pgsql: Don't retreat slot's confirmed_flush LSN.

Don't retreat slot's confirmed_flush LSN.

Prevent moving the confirmed_flush backwards, as this could lead to data
duplication issues caused by replicating already replicated changes.

This can happen when a client acknowledges an LSN it doesn't have to do
anything for, and thus didn't store persistently. After a restart, the
client can send the prior LSN that it stored persistently as an
acknowledgement, but we need to ignore such an LSN to avoid retreating
confirm_flush LSN.

Diagnosed-by: Zhijie Hou <houzj(dot)fnst(at)fujitsu(dot)com>
Author: shveta malik <shveta(dot)malik(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Tested-by: Nisha Moond <nisha(dot)moond412(at)gmail(dot)com>
Backpatch-through: 13
Discussion: https://postgr.es/m/CAJpy0uDZ29P=BYB1JDWMCh-6wXaNqMwG1u1mB4=10Ly0x7HhwQ@mail.gmail.com
Discussion: https://postgr.es/m/OS0PR01MB57164AB5716AF2E477D53F6F9489A@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e323d9df002debb103a58ba2479368c46804a9b6

Modified Files
--------------
src/backend/replication/logical/logical.c | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)