[ServiceModel] Make TcpReplyChannel.OnClose thread-safe

We were seeing the following unhandled exception on Jenkins:

````
Unhandled exceptions:
1) MonoTests.System.ServiceModel.OperationContextTest.Current : System.NullReferenceException: Object reference not set to an instance of an object
  at System.ServiceModel.Channels.NetTcp.TcpReplyChannel.OnClose (TimeSpan timeout) [0x00000] in /media/ssd/jenkins/workspace/test-mono-mainline/label/debian-armhf/mcs/class/System.ServiceModel/System.ServiceModel.Channels.NetTcp/TcpReplyChannel.cs:141
  at System.ServiceModel.Channels.CommunicationObject.Close (TimeSpan timeout) [0x0001c] in /media/ssd/jenkins/workspace/test-mono-mainline/label/debian-armhf/mcs/class/System.ServiceModel/System.ServiceModel.Channels/CommunicationObject.cs:140
  at System.ServiceModel.Channels.CommunicationObject.Close () [0x00000] in /media/ssd/jenkins/workspace/test-mono-mainline/label/debian-armhf/mcs/class/System.ServiceModel/System.ServiceModel.Channels/CommunicationObject.cs:131
  at System.ServiceModel.Dispatcher.ListenerLoopManager.TryReceiveRequestDone (IAsyncResult result) [0x00027] in /media/ssd/jenkins/workspace/test-mono-mainline/label/debian-armhf/mcs/class/System.ServiceModel/System.ServiceModel.Dispatcher/ChannelDispatcher.cs:583
```

This happens because multiple threads can call OnClose() and so `client` can be null.

Fixed this by taking code from HttpReplyChannel.OnClose which handled this already and also fixed the race condition there (the checks need to happen under a lock).

2 files changed, 15 insertions, 3 deletions

diff --git a/mcs/class/System.ServiceModel/System.ServiceModel.Channels.Http/HttpReplyChannel.cs b/mcs/class/System.ServiceModel/System.ServiceModel.Channels.Http/HttpReplyChannel.cs
index 03e7cad1f1c..098b622aa83 100644
--- a/mcs/class/System.ServiceModel/System.ServiceModel.Channels.Http/HttpReplyChannel.cs
+++ b/mcs/class/System.ServiceModel/System.ServiceModel.Channels.Http/HttpReplyChannel.cs
@@ -104,12 +104,15 @@ namespace System.ServiceModel.Channels.Http
 		}
 
 		bool close_started;
+		object close_lock = new object ();
 
 		protected override void OnClose (TimeSpan timeout)
 		{
-			if (close_started)
-				return;
-			close_started = true;
+			lock (close_lock) {
+				if (close_started)
+					return;
+				close_started = true;
+			}
 			DateTime start = DateTime.Now;
 
 			// FIXME: consider timeout
diff --git a/mcs/class/System.ServiceModel/System.ServiceModel.Channels.NetTcp/TcpReplyChannel.cs b/mcs/class/System.ServiceModel/System.ServiceModel.Channels.NetTcp/TcpReplyChannel.cs
index f597167e9d1..b005ad6c18a 100644
--- a/mcs/class/System.ServiceModel/System.ServiceModel.Channels.NetTcp/TcpReplyChannel.cs
+++ b/mcs/class/System.ServiceModel/System.ServiceModel.Channels.NetTcp/TcpReplyChannel.cs
@@ -135,9 +135,18 @@ namespace System.ServiceModel.Channels.NetTcp
 		{
 			throw new NotImplementedException ();
 		}
+		
+		bool close_started;
+		object close_lock = new object ();
 
 		protected override void OnClose (TimeSpan timeout)
 		{
+			lock (close_lock) {
+				if (close_started)
+					return;
+				close_started = true;
+			}
+
 			client.Close ();
 			client = null;
 			base.OnClose (timeout);