Re: prevent immature WAL streaming

On Fri, Nov 26, 2021 at 1:42 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> > On 2021-Nov-25, Tom Lane wrote:
> >> Really? AFAICS the WAL record contains the correct value, or at least
> >> we should define that one as being correct, for precisely this reason.
>
> > I don't know what is the correct value for a record that comes exactly
> > after the page header. But here's a patch that fixes the problem; and
> > if a standby replays WAL written by an unpatched primary, it will be
> > able to read past instead of dying of FATAL.
>
> Meh ... but given the simplicity of the write-side fix, maybe changing
> it is appropriate.
>
> However, this seems too forgiving:
>
> + if (xlrec->overwritten_lsn != state->overwrittenRecPtr &&
> + xlrec->overwritten_lsn - SizeOfXLogShortPHD != state->overwrittenRecPtr &&
> + xlrec->overwritten_lsn - SizeOfXLogLongPHD != state->overwrittenRecPtr)
>

Unless I am missing something, I am not sure why need this adjustment
if we are going to use state->currRecPtr value which doesn't seem to
be changing at all. AFAICU, state->currRecPtr will be unchanged value
whether going to set overwrittenRecPtr or abortedRecPtr. Do primary
and standby see state->currRecPtr differently, I guess not, never?

Regards,
Amul