pgsql: Fix incorrect WAL description for PREPARE TRANSACTION record.

Fix incorrect WAL description for PREPARE TRANSACTION record.

Since commit 8b1dccd37c7, the PREPARE TRANSACTION WAL record includes
information about dropped statistics entries. However, the WAL resource
manager description function for PREPARE TRANSACTION record failed to
parse this information correctly and always assumed there were
no such entries.

As a result, for example, pg_waldump could not display the dropped
statistics entries stored in PREPARE TRANSACTION records.

The root cause was that ParsePrepareRecord() did not set the number of
statistics entries to drop on commit or abort. These values remained
zero-initialized and were never updated from the parsed record.

This commit fixes the issue by properly setting those values during parsing.
With this fix, pg_waldump can now correctly report dropped statistics
entries in PREPARE TRANSACTION records.

Back-patch to v15, where commit 8b1dccd37c7 was introduced.

Author: Daniil Davydov <3danissimo(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAJDiXgh-6Epb2XiJe4uL0zF-cf0_s_7Lw1TfEHDMLzYjEmfGOw@mail.gmail.com
Backpatch-through: 15

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0d2063585876e1333956ae8e3b5efae4b0cff083

Modified Files
--------------
src/backend/access/rmgrdesc/xactdesc.c | 2 ++
1 file changed, 2 insertions(+)