Current LSN : Account for NULL LSN value being returned

[Amogh-Bharadwaj]

// ParseLSN parses the given XXX/XXX text format LSN used by PostgreSQL.
func ParseLSN(s string) (LSN, error) {
	var upperHalf uint64
	var lowerHalf uint64
	var nparsed int
	nparsed, err := fmt.Sscanf(s, "%X/%X", &upperHalf, &lowerHalf)
	if err != nil {
		return 0, fmt.Errorf("failed to parse LSN: %w", err)
	}

	if nparsed != 2 {
		return 0, fmt.Errorf("failed to parsed LSN: %s", s)
	}

	return LSN((upperHalf << 32) + lowerHalf), nil
}

It was recently seen for a read replica mirror that the above pglogrepl function we use to get the current LSN returned an EOF error at the first check - indicating that the LSN string we were giving it was not of the right format.

Upon research it seems like the functions in the query we run:

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

can return NULL as seen here for example.

It seems that this can occur when there is no more WAL activity to collect, not sure

This PR simply adds more logging around this.