I said:
> Mmm ... maybe. Is this safe if a checkpoint is currently in progress?
> Seems like you could look at RedoRecPtr and decide you are okay, but you
> really are not if checkpointer has already dumped sequence' disk
> buffer and will later set RedoRecPtr to a value beyond the old LSN.
Oh, wait, I take that back: the checkpointer advances RedoRecPtr
*before* it starts to dump disk buffers.
I'm still worried about whether we shouldn't try to simplify, rather
than add complexity.
regards, tom lane