
 					connectionsToClose.Add (cnc.Connection);
-					cnc.Connection = null;
+					connections.Remove (node);
 				}
 
 				recycled = connections.Count == 0;
@@ -224,7 +223,10 @@ internal bool TryRecycle (TimeSpan maxIdleTime, ref DateTime idleSince)
 		}
 
 		class ConnectionState : IWebConnectionState {
-			public WebConnection Connection;
+			public WebConnection Connection {
+				get;
+				private set;
+			}
 
 			public WebConnectionGroup Group {
 				get;