BUG #14228: replication slot catalog_xmin not cleared on slot reuse

The following bug has been logged on the website:

Bug reference: 14228
Logged by: Andrew Gierth
Email address: andrew(at)tao11(dot)riddles(dot)org(dot)uk
PostgreSQL version: 9.6beta2
Operating system: any
Description:

[My analysis of a bug reported on IRC; please CC original reporter: md at
chewy dot com]

Reported on pg 9.4.6 but also reproduced on 9.6beta2

When creating a physical replication slot, the catalog_xmin field of the new
slot is not initialized. If the slot storage had previously been used for a
logical slot, the old catalog_xmin will remain in place and interfere with
vacuum.

Trivially reproducible:

select pg_create_logical_replication_slot('test','test_decoding');
select pg_drop_replication_slot('test');
select pg_create_physical_replication_slot('test');
select * from pg_replication_slots;
slot_name | plugin | slot_type | datoid | database | active | active_pid |
xmin | catalog_xmin | restart_lsn | confirmed_flush_lsn
-----------+--------+-----------+--------+----------+--------+------------+------+--------------+-------------+---------------------
test | | physical | | | f | |
| 546 | | 0/1525438
(1 row)