Replication should now work from PG 16 Standbys

[saisrirampur]

I tried PeerDB on Read Replicas in PG 16:

1. Initial Load worked as expected
2. SyncFlows were erroring out:

ERROR:  recovery is in progress
HINT:  WAL control functions cannot be executed during recovery.

The reason was pg_current_wal_lsn() cannot be run on a standby. I changed that function to pg_last_wal_replay_lsn() if Replica.

[saisrirampur]

@Amogh-Bharadwaj I am still seeing issues on the UI because to get slot information because we use pg_current_wal_lsn() in the GetSlotInfo function. That needs separate fixing.

[serprex]

Can you try keeping it down to a single query roundtrip using

CASE WHEN pg_is_in_recovery()
  THEN pg_last_wal_receive_lsn()
  ELSE pg_current_wal_lsn()
END

GetSlotInfo could use the same expression

[saisrirampur]

Below works @serprex. Just checked on a standby server. This PR introduces isReplica() function which might be useful to be present in code otherwise. However I'll leave it to you on deciding the right approach.

CASE WHEN pg_is_in_recovery()
  THEN pg_last_wal_receive_lsn()
  ELSE pg_current_wal_lsn()
END

[serprex]

Tested. Problem: CREATE PUBLICATION has to be run on primary, not standby. So a followup will need to be made where mirror can be configured to create publication on a different peer than source. But in the meantime one can create the publication slot for peerdb based on error message & things work after that