Re: Follow-up fixes for online wal_level change

On 7/16/2026 8:09 AM, Masahiko Sawada wrote:
> Hi all,
>
> While reviewing the online wal_level change feature (commit
> 67c20979ce7), I found an assertion failure during crash recovery.
> Also, I noticed that pg_controldata doesn't show the logical decoding
> status. Patches attached.
>
> 0001 fixes the assertion failure that happens in the following scenario:
>
> 1. Run the server with wal_level = 'replica' and create a logical
> slot, which writes an XLOG_LOGICAL_DECODING_STATUS_CHANGE record that
> activates logical decoding.
> 2. Drop the slot, and crash the server before the checkpointer
> deactivates logical decoding.
> 3. Restart the server with wal_level = 'minimal'.

I can reproduce this crash following above procedure. But in step 3, I also needed to set max_wal_senders=0, otherwise the serve failed to start with error: FATAL: WAL streaming ("max_wal_senders" > 0) requires "wal_level" to be "replica" or “logical".

> Crash recovery replays the status change record and activates logical
> decoding, then UpdateLogicalDecodingStatusEndOfRecovery() fails the
> assertion (Assert(!IsXLogLogicalInfoEnabled() &&
> !IsLogicalDecodingEnabled())) that logical decoding is never active
> with wal_level='minimal'.
>
> I think that replaying the status change record itself is correct, as
> it reflects the status at the time the record was written. The problem
> is that the end-of-recovery code assumed that this cannot happen,
> instead of adjusting the status. Therefore, the fix removes the
> wal_level='minimal' special case from
> UpdateLogicalDecodingStatusEndOfRecovery() so that it recomputes the
> status as usual.
>
> 0002 adds the logical decoding status stored in the checkpoint record
> to the pg_controldata output, which I found useful while investigating
> the above issue.
>
> Both patches are intended to be backpatched to v19.
>
> Regards,

Both 0001 and 0002 LGTM.

Regards,
Guoqing Yang