pgsql: Check CREATE_REPLICATION_SLOT response shape in libpqwalreceiver

Check CREATE_REPLICATION_SLOT response shape in libpqwalreceiver

Previously, libpqrcv_create_slot() checked only that
CREATE_REPLICATION_SLOT returned PGRES_TUPLES_OK before reading
values from the first row. If the server unexpectedly returned an
invalid result, such as zero rows, PQgetvalue() could return NULL,
leading to a crash while parsing the LSN.

Other replication commands, such as IDENTIFY_SYSTEM, already validate
the response shape before accessing result values, but
CREATE_REPLICATION_SLOT did not.

Fix this by verifying that CREATE_REPLICATION_SLOT response contains
exactly one row with four fields, and report a protocol violation otherwise.

Backpatch to all supported versions.

Bug: #19547
Reported-by: Yuelin Wang <1217816127(at)qq(dot)com>
Author: Kenny Chen <kennychen851228(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/19547-f7986f668f71e788@postgresql.org
Discussion: https://postgr.es/m/CAPXstDtW2iqe+DJAOTQTX+rRziJp2UhZSo1+HRj1COAtbu+nKw@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d7bb9f643b374112cf806b40def8e3783493ebc2

Modified Files
--------------
src/backend/replication/libpqwalreceiver/libpqwalreceiver.c | 8 ++++++++
1 file changed, 8 insertions(+)