Re: pg_logical_slot_get_changes waits continously for a partial WAL record spanning across 2 pages

On Thu, Jun 26, 2025 at 11:47 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Thu, Jun 26, 2025 at 08:48:32AM +0530, Dilip Kumar wrote:
> > On Thu, Jun 26, 2025 at 6:22 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> >> So you are suggesting the addition of an extra ReadPageInternal() that
> >> forces a read of only the read, perform the checks on the header, then
> >> read the rest. After reading SizeOfXLogShortPHD worth of data,
> >> shouldn't the checks on xlp_rem_len be done a bit earlier than what
> >> you are proposing in this patch?
> >
> > I did not get the point, IMHO it has to be validated after the record
> > on the next page has been read.
>
> As proposed in the patch, the first ReadPageInternal() reads
> SizeOfXLogShortPHD worth of data (right?), so we should have access to
> XLogPageHeaderData.xlp_rem_len before attempting the second
> ReadPageInternal() worth Min(total_len - gotlen + SizeOfXLogShortPHD,
> XLOG_BLCKSZ) of data. The check that fails with a "invalid contrecord
> length %u" relies on xlp_rem_len, total_len, RecPtr and gotlen. We
> know all this information before the second ReadPageInternal().

Yeah you are right, this can be moved before the second
ReadPageInternal() as we have already read XLogPageHeader.

--
Regards,
Dilip Kumar
Google