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

Hi, Alexander, Here, I made some improvements according to your 
discussion with Heikki.

On 2024-04-11 18:09, Alexander Korotkov wrote:
> On Thu, Apr 11, 2024 at 1:46 AM Heikki Linnakangas <hlinnaka@iki.fi> 
> wrote:
>> In a nutshell, it's possible for the loop in WaitForLSN to exit 
>> without
>> cleaning up the process from the heap. I was able to hit that by 
>> adding
>> a delay after the addLSNWaiter() call:
>> 
>> > TRAP: failed Assert("!procInfo->inHeap"), File: "../src/backend/commands/waitlsn.c", Line: 114, PID: 1936152
>> > postgres: heikki postgres [local] CALL(ExceptionalCondition+0xab)[0x55da1f68787b]
>> > postgres: heikki postgres [local] CALL(+0x331ec8)[0x55da1f204ec8]
>> > postgres: heikki postgres [local] CALL(WaitForLSN+0x139)[0x55da1f2052cc]
>> > postgres: heikki postgres [local] CALL(pg_wal_replay_wait+0x18b)[0x55da1f2056e5]
>> > postgres: heikki postgres [local] CALL(ExecuteCallStmt+0x46e)[0x55da1f18031a]
>> > postgres: heikki postgres [local] CALL(standard_ProcessUtility+0x8cf)[0x55da1f4b26c9]
>> 
>> I think there's a similar race condition if the timeout is reached at
>> the same time that the startup process wakes up the process.
> 
> Thank you for catching this.  I think WaitForLSN() just needs to call
> deleteLSNWaiter() unconditionally after exit from the loop.

Fix and add injection point test on this race condition.

> On Thu, Apr 11, 2024 at 1:46 AM Heikki Linnakangas <hlinnaka@iki.fi> 
> wrote:
>> The docs could use some-copy-editing, but just to point out one issue:
>> 
>> > There are also procedures to control the progress of recovery.
>> 
>> That's copy-pasted from an earlier sentence at the table that lists
>> functions like pg_promote(), pg_wal_replay_pause(), and
>> pg_is_wal_replay_paused(). The pg_wal_replay_wait() doesn't control 
>> the
>> progress of recovery like those functions do, it only causes the 
>> calling
>> backend to wait.

Fix documentation and add extra tests on multi-standby replication
and cascade replication.

-- 
Ivan Kartyshov
Postgres Professional: www.postgrespro.com