CSHARP-3462: Unobserved task exception on connection failure raises its head again (#492)

Author: MikalaiMazurenka

src/MongoDB.Driver.Core/Core/Connections/BinaryConnection.cs

@@ -357,7 +357,7 @@ private IByteBuffer ReceiveBuffer(int responseTo, CancellationToken cancellation
                     Task.WaitAny(messageTask, receiveLockRequest.Task);
                     if (messageTask.IsCompleted)
                     {
-                        return _dropbox.RemoveMessage(responseTo);
+                        return _dropbox.RemoveMessage(responseTo); // also propagates exception if any
                     }
 
                     receiveLockRequest.Task.GetAwaiter().GetResult(); // propagate exceptions
@@ -371,12 +371,11 @@ private IByteBuffer ReceiveBuffer(int responseTo, CancellationToken cancellation
                         catch (Exception ex)
                         {
                             _dropbox.AddException(ex);
-                            throw;
                         }
 
                         if (messageTask.IsCompleted)
                         {
-                            return _dropbox.RemoveMessage(responseTo);
+                            return _dropbox.RemoveMessage(responseTo); // also propagates exception if any
                         }
 
                         cancellationToken.ThrowIfCancellationRequested();
@@ -428,7 +427,7 @@ private async Task<IByteBuffer> ReceiveBufferAsync(int responseTo, CancellationT
                     await Task.WhenAny(messageTask, receiveLockRequest.Task).ConfigureAwait(false);
                     if (messageTask.IsCompleted)
                     {
-                        return _dropbox.RemoveMessage(responseTo);
+                        return _dropbox.RemoveMessage(responseTo); // also propagates exception if any
                     }
 
                     receiveLockRequest.Task.GetAwaiter().GetResult(); // propagate exceptions
@@ -442,12 +441,11 @@ private async Task<IByteBuffer> ReceiveBufferAsync(int responseTo, CancellationT
                         catch (Exception ex)
                         {
                             _dropbox.AddException(ex);
-                            throw;
                         }
 
                         if (messageTask.IsCompleted)
                         {
-                            return _dropbox.RemoveMessage(responseTo);
+                            return _dropbox.RemoveMessage(responseTo); // also propagates exception if any
                         }
 
                         cancellationToken.ThrowIfCancellationRequested();

tests/MongoDB.Driver.Core.Tests/Core/Connections/BinaryConnectionTests.cs

@@ -468,6 +468,66 @@ public void ReceiveMessage_should_handle_out_of_order_replies(
             }
         }
 
+        [Theory]
+        [ParameterAttributeData]
+        public void ReceiveMessage_should_not_produce_unobserved_task_exceptions_on_fail(
+            [Values(false, true)] bool async)
+        {
+            var unobservedTaskExceptionRaised = false;
+            var mockStream = new Mock<Stream>();
+            EventHandler<UnobservedTaskExceptionEventArgs> eventHandler = (s, args) =>
+            {
+                unobservedTaskExceptionRaised = true;
+                args.SetObserved();
+            };
+
+            try
+            {
+                TaskScheduler.UnobservedTaskException += eventHandler;
+                var encoderSelector = new ReplyMessageEncoderSelector<BsonDocument>(BsonDocumentSerializer.Instance);
+
+                _mockStreamFactory
+                    .Setup(f => f.CreateStream(_endPoint, CancellationToken.None))
+                    .Returns(mockStream.Object);
+
+                if (async)
+                {
+                    mockStream
+                        .Setup(s => s.ReadAsync(It.IsAny<byte[]>(), It.IsAny<int>(), It.IsAny<int>(), It.IsAny<CancellationToken>()))
+                        .Throws(new SocketException());
+                }
+                else
+                {
+                    mockStream
+                        .Setup(s => s.Read(It.IsAny<byte[]>(), It.IsAny<int>(), It.IsAny<int>()))
+                        .Throws(new SocketException());
+                }
+
+                _subject.Open(CancellationToken.None);
+
+                Exception exception;
+                if (async)
+                {
+                    exception = Record.Exception(() => _subject.ReceiveMessageAsync(1, encoderSelector, _messageEncoderSettings, CancellationToken.None).GetAwaiter().GetResult());
+                }
+                else
+                {
+                    exception = Record.Exception(() => _subject.ReceiveMessage(1, encoderSelector, _messageEncoderSettings, CancellationToken.None));
+                }
+                exception.Should().BeOfType<MongoConnectionException>();
+
+                GC.Collect(); // Collects the unobserved tasks
+                GC.WaitForPendingFinalizers(); // Assures finilizers are executed
+
+                unobservedTaskExceptionRaised.Should().BeFalse();
+            }
+            finally
+            {
+                TaskScheduler.UnobservedTaskException -= eventHandler;
+                mockStream.Object?.Dispose();
+            }
+        }
+
         [Theory]
         [ParameterAttributeData]
         public void ReceiveMessage_should_throw_network_exception_to_all_awaiters(