Capture stacktrace of exceptions thrown from async (#524)

* Capture stacktrace of exceptions thrown from async

* tidy

src/Machine.Specifications.Specs/Runner/AsyncDelegateRunnerSpecs.cs

@@ -68,6 +68,11 @@ public class when_running_async_specifications_with_exceptions : RunnerSpecs
 
  It should_have_failures = () =>
  results.Should().Match(x => x.All(y => !y.Passed));
+
+ It should_have_exception_details = () =>
+ results.Should().Match(x => x.All(r => r.Exception.TypeName == nameof(InvalidOperationException) &&
+ r.Exception.Message == "something went wrong" &&
+ r.Exception.StackTrace.Contains(typeof(AsyncSpecificationsWithExceptions).FullName)));
  }
 
 #if NETCOREAPP

src/Machine.Specifications/Runner/Impl/AsyncSynchronizationContext.cs

@@ -1,5 +1,6 @@
 #if !NET35
 using System;
+using System.Runtime.ExceptionServices;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -13,7 +14,11 @@ internal class AsyncSynchronizationContext : SynchronizationContext
 
  private int callCount;
 
+#if !NET40
+ private ExceptionDispatchInfo exception;
+#else
  private Exception exception;
+#endif
 
  public AsyncSynchronizationContext(SynchronizationContext inner)
  {
@@ -28,7 +33,11 @@ private void Execute(SendOrPostCallback callback, object state)
  }
  catch (Exception ex)
  {
+#if !NET40
+ exception = ExceptionDispatchInfo.Capture(ex);
+#else
  exception = ex;
+#endif
  }
  finally
  {
@@ -89,11 +98,19 @@ public override void Send(SendOrPostCallback d, object state)
  }
  catch (Exception ex)
  {
+#if !NET40
+ exception = ExceptionDispatchInfo.Capture(ex);
+#else
  exception = ex;
+#endif
  }
  }
 
+#if !NET40
+ public ExceptionDispatchInfo WaitAsync()
+#else
  public Exception WaitAsync()
+#endif
  {
  events.Wait();
 

src/Machine.Specifications/Runner/Impl/DelegateRunner.cs

@@ -44,7 +44,11 @@ public void Execute()
 
  if (exception != null)
  {
+#if !NET40
+ exception.Throw();
+#else
  throw exception;
+#endif
  }
  }
  finally