Unexpected Standby Shutdown on sync_replication_slots change

Hello,

I'm not sure if it's a bug but I've encountered an unexpected behavior when dynamically changing the sync_replication_slots parameter on a PostgreSQL 17 standby server. Instead of logging an error and continuing to run, the standby 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

1. Set up a physical replication between two PostgreSQL 17.5 instances.

2. Ensure wal_level on the primary (and consequently on the standby) is set to replica.

3. Start both the primary and standby instances, confirming replication is active.

4. On the standby instance, dynamically change the sync_replication_slots parameter (I have run the following query: ALTER SYSTEM 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 behaves when 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 anticipated behavior. The documentation for sync_replication_slots also doesn't indicate that a misconfiguration or incompatible wal_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 instance immediately 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