Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

On Mon, 1 Dec 2025 at 12:33, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Mon, Dec 1, 2025 at 12:20 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> I've attached the patch. All the comments I got so far have been
> addressed in this version unless I'm missing something.

Few comments:
1) We have an invalidation test at 044_invalidate_inactive_slots.pl
which does an equivalent of this, I felt this can be removed and just
the test_wal_level can be done from 044_invalidate_inactive_slots:
+# Add other settings to test if we disable logical decoding when
invalidating the last
+# logical slot.
+$primary->append_conf(
+ 'postgresql.conf',
+ qq[
+min_wal_size = 32MB
+max_wal_size = 32MB
+max_slot_wal_keep_size = 16MB
+]);
+$primary->start;
+
+# Advance WAL and check if the slot gets invalidated.
+$primary->advance_wal(2);
+$primary->safe_psql('postgres', qq[CHECKPOINT]);
+is( $primary->safe_psql(
+ 'postgres',
+ qq[
+select invalidation_reason = 'wal_removed' from pg_replication_slots
where slot_name = 'test_slot';
+ ]),
+ 't',
+ 'test_slot gets invalidated due to wal_removed');
+
+# Check if logical decoding is disabled after invalidating the last
logical slot.
+wait_for_logical_decoding_disabled($primary);
+test_wal_level($primary, "replica|replica",
+ "effective_wal_level got decreased to 'replica' after
invalidating the last logical slot"
+);

+# Recreate the logical slot to enable logical decoding again.
+$primary->safe_psql('postgres',
+ qq[select pg_drop_replication_slot('test_slot')]);
+$primary->safe_psql('postgres',
+ qq[select pg_create_logical_replication_slot('test_slot', 'pgoutput')]);

2) What are we verifying here, as a logical replication slot can be
created without promotion too:
# Confirm if we can create a logical slot after the promotion.
$standby1->safe_psql('postgres',
qq[select pg_create_logical_replication_slot('standby1_slot2', 'pgoutput')]
);

If it is not required we can remove it.

3) This message seems wrong:
+ "cannot server with wal_level='minimal' as there is in-use
logical slot");
+
+my $logfile = slurp_file($primary->logfile());

Should it be:
cannot start server with wal_level='minimal'

Regards,
Vignesh