Fix ScheduledTaskExecutor deadlock when TrySetResult runs continuations inline (#2953)

* Add failing test for inline continuation deadlock

* Make sure we run task continuations async

Resolves #2948.

Author: crnhrv

src/Polly.Core/CircuitBreaker/Controller/ScheduledTaskExecutor.cs

@@ -26,7 +26,7 @@ public Task ScheduleTask(Func<Task> taskFactory)
         }
 #endif
 
-        var source = new TaskCompletionSource<object>();
+        var source = new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously);
 
         _tasks.Enqueue(new Entry(taskFactory, source));
         _semaphore.Release();

test/Polly.Core.Tests/CircuitBreaker/Controller/ScheduledTaskExecutorTests.cs

@@ -129,6 +129,27 @@ public void Dispose_WhenScheduledTaskExecuting()
 #pragma warning restore xUnit1031
     }
 
+    [Fact]
+    public void ScheduleTask_InlineContinuationDoesNotDeadlock()
+    {
+        var timeout = TimeSpan.FromMilliseconds(250);
+        using var scheduler = new ScheduledTaskExecutor();
+
+        var firstTask = scheduler.ScheduleTask(() => Task.CompletedTask);
+
+        var continuationTask = firstTask.ContinueWith(
+            _ =>
+            {
+                var secondTask = scheduler.ScheduleTask(() => Task.CompletedTask);
+                secondTask.Wait(timeout);
+            },
+            CancellationToken,
+            TaskContinuationOptions.ExecuteSynchronously,
+            TaskScheduler.Default);
+
+        continuationTask.Wait(timeout).ShouldBeTrue();
+    }
+
     [Fact]
     public async Task Dispose_EnsureNoBackgroundProcessing()
     {