pgsql: Fix a race in 035_standby_logical_decoding.pl

Fix a race in 035_standby_logical_decoding.pl

Buildfarm animals francolin and kestrel reported intermittent failures
in 035_standby_logical_decoding.pl.

The test checks that replaying DROP DATABASE on a standby terminates
pg_recvlogical with the expected recovery-conflict message and removes
the database's replication slots. To exercise this while pg_recvlogical
is streaming, the test must wait for streaming to start before dropping
the database on the primary.

Previously, the test treated an active slot in pg_replication_slots as
confirmation that pg_recvlogical was streaming. However, the slot can
become active before pg_recvlogical has entered streaming mode. If
DROP DATABASE is replayed during this window, pg_recvlogical can report
a connection-closed error instead of the recovery-conflict message,
causing the test to fail.

Run pg_recvlogical with --verbose and wait for "streaming initiated"
instead of checking whether the slot is active. This ensures that
pg_recvlogical has entered streaming mode before the database is
dropped on the primary.

Back-patch to v16, where this test was introduced.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/a50ff89b-fb2c-4912-8f89-50dd0bd0916b@gmail.com
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c1b3845ade341d61ed5749f428e032ad75b755b7

Modified Files
--------------
src/test/recovery/t/035_standby_logical_decoding.pl | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)