DirectRunner does not update reference to currentRestriction when running in SDF

I have an SDF implementation that looks like so:

class MyRestrictionTracker {
  MyRestriction restriction;

  currentRestriction() { return restriction; }

  tryClaim(MyPosition position) {
    this.restriction = new MyRestriction(position)
  }
}

I ran this on the DirectRunner, and the restriction would never advance: It would get stuck on the very first value.

I also ran this on DataflowRunner, and the problem did not exist there: This ran fine.

I was able to fix this on the DirectRunner (it works well on Dataflow as well) by changing the restriction to be mutable. Something like this:

class MyRestrictionTracker {
  MyRestriction restriction;

  currentRestriction() { return restriction; }

  tryClaim(MyPosition position) {
    this.restriction.position = position;
  }
}

This looks like an execution issue with SDF on DirectRunner: The DirectRunner is likely storing a reference to `currentRestriction()` and never updating it as it runs.

I'm happy to fix this on the DirectRunner - I would just like to find pointers : )