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

On 11/04/2024 18:09, Alexander Korotkov wrote:
> On Thu, Apr 11, 2024 at 1:46 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> On 07/04/2024 00:52, Alexander Korotkov wrote:
>>>         * At first, we check that pg_wal_replay_wait() is called in a non-atomic
>>>         * context.  That is, a procedure call isn't wrapped into a transaction,
>>>         * another procedure call, or a function call.
>>>         *
>>
>> It's pretty unfortunate to have all these restrictions. It would be nice
>> to do:
>>
>> select pg_wal_replay_wait('0/1234'); select * from foo;
> 
> This works for me, except that it needs "call" not "select".
> 
> # call pg_wal_replay_wait('0/1234'); select * from foo;
> CALL
>   i
> ---
> (0 rows)

If you do that from psql prompt, it works because psql parses and sends 
it as two separate round-trips. Try:

psql postgres -p 5433 -c "call pg_wal_replay_wait('0/4101BBB8'); select 1"
ERROR:  pg_wal_replay_wait() must be only called in non-atomic context
DETAIL:  Make sure pg_wal_replay_wait() isn't called within a 
transaction, another procedure, or a function.

>> This assumption that PortalRunUtility() can tolerate us popping the
>> snapshot sounds very fishy. I haven't looked at what's going on there,
>> but doesn't sound like a great assumption.
> 
> This is what PortalRunUtility() says about this.
> 
> /*
>   * Some utility commands (e.g., VACUUM) pop the ActiveSnapshot stack from
>   * under us, so don't complain if it's now empty.  Otherwise, our snapshot
>   * should be the top one; pop it.  Note that this could be a different
>   * snapshot from the one we made above; see EnsurePortalSnapshotExists.
>   */
> 
> So, if the vacuum pops a snapshot when it needs to run without a
> snapshot, then it's probably OK for other utilities.  But I agree this
> decision needs some consensus.

Ok, so it's at least somewhat documented that it's fine.

>> Overall, this feature doesn't feel quite ready for v17, and IMHO should
>> be reverted. It's a nice feature, so I'd love to have it fixed and
>> reviewed early in the v18 cycle.
> 
> Thank you for your review.  I've reverted this. Will repost this for early v18.

Thanks Alexander for working on this.

-- 
Heikki Linnakangas
Neon (https://neon.tech)