Catalog_xmin is not advanced when a logical slot is lost

Hi Hackers,

forking this thread from the discussion [1] as suggested by Amit.

Catalog_xmin is not advanced when a logical slot is invalidated (lost)
until the invalidated slot is dropped. This patch ignores invalidated slots
while computing the oldest xmin. Attached a small patch to address this and
the output after the patch is as shown below.

postgres=# select * from pg_replication_slots;
slot_name | plugin | slot_type | datoid | database | temporary |
active | active_pid | xmin | catalog_xmin | restart_lsn |
confirmed_flush_lsn | wal_status | safe_wal_size | two_phase
-----------+---------------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------+------------+---------------+-----------
s2 | test_decoding | logical | 5 | postgres | f | f
| | | 771 | 0/30466368 | 0/304663A0
| reserved | 28903824 | f
(1 row)

postgres=# create table t2(c int, c1 char(100));
CREATE TABLE
postgres=# drop table t2;
DROP TABLE
postgres=# vacuum pg_class;
VACUUM
postgres=# select n_dead_tup from pg_stat_all_tables where relname =
'pg_class';
n_dead_tup
------------
2
(1 row)

postgres=# select * from pg_stat_replication;
pid | usesysid | usename | application_name | client_addr |
client_hostname | client_port | backend_start | backend_xmin | state |
sent_lsn | write_lsn | flush_lsn | replay_lsn | write_lag | flush_lag |
replay_lag | sync_pri
ority | sync_state | reply_time
-----+----------+---------+------------------+-------------+-----------------+-------------+---------------+--------------+-------+----------+-----------+-----------+------------+-----------+-----------+------------+---------
------+------------+------------
(0 rows)

postgres=# insert into t1 select * from t1;
INSERT 0 2097152
postgres=# checkpoint;
CHECKPOINT
postgres=# select * from pg_replication_slots;
slot_name | plugin | slot_type | datoid | database | temporary |
active | active_pid | xmin | catalog_xmin | restart_lsn |
confirmed_flush_lsn | wal_status | safe_wal_size | two_phase
-----------+---------------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------+------------+---------------+-----------
s2 | test_decoding | logical | 5 | postgres | f | f
| | | 771 | | 0/304663A0
| lost | | f
(1 row)

postgres=# vacuum pg_class;
VACUUM
postgres=# select n_dead_tup from pg_stat_all_tables where relname =
'pg_class';
n_dead_tup
------------
0
(1 row)

[1]
https://www.postgresql.org/message-id/flat/CAKrAKeW-sGqvkw-2zKuVYiVv%3DEOG4LEqJn01RJPsHfS2rQGYng%40mail.gmail.com

Thanks,
Sirisha