Re: BUG: Cascading standby fails to reconnect after falling back to archive recovery

On Fri, Mar 20, 2026 at 8:40 PM Marco Nenciarini
<marco.nenciarini@enterprisedb.com> wrote:
>
> On Fri, Mar 20, 2026 at 4:33 AM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> >
> > After taking a closer look, I'm less certain about this. I'll
> > investigate further. Could you also explain why you think this is the
> > case?
>
> The mechanism is in RequestXLogStreaming (walreceiverfuncs.c, around
> line 276): it explicitly truncates recptr to the segment start before
> passing it to the walreceiver.  So even when both nodes have replayed
> the same records, the cascade's startpoint lands at the beginning of
> the next segment while the upstream's GetStandbyFlushRecPtr returns
> replayPtr somewhere inside the current one.
>
> I covered this in more detail in my reply to your previous message.
>
> Best regards,
> Marco
>

if (XLogSegmentOffset(recptr, wal_segment_size) != 0)
    recptr -= XLogSegmentOffset(recptr, wal_segment_size);

I think this rounds down to the start of segment that contains
targetPagePtr + reqLen. It does not round up.  So if both standby have
replayed the same record, the cascade's startpoint would land at the
beginning of the current segment, which will provide a legitimate LSN
to upstream server. This case would be fine. Am I missing something
here?

-- 
Best,
Xuneng