Make ThreadingUtilities.ExecuteTasksOnNewThreads await on the operation completion.

Author: Oleksandr Poliakov

tests/MongoDB.Bson.TestHelpers/Threading/ThreadingUtilities.cs

@@ -110,7 +110,7 @@ public static async Task<Exception[]> ExecuteTasksOnNewThreadsCollectExceptions(
                 }
             });
 
-            var taskAll = Task.WhenAll(allTasks);
+            var taskAll = Task.WhenAll(allTasks.Select(t => t.Result));
             if (await Task.WhenAny(taskAll, Task.Delay(timeoutMilliseconds)) != taskAll)
             {
                 exceptions.Add(new TimeoutException());

tests/MongoDB.Driver.Tests/TestClassDoNotMerge.cs

@@ -0,0 +1,46 @@
+using System.Threading;
+using System.Threading.Tasks;
+using MongoDB.Bson.TestHelpers;
+using MongoDB.TestHelpers.XunitExtensions;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace MongoDB.Driver.Tests
+{
+    // TODO: DO NOT MERGE THIS CLASS!
+    // It exists for problem repro purposes only
+    public class TestClassDoNotMerge
+    {
+        private readonly ITestOutputHelper _output;
+
+        public TestClassDoNotMerge(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        [Theory]
+        [ParameterAttributeData]
+        public async Task RunTasksInParallelTest([Values(false, true)] bool async)
+        {
+            _output.WriteLine($"Test async={async}: before parallel");
+
+            await ThreadingUtilities.ExecuteTasksOnNewThreads(10, async i =>
+            {
+                if (async)
+                {
+                    await Task.Delay(100);
+                }
+                else
+                {
+                    Thread.Sleep(100);
+                }
+                _output.WriteLine($"I'm task number {i}");
+            });
+
+            _output.WriteLine($"Test async={async}: after parallel");
+
+            await Task.Delay(1_000);
+            _output.WriteLine($"Test async={async}: done");
+        }
+    }
+}