Re: Unexpected Standby Shutdown on sync_replication_slots change

On Fri, Jul 25, 2025 at 12:55 AM Fujii Masao <masao.fujii@gmail.com> wrote:
>
> On Thu, Jul 24, 2025 at 10:54 PM Hugo DUBOIS <hdubois@scaleway.com> wrote:
> >
> > Hello,
> >
> > I'm not sure if it's a bug but I've encountered an unexpected behavior when dynamically changing the
sync_replication_slotsparameter on a PostgreSQL 17 standby server. Instead of logging an error and continuing to run,
thestandby instance shuts down with a FATAL error, which is not the anticipated behavior for a dynamic parameter
change,especially when the documentation doesn't indicate such an outcome. 
> >
> > Steps to Reproduce
> >
> > Set up a physical replication between two PostgreSQL 17.5 instances.
> >
> > Ensure wal_level on the primary (and consequently on the standby) is set to replica.
> >
> > Start both the primary and standby instances, confirming replication is active.
> >
> > On the standby instance, dynamically change the sync_replication_slots parameter (I have run the following query:
ALTERSYSTEM SET sync_replication_slots = 'on'; followed by SELECT pg_reload_conf();) 
> >
> > Expected Behavior
> >
> > I expected the standby instance to continue running and log an error message (similar to how hot_standby_feedback
behaveswhen not enabled, e.g., a loop of LOG: replication slot synchronization requires "hot_standby_feedback" to be
enabled).A FATAL error leading to an unexpected shutdown for a dynamic parameter change on a running standby is not the
anticipatedbehavior. The documentation for sync_replication_slots also doesn't indicate that a misconfiguration or
incompatiblewal_level would lead to a shutdown. 
> >
> > Actual Behavior
> >
> > Upon attempting to set sync_replication_slots to on on the standby with wal_level set to replica, the standby
instanceimmediately shuts down with the following log messages: 
> >
> > LOG:  database system is ready to accept read-only connections
> > LOG:  started streaming WAL from primary at 0/3000000 on timeline 1
> > LOG:  received SIGHUP, reloading configuration files
> > LOG:  parameter "sync_replication_slots" changed to "on"
> > FATAL:  replication slot synchronization requires "wal_level" >= "logical"
> >
> > Environment
> >
> > PostgreSQL Version: 17.5
>
> Thanks for the report!
>
> I was able to reproduce the issue even on the latest master (v19dev).
> I agree that the current behavior—where changing a GUC parameter can
> cause the server to shut down—is unexpected and should be avoided.
>
> From what I’ve seen in the code, the problem stems from postmaster
> calling ValidateSlotSyncParams() before starting the slot sync worker.
> That function raises an ERROR if wal_level is not logical while
> sync_replication_slots is enabled. Since ERROR is treated as FATAL
> in postmaster, it causes the server to exit.
>
> To fix this, we could modify ValidateSlotSyncParams() so it doesn’t
> raise an ERROR in this case, as follows.
>
>  ValidateSlotSyncParams(int elevel)
>  {
>   /*
>   * Logical slot sync/creation requires wal_level >= logical.
> - *
> - * Since altering the wal_level requires a server restart, so error out in
> - * this case regardless of elevel provided by caller.
>   */
>   if (wal_level < WAL_LEVEL_LOGICAL)
> - ereport(ERROR,
> + {
> + ereport(elevel,
>   errcode(ERRCODE_INVALID_PARAMETER_VALUE),
>   errmsg("replication slot synchronization requires \"wal_level\" >=
> \"logical\""));
> + return false;
> + }

I've created a patch to implement the above—attached.

Note that this patch does not change the existing behavior when
the misconfiguration (sync_replication_slots enabled but wal_level not
set to logical) is detected at server startup. In that case, the server
still shuts down with a FATAL error, which is consistent with other
settings like summarize_wal.

Regards,

--
Fujii Masao