CSHARP-2564: Wrap call to TrySetResult in Task.Run.

Author: rstam

src/MongoDB.Driver.Core/Core/Clusters/Cluster.cs

@@ -308,7 +308,9 @@ protected void UpdateClusterDescription(ClusterDescription newClusterDescription
             }
 
             OnDescriptionChanged(oldClusterDescription, newClusterDescription);
-            oldDescriptionChangedTaskCompletionSource.TrySetResult(true);
+
+            // TODO: use RunContinuationsAsynchronously instead once we require a new enough .NET Framework
+            Task.Run(() => oldDescriptionChangedTaskCompletionSource.TrySetResult(true));
         }
 
         private string BuildTimeoutExceptionMessage(TimeSpan timeout, IServerSelector selector, ClusterDescription clusterDescription)

tests/MongoDB.Driver.Tests/Jira/CSharp2564Tests.cs

@@ -0,0 +1,65 @@
+/* Copyright 2019-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;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using MongoDB.Bson;
+using MongoDB.Driver.Core.Clusters;
+using MongoDB.Driver.Core.Configuration;
+using MongoDB.Driver.Core.Events;
+using MongoDB.Driver.Core.Misc;
+using MongoDB.Driver.Core.Servers;
+using MongoDB.Driver.Core.TestHelpers.XunitExtensions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Jira
+{
+    public class CSharp2564Tests
+    {
+        [SkippableFact]
+        public async Task Misbehaved_async_method_should_not_deadlock_server_selection()
+        {
+            RequireServer.Check().ClusterType(ClusterType.ReplicaSet).VersionGreaterThanOrEqualTo("3.6");
+
+            // note: the code below deadlocks because await StartSessionAsync continues when UpdateClusterDescription in Cluster called TrySetResult after finding the secondary
+            // but then the sync call to RunCommand blocks waiting for a primary and the call to TrySetResult never returns
+            // which in turn prevents SDAM from unwinding back to process the next queued heartbeat event so the primary is never found
+
+            var primary = CoreTestConfiguration.Cluster.Description.Servers.Where(s => s.Type == ServerType.ReplicaSetPrimary).Single();
+            void clusterConfigurator(ClusterBuilder builder)
+            {
+                builder.Subscribe((ServerHeartbeatSucceededEvent heartbeatEvent) =>
+                {
+                    // ensure that the primary heartbeat is the last to be processed by introducing a small artificial delay
+                    if (EndPointHelper.Equals(heartbeatEvent.ConnectionId.ServerId.EndPoint, primary.EndPoint))
+                    {
+                        Thread.Sleep(TimeSpan.FromSeconds(1));
+                    }
+                });
+            }
+
+            using (var client = DriverTestConfiguration.CreateDisposableClient(clusterConfigurator))
+            {
+                using (var session = await client.StartSessionAsync().ConfigureAwait(false))
+                {
+                    var adminDatabase = client.GetDatabase("admin");
+                    adminDatabase.RunCommand<BsonDocument>(session, "{ ping : 1 }"); // this async method is misbehaving by calling a blocking sync method
+                }
+            }
+        }
+    }
+}