[Bug+fix] Inconsistency in sync/async statemachine execution

[vrenken]

Hi,

during the easter holidays the wetter was bad which gave me some time to work on a PlantUML 2 Stateless Roslyn code generator.

It's amazing to see how such a great library as Stateless benefits when being configured in a more visually way - It completely speeds up the whole development process as the mental model is far easier to grasp.

However, during this testing I also wanted to make sure that both synchronous and asynchronous state machines work as expected, and in doing so I think I might have spotted an issue in how asynchronous calls in Stateless work with superstates/substates.

It's best to demonstrate with two 'identical' state machines.

First the synchronous one (which works as expected):

  [Fact]
  public void SuperStateShouldNotExitOnSubStateTransition_WhenUsingSyncTriggers()
  {
      // Arrange.
      var sm = new StateMachine<State, Trigger>(State.A);
      var record = new List<string>();

      sm.Configure(State.A)
          .OnEntry(() => record.Add("Entered state A"))
          .OnExit(() => record.Add("Exited state A"))
          .Permit(Trigger.X, State.B);
      
      sm.Configure(State.B) // Our super state.
          .InitialTransition(State.C)
          .OnEntry(() => record.Add("Entered super state B"))
          .OnExit(() => record.Add("Exited super state B"));

      sm.Configure(State.C) // Our first sub state.
          .OnEntry(() => record.Add("Entered sub state C"))
          .OnExit(() => record.Add("Exited sub state C"))
          .Permit(Trigger.Y, State.D)
          .SubstateOf(State.B);
      sm.Configure(State.D) // Our second sub state.
          .OnEntry(() => record.Add("Entered sub state D"))
          .OnExit(() => record.Add("Exited sub state D"))
          .SubstateOf(State.B);

      
      // Act.
      sm.Fire(Trigger.X);
      sm.Fire(Trigger.Y);
      
      // Assert.
      Assert.Equal("Exited state A", record[0]);
      Assert.Equal("Entered super state B", record[1]);
      Assert.Equal("Entered sub state C", record[2]);
      Assert.Equal("Exited sub state C", record[3]);
      Assert.Equal("Entered sub state D", record[4]);
  }

And the asynchronous one (which doesn't work as expected)

  [Fact]
  public async Task SuperStateShouldNotExitOnSubStateTransition_WhenUsingAsyncTriggers()
  {
      // Arrange.
      var sm = new StateMachine<State, Trigger>(State.A);
      var record = new List<string>();

      sm.Configure(State.A)
          .OnEntryAsync(() => Task.Run(() => record.Add("Entered state A")))
          .OnExitAsync(() => Task.Run(() => record.Add("Exited state A")))
          .Permit(Trigger.X, State.B);
      
      sm.Configure(State.B) // Our super state.
          .InitialTransition(State.C)
          .OnEntryAsync(() => Task.Run(() => record.Add("Entered super state B")))
          .OnExitAsync(() => Task.Run(() => record.Add("Exited super state B")));

      sm.Configure(State.C) // Our first sub state.
          .OnEntryAsync(() => Task.Run(() => record.Add("Entered sub state C")))
          .OnExitAsync(() => Task.Run(() => record.Add("Exited sub state C")))
          .Permit(Trigger.Y, State.D)
          .SubstateOf(State.B);
      sm.Configure(State.D) // Our second sub state.
          .OnEntryAsync(() => Task.Run(() => record.Add("Entered sub state D")))
          .OnExitAsync(() => Task.Run(() => record.Add("Exited sub state D")))
          .SubstateOf(State.B);

      
      // Act.
      await sm.FireAsync(Trigger.X).ConfigureAwait(false);
      await sm.FireAsync(Trigger.Y).ConfigureAwait(false);
      
      // Assert.
      Assert.Equal("Exited state A", record[0]);
      Assert.Equal("Entered super state B", record[1]);
      Assert.Equal("Entered sub state C", record[2]);
      Assert.Equal("Exited sub state C", record[3]);
      Assert.Equal("Entered sub state D", record[4]); // Before the patch the actual result was "Exited super state B"
  }

Both state machines follow the same transitions as specified below, but in case of the asynchronous executing one It seems that superstate B is exiting even on a transition between its substates.

This is caused by an inconsistency in ExitAsync (in StateRepresentation.Async.cs) compared to Exit (in StateRepresentation.cs). There seem to be some checks missing.
I've got a small correction ready and two additional unit tests to prove its feasibility and correctness.

Happy to assist Stateless with a PR to fix this: #445

[HenningNT]

Thank you, I'll get this released as soon as I have time ;-)