Re: recovering from "found xmin ... from before relfrozenxid ..."

You really cannot
modify the buffer like this and then decide, oops, never mind, I think
I won't mark it dirty or write WAL for the changes. If you do that,
the buffer is still in memory, but it's now been modified. A
subsequent operation that modifies it will start with the altered
state you created here, quite possibly leading to WAL that cannot be
correctly replayed on the standby. In other words, you've got to
decide for certain whether you want to proceed with the operation
*before* you enter the critical section.