Re: Fix assert failure when decoding XLOG_PARAMETER_CHANGE on primary

On Fri, Jan 24, 2025 at 4:05 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> When a standby replays a XLOG_PARAMETER_CHANGE record that lowers
> wal_level from logical, we invalidate all logical slots only when the
> standby is in hot standby mode:
>
> if (InRecovery && InHotStandby &&
> xlrec.wal_level < WAL_LEVEL_LOGICAL &&
> wal_level >= WAL_LEVEL_LOGICAL)
> InvalidateObsoleteReplicationSlots(RS_INVAL_WAL_LEVEL,
> 0, InvalidOid,
> InvalidTransactionId);
>
> However, it's possible that this record is replayed when not in hot
> standby mode and the slot is used after the promotion. In this case,
> the following Assert in xlog_decode() fails:
>
> /*
> * This can occur only on a standby, as a primary would
> * not allow to restart after changing wal_level < logical
> * if there is pre-existing logical slot.
> */

Shouldn't we do similar to what this comment indicates on standby? We
can disallow to start the server as standby, if the hot_standby is off
and there is a pre-existing logical slot.

> Assert(RecoveryInProgress());
> ereport(ERROR,
> (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> errmsg("logical decoding on standby requires \"wal_level\" >=
> \"logical\" on the primary")));
>
> Here is brief steps to reproduce this issue:
>
> 1. setup the primary and the standby servers (with hot_standby=on).
> 2. create a logical slot on the standby.
> 3. on standby, set hot_standby=off and restart.
> 4. on primary, lower wal_level to replica and restart.
> 5. promote the standby and execute the logical decoding.
>
> I've attached a small patch to fix this issue. With the patch, we
> invalidate all logical slots even when not in hot standby, that is,
> the patch just removes InHotStandby from the condition. Thoughts?
>

This can fix the scenario you described but I am not sure it is a good
idea. We can consider the fix on these lines if you don't like the
above suggestion.

--
With Regards,
Amit Kapila.