To properly simulate OAuth tokens expiring and being refreshed, need the ability within a Groovy post processor to do the following when the response status is a 401:
* Invoke a HTTP transaction to obtain the updated token (would be great if this could be done by dispatching off to another sampler, but it can also be achieved by using URLConnection)
* Re-execute the current HTTP sampler with the updated Authorization token

There is no current way to re-execute the current sampler from within the Groovy post procesor.

There is also no way to have a pre-processor not fire the current transaction and just skip/continue to the next sampler or to the start of the current loop.