Horton

Re: [HACKERS] make async slave to wait for lsn to be replayed

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: Картышов Иван <i.kartyshov@postgrespro.ru>
Cc: pgsql-hackers@lists.postgresql.org
Date: 2023-11-27T00:08:26Z
Lists: pgsql-hackers 
On Mon, Nov 20, 2023 at 1:10 PM Картышов Иван
<i.kartyshov@postgrespro.ru> wrote:
> Alexander, thank you for your review and pointing this issues. According to
> them I made some fixes and rebase all patch.
>
> But I can`t repeat your ERROR. Not with hot_standby_feedback = on nor
> hot_standby_feedback = off.
>
> master: create table test as (select i from generate_series(1,10000) i);
> slave conn1: select pg_wal_replay_pause();
> master: delete from test;
> master: vacuum test;
> master: select pg_current_wal_lsn();
> slave conn2: select pg_wait_lsn('the value from previous query'::pg_lsn, 0);
> slave conn1: select pg_wal_replay_resume();
> slave conn2: ERROR: canceling statement due to conflict with recovery
> DETAIL: User query might have needed to see row versions that must be removed.
>
> Also I use little hack to work out of snapshot similar to SnapshotResetXmin.
>
> Patch rebased and ready for review.

I've retried my case with v6 and it doesn't fail anymore.  But I
wonder how safe it is to reset xmin within the user-visible function?
We have no guarantee that the function is not called inside the
complex query.  Then how will the rest of the query work with xmin
reset?  Separate utility statement still looks like more safe option
for me.

------
Regards,
Alexander Korotkov

Commits

  1. Ensure standby promotion point in 043_wal_replay_wait.pl

  2. Minor cleanup related to pg_wal_replay_wait() procedure

  3. Adjust pg_wal_replay_wait() procedure behavior on promoted standby

  4. pg_wal_replay_wait(): Fix typo in the doc

  5. Implement pg_wal_replay_wait() stored procedure

  6. Revert: Implement pg_wal_replay_wait() stored procedure

  7. Call WaitLSNCleanup() in AbortTransaction()

  8. Clarify what is protected by WaitLSNLock

  9. Use an LWLock instead of a spinlock in waitlsn.c

  10. Use the pairing heap instead of a flat array for LSN replay waiters

  11. Minor improvements for waitlsn.c

  12. Make the order of the header file includes consistent