Re: test: avoid redundant standby catchup in 049_wait_for_lsn

Hi Jobin,

On Tue, Sep 29, 2026 at 7:47 PM Jobin Augustine <jobin(dot)augustine(at)percona(dot)com>
wrote:

> Hi Xuneng, hackers,
>
> I ran into the same pg_current_wal_insert_lsn() page-boundary problem
> discussed in this thread, independently, outside the regression tests,
> while evaluating WAIT FOR for read-your-writes on a standby using a custom
> coded application to test the usability of this feature.
>
> At this boundary, pg_current_wal_insert_lsn() returns the next insertion
>> address, which is after the header gap. The test thus waited for a position
>> beyond the existing WAL, requiring another WAL record to make progress. The
>> unrelated progression comes from the primary's background writer who
>> generated a RUNNING_XACTS record, advancing WAL to 0/0301A050, and then
>> notified the waiters.
>
>
> About 1 in 2000 waits never succeeded and ran to the full TIMEOUT
> Since all my sessions were using WAIT FOR LSN, it was a complete stall,
> whenever it happens.
>

Interestingly, is this a pressure test? Can you please elaborate a bit more
on how the application tests it? The stall in the TAP test 049 persisted
for ~8s because of no WAL activity in that test env, and it was saved by a
later RUNNING_XACTS record generated by bgwriter. In practice, I expected
the problem to be harder to notice since WAL is more active.

> A client side workaround similar to the following is a temporary solution
> for me for my tests:
> CREATE FUNCTION wal_insert_end_lsn(l pg_lsn DEFAULT
> pg_current_wal_insert_lsn())
> RETURNS pg_lsn LANGUAGE sql STABLE AS $$
> SELECT CASE
> WHEN (l - '0/0'::pg_lsn) %
> pg_size_bytes(current_setting('wal_segment_size'))
> < current_setting('wal_block_size')::int -- first page of a
> segment
> THEN CASE WHEN (l - '0/0'::pg_lsn) %
> current_setting('wal_block_size')::int = 40
> THEN l - 40 ELSE l END
> ELSE CASE WHEN (l - '0/0'::pg_lsn) %
> current_setting('wal_block_size')::int = 24
> THEN l - 24 ELSE l END
> END
> $$;
>
> A server-side fix would be better
> Could WAIT FOR itself also tolerate such targets? When the target points
> exactly past a page header treat it as the page boundary.
>
>
I think we better fix the page/segment header issue on the server side.

--
Regards,
Xuneng Zhou
HighGo Software Co., Ltd.