NestedTopLevel transactions do not rollback state on exception

The attached file is a reproducer. Drop it into the STM tests directory and run to see the failure.

It creates two transactional objects A and B. B is further annotated with @NestedTopLevel. The test:

• starts a transaction A;
• calls a write method on A;
• then calls a write method on B which throws an exception;
• aborts or commits A (either outcome produces the error)

Since the container starts a top-level transaction for the call to the method on B I would expect it to abort it if the B method throws an exception. What I find is that it will always call commit.

This behaviour limits the usefulness of @NestedTopLevel when such methods are called inside an existing transaction since there is no mechanism for rolling back changes.