Change how EndSegment is called for Tasks.

If the Task has already completed, we call EndSegment (synchronously) immediately. Otherwise, we always continue with TaskContinuationOptions.ExecuteSynchronously, which runs our continuation as soon as possible (on the thread that completed the task).

This avoids task continuations being delayed until much later in the ASP.NET pipeline (due to when AspNetSynchronizationContext posts them), which causes inaccurate timings.

Author: bgrainger

src/Agent/CHANGELOG.md

@@ -14,6 +14,7 @@ Enables nullable reference types that are part of the C# 8.0 language specificat
 
 ### Fixes
 * Fixes an issue that may cause `InvalidCastException` due to an assembly version mismatch in Mvc3 instrumentation.
+* Fixes an async timing issue that can cause the end time of `Task`-returning methods to be determined incorrectly.
 
 ## [8.31] - 2020-08-17
 ### New Features

src/Agent/NewRelic/Agent/Extensions/NewRelic.Agent.Extensions/Providers/Wrapper/Delegates.cs

@@ -152,22 +152,18 @@ private static void OnSuccess<T>(Task task, IAgent agent, ISegment segment, bool
                 transaction.Hold();
             }
 
-            if (options == TaskContinueWithOption.None)
+            if (task.IsCompleted)
+            {
+                EndSegment(task);
+            }
+            else if (options == TaskContinueWithOption.None)
             {
                 if (!continuationOptions.HasValue) task.ContinueWith(EndSegment);
                 else task.ContinueWith(EndSegment, continuationOptions.Value);
             }
             else
             {
-                var context = SynchronizationContext.Current;
-                if (context != null)
-                {
-                    task.ContinueWith(EndSegment, TaskScheduler.FromCurrentSynchronizationContext());
-                }
-                else
-                {
-                    task.ContinueWith(EndSegment, TaskContinuationOptions.ExecuteSynchronously);
-                }
+                task.ContinueWith(EndSegment, CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
             }
 
             void EndSegment(Task completedTask)