From: Rodrigo Kumpera <kumpera@gmail.com>
Date: Thu, 19 Jun 2014 15:45:17 +0000 (-0400)
Subject: Merge pull request #1074 from esdrubal/bug18421
X-Git-Url: http://wien.tomnetworks.com/gitweb/?a=commitdiff_plain;h=21f79f8d6e420dc967127c35c00a375f5d10346e;hp=feec6280267e8bdcced861653e5eed0a75f9ea95;p=mono.git

Merge pull request #1074 from esdrubal/bug18421

Changed TryPeek to handle race condition with Dequeue.
---

diff --git a/mcs/class/corlib/System.Collections.Concurrent/ConcurrentQueue.cs b/mcs/class/corlib/System.Collections.Concurrent/ConcurrentQueue.cs
index aff2492e021..ea8984394b4 100644
--- a/mcs/class/corlib/System.Collections.Concurrent/ConcurrentQueue.cs
+++ b/mcs/class/corlib/System.Collections.Concurrent/ConcurrentQueue.cs
@@ -132,14 +132,24 @@ namespace System.Collections.Concurrent
 		
 		public bool TryPeek (out T result)
 		{
-			Node first = head.Next;	
+			result = default (T);
+			bool update = true;
+			
+			while (update)
+			{
+				Node oldHead = head;
+				Node oldNext = oldHead.Next;
 
-			if (first == null) {
-				result = default (T);
-				return false;
-			}
+				if (oldNext == null) {
+					result = default (T);
+					return false;
+				}
 
-			result = first.Value;
+				result = oldNext.Value;
+				
+				//check if head has been updated
+				update = head != oldHead;
+			}
 			return true;
 		}
 		
diff --git a/mcs/class/corlib/Test/System.Collections.Concurrent/ConcurrentQueueTests.cs b/mcs/class/corlib/Test/System.Collections.Concurrent/ConcurrentQueueTests.cs
index 51aeeeb8e89..91436a05b21 100644
--- a/mcs/class/corlib/Test/System.Collections.Concurrent/ConcurrentQueueTests.cs
+++ b/mcs/class/corlib/Test/System.Collections.Concurrent/ConcurrentQueueTests.cs
@@ -30,6 +30,7 @@ using System.Collections.Generic;
 using System.Collections.Concurrent;
 
 using NUnit.Framework;
+using MonoTests.System.Threading.Tasks;
 
 namespace MonoTests.System.Collections.Concurrent
 {
@@ -115,6 +116,34 @@ namespace MonoTests.System.Collections.Concurrent
 			CollectionStressTestHelper.RemoveStressTest (new ConcurrentQueue<int> (), CheckOrderingType.InOrder);
 		}
 		
+		[Test]
+		public void StressTryPeekTestCase ()
+		{
+			ParallelTestHelper.Repeat (delegate {
+				var queue = new ConcurrentQueue<object> ();
+				queue.Enqueue (new object());
+				
+				const int threads = 10;
+				int threadCounter = 0;
+				bool success = true;
+				
+				ParallelTestHelper.ParallelStressTest (queue, (q) => {
+					int threadId = Interlocked.Increment (ref threadCounter);
+					object temp;
+					if (threadId < threads)
+					{
+						while (queue.TryPeek (out temp))
+							if (temp == null)
+								success = false;
+					} else {
+						queue.TryDequeue (out temp);
+					}
+				}, threads);
+				
+				Assert.IsTrue (success, "TryPeek returned unexpected null value.");
+			}, 10);
+		}
+		
 		[Test]
 		public void CountTestCase()
 		{