Re: test: avoid redundant standby catchup in 049_wait_for_lsn

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.

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.

Thanks for tracking this down in the test. It saved me from chasing what
looked like a WAIT FOR wakeup bug.

Thanks and Regards,
Jobin.

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