Workflow host observers are notified before internal updates are finished

Scenario

Say I am hosting a Workflow in a UIViewController using a WorkflowHostingController. In the view controller, I am observing the output of my Workflow with something like:

hostingController.output.take(during: lifetime).observeValues { [weak self] output in
    guard let self else { return }

    switch output {
    case .somethingChanged(let foo):
        self.foo = foo
    }
}

If I need to pass the new value of foo back into the Workflow, and I try to update the Workflow synchronously, e.g. by calling:

hostingController.update(workflow: MyWorkflow(foo: foo))

then this consistently results in a crash: Fatal error: EventPipe can only be enabled from the `pending` state

A suggestion by @jamieQ is that "we could potentially delay notifying the workflow host observers until after all the internal updates were finished."

Workarounds

We can avoid the crash by delaying event handling, either when observing the Workflow output:

hostingController.output.take(during: lifetime).observe(on: QueueScheduler.main).observeValues {
    // ...
}

or when updating the Workflow:

DispatchQueue.main.async { [weak self] in
    guard let self else { return }
    self.hostingController.update(workflow: MyWorkflow(foo: foo))
}

Start by reproducing the synchronous output observation and WorkflowHostingController.update(workflow:) sequence described in the issue, then inspect the WorkflowHostingController notification flow and EventPipe state transitions. Done means a host observer can synchronously update the Workflow without the “EventPipe can only be enabled from the pending state” crash.