... BTW, with respect to the documentation angle, it seems to me
that it'd be better if GenericXLogRegister were renamed to
GenericXLogRegisterBuffer, or perhaps GenericXLogRegisterPage.
I think this would make the documentation clearer, and it would
also make it easier to add other sorts of Register actions later,
if we ever think of some (which seems not unlikely, really).
Another thing to think about is whether we're going to regret
hard-wiring the third argument as a boolean. Should we consider
making it a bitmask of flags, instead? It's not terribly hard
to think of other flags we might want there in future; for example
maybe something to tell GenericXLogFinish whether it's worth trying
to identify data movement on the page rather than just doing the
byte-by-byte delta calculation.
regards, tom lane