Pushed into bb-lf-iterator branch.
svoj, please see if the new API is ok for you

API as such is alright. I just hope this extra if won't be that expensive.

Thanks. I hope not, it's inside unlikely() so in the normal use case it should be cheap. And I've removed one if in the previous changeset, which should compensate to an extent. Old if was actually worse — it was comparing volatile shared values and under high concurrency the expression could often return different results. New if expression uses only local variables and is false almost always.

svoj, please push it into 10.1 when and as you see fit. It's all your.