CSHARP-2188: Decrease likelihood of session leaks.

Author: rstam

src/MongoDB.Driver.Core/Core/Operations/AsyncCursor.cs

@@ -46,7 +46,7 @@ public class AsyncCursor<TDocument> : IAsyncCursor<TDocument>
         // fields
         private readonly int? _batchSize;
         private readonly CollectionNamespace _collectionNamespace;
-        private readonly IChannelSource _channelSource;
+        private IChannelSource _channelSource;
         private int _count;
         private IReadOnlyList<TDocument> _currentBatch;
         private long _cursorId;
@@ -103,12 +103,7 @@ public AsyncCursor(
             }
             _count = _firstBatch.Count;
 
-            // if we aren't going to need the channel source we can go ahead and Dispose it now
-            if (_cursorId == 0 && _channelSource != null)
-            {
-                _channelSource.Dispose();
-                _channelSource = null;
-            }
+            DisposeChannelSourceIfNoLongerNeeded();
         }
 
         // properties
@@ -277,6 +272,15 @@ protected virtual void Dispose(bool disposing)
             _disposed = true;
         }
 
+        private void DisposeChannelSourceIfNoLongerNeeded()
+        {
+            if (_channelSource != null && _cursorId == 0)
+            {
+                _channelSource.Dispose();
+                _channelSource = null;
+            }
+        }
+
         private CursorBatch<TDocument> GetNextBatch(CancellationToken cancellationToken)
         {
             using (EventContext.BeginOperation(_operationId))
@@ -374,6 +378,8 @@ private void SaveBatch(CursorBatch<TDocument> batch)
 
             _currentBatch = documents;
             _cursorId = batch.CursorId;
+
+            DisposeChannelSourceIfNoLongerNeeded();
         }
 
         private void ThrowIfDisposed()

tests/MongoDB.Bson.TestHelpers/MongoDB.Bson.TestHelpers.csproj

@@ -87,6 +87,7 @@
     <Compile Include="EqualityComparers\ReferenceEqualsEqualityComparer.cs" />
     <Compile Include="IO\NullBsonStream.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="Reflector.cs" />
     <Compile Include="UnicodeHelper.cs" />
     <Compile Include="XunitExtensions\AssertionException.cs" />
     <Compile Include="XunitExtensions\RequireEnvironment.cs" />

tests/MongoDB.Bson.TestHelpers/Reflector.cs

@@ -0,0 +1,37 @@
+/* Copyright 2018-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System.Linq;
+using System.Reflection;
+
+namespace MongoDB.Bson.TestHelpers
+{
+    public static class Reflector
+    {
+        public static object GetFieldValue(object obj, string name)
+        {
+            var fieldInfo = obj.GetType().GetField(name, BindingFlags.NonPublic | BindingFlags.Instance);
+            return fieldInfo.GetValue(obj);
+        }
+
+        public static object Invoke(object obj, string name)
+        {
+            var methodInfo = obj.GetType().GetMethods(BindingFlags.NonPublic | BindingFlags.Instance)
+                .Where(m => m.Name == name && m.GetParameters().Length == 0)
+                .Single();
+            return methodInfo.Invoke(obj, new object[] { });
+        }
+    }
+}

tests/MongoDB.Driver.Core.Tests/Core/Bindings/ChannelSourceHandleTests.cs

@@ -17,6 +17,7 @@
 using System.Reflection;
 using System.Threading;
 using FluentAssertions;
+using MongoDB.Bson.TestHelpers;
 using MongoDB.Bson.TestHelpers.XunitExtensions;
 using MongoDB.Driver.Core.Bindings;
 using MongoDB.Driver.Core.Misc;
@@ -155,10 +156,8 @@ public void Disposing_of_last_handle_should_dispose_of_connectioSource()
 
     internal static class ChannelSourceHandleReflector
     {
-        public static ReferenceCounted<IChannelSource> _reference(this ChannelSourceHandle obj)
-        {
-            var fieldInfo = typeof(ChannelSourceHandle).GetField("_reference", BindingFlags.NonPublic | BindingFlags.Instance);
-            return (ReferenceCounted<IChannelSource>)fieldInfo.GetValue(obj);
-        }
+        // private fields
+        public static bool _disposed(this ChannelSourceHandle obj) => (bool)Reflector.GetFieldValue(obj, nameof(_disposed));
+        public static ReferenceCounted<IChannelSource> _reference(this ChannelSourceHandle obj) => (ReferenceCounted<IChannelSource>)Reflector.GetFieldValue(obj, nameof(_reference));
     }
 }