pgsql: Fix assertion failure during decoding from synced slots.

Fix assertion failure during decoding from synced slots.

The slot synchronization skips updating the confirmed_flush LSN of the
local slot if the local slot has a newer catalog_xmin or restart_lsn, but
still allows updating the two_phase and two_phase_at fields of the slot.
This opens up a window for the prepared transactions between old
confirmed_flush LSN and two_phase_at to unexpectedly get decoded and sent
to the downstream after promotion. Then, while decoding the commit
prepared the assert will fail, which expects that the prepare hasn't been
sent to the downstream.

The fix is to skip updating the other slot fields when we are skipping to
update the confirmed_flush LSN of the slot.

We didn't backpatch this commit as two_phase_at was not synced in back
branches, which means prepared transactions won't be unexpectedly sent to
downstream.

We discovered this problem while analyzing BF failure reported in the
discussion link.

Reliably reproducing this issue without a debugger is difficult. Given
its rarity, adding specific injection point to test it doesn't seem
worthwhile, so we won't be adding a dedicated test case.

Author: Zhijie Hou <houzj(dot)fnst(at)fujitsu(dot)com>
Reviewed-by: shveta malik <shveta(dot)malik(at)gmail(dot)com>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Discussion: https://postgr.es/m/OS0PR01MB5716B44052000EB91EFAE60E94BC2@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3ff2a1f0c9e8d016f3288f4da3962eb1f1592cd8

Modified Files
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src/backend/replication/logical/slotsync.c | 39 +++++++++++++++++++++---------
1 file changed, 27 insertions(+), 12 deletions(-)