[System.ServiceModel] Fix a race condition in HttpRequestChannel. (#13304)

Concurrent request completions could cause following exception:

```
    FATAL UNHANDLED EXCEPTION: System.ArgumentOutOfRangeException:
        Index was out of range. Must be non-negative and less than
        the size of the collection.
        Parameter name: index
       at System.Collections.Generic.List`1[T].RemoveAt (System.Int32 index) [0x00009] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Collections.Generic.List`1[T].Remove (T item) [0x0000c] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.ServiceModel.Channels.HttpRequestChannel+HttpChannelRequestAsyncResult.Cleanup () [0x00011] in <28c4695205ef47378f5370502b066592>:0
       at System.ServiceModel.Channels.HttpRequestChannel+HttpChannelRequestAsyncResult.set_IsCompleted (System.Boolean value) [0x00034] in <28c4695205ef47378f5370502b066592>:0
       at System.ServiceModel.Channels.HttpRequestChannel+HttpChannelRequestAsyncResult.Complete (System.Exception ex) [0x0001a] in <28c4695205ef47378f5370502b066592>:0
       at System.ServiceModel.Channels.HttpRequestChannel+<>c__DisplayClass7_1.<BeginProcessRequest>b__0 (System.IAsyncResult r) [0x000bd] in <28c4695205ef47378f5370502b066592>:0

    This stack was captured after some changes in AwaitTaskContinuation.RunOrScheduleAction.
    Unfortunately the current implementation of AwaitTaskContinuation.RunOrScheduleAction re-throws the above
    exception with a wrong stack:

       at System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction (System.Action action, System.Boolean allowInlining, System.Threading.Tasks.Task& currentTask) [0x00023] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.Task.FinishContinuations () [0x00052] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.Task.FinishStageThree () [0x0003c] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.Task.FinishStageTwo () [0x000a1] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.Task.Finish (System.Boolean bUserDelegateExecuted) [0x00003] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.Task.TrySetException (System.Object exceptionObject) [0x00023] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1[TResult].SetException (System.Exception exception) [0x00029] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Net.HttpWebRequest+<RunWithTimeoutWorker>d__241`1[T].MoveNext () [0x000e0] in <3e9b3e26c4694baab3f689687ad40612>:0
       at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext (System.Object stateMachine) [0x00000] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00071] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00000] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.Run () [0x00024] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction (System.Action action, System.Boolean allowInlining, System.Threading.Tasks.Task& currentTask) [0x00023] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.Task.FinishContinuations () [0x00052] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.Task.FinishStageThree () [0x0003c] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Threading.Tasks.Task`1[TResult].TrySetResult (TResult result) [0x0004f] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1[TResult].SetResult (TResult result) [0x0003d] in <4a8a0eaa024f41829d3435c07e5c7edb>:0
       at System.Net.ServicePointScheduler+<WaitAsync>d__45.MoveNext () [0x000f7] in <3e9b3e26c4694baab3f689687ad40612>:0
       at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext (System.Object stateMachine) [0x00000] in <4a8a0
```

On an Android Xamarin installation this causes an app crash. Since no user code is active the exception cannot be caught.

1 files changed, 25 insertions, 5 deletions

diff --git a/mcs/class/System.ServiceModel/System.ServiceModel.Channels/HttpRequestChannel.cs b/mcs/class/System.ServiceModel/System.ServiceModel.Channels/HttpRequestChannel.cs
index 4e4d73f9fb1..c5a96fe3314 100644
--- a/mcs/class/System.ServiceModel/System.ServiceModel.Channels/HttpRequestChannel.cs
+++ b/mcs/class/System.ServiceModel/System.ServiceModel.Channels/HttpRequestChannel.cs
@@ -41,7 +41,8 @@ namespace System.ServiceModel.Channels
 	{
 		HttpChannelFactory<IRequestChannel> source;
 
-		List<WebRequest> web_requests = new List<WebRequest> ();
+		object locker = new object();
+		List<WebRequest> web_requests = new List<WebRequest> ();	//synced by locker
 
 		// Constructor
 
@@ -86,7 +87,12 @@ namespace System.ServiceModel.Channels
 			}
 
 			var web_request = (HttpWebRequest) HttpWebRequest.Create (destination);
-			web_requests.Add (web_request);
+
+			lock (locker)
+			{
+				web_requests.Add (web_request);
+			}
+		
 			result.WebRequest = web_request;
 			web_request.Method = "POST";
 			web_request.ContentType = Encoder.ContentType;
@@ -350,9 +356,20 @@ namespace System.ServiceModel.Channels
 
 		protected override void OnAbort ()
 		{
-			foreach (var web_request in web_requests.ToArray ())
+			WebRequest[] current_web_requests;
+
+			lock (locker)
+			{
+				current_web_requests = web_requests.ToArray();
+			}
+
+			foreach (var web_request in current_web_requests)
 				web_request.Abort ();
-			web_requests.Clear ();
+
+			lock(locker)
+			{
+				web_requests.Clear ();
+			}
 		}
 
 		// Close
@@ -497,7 +514,10 @@ namespace System.ServiceModel.Channels
 			
 			void Cleanup ()
 			{
-				owner.web_requests.Remove (WebRequest);
+				lock (owner.locker)
+				{
+					owner.web_requests.Remove (WebRequest);
+				}
 			}
 		}
 	}