Horton

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

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>, shveta malik <shveta.malik@gmail.com>
Date: 2025-07-29T05:02:35Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix regression test failure when wal_level is set to minimal.

  2. Toggle logical decoding dynamically based on logical slot presence.

  3. Disallow server start with sync_replication_slots = on and wal_level < logical.

On Fri, Jul 25, 2025 at 11:45 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> Thank you for testing the patch!
>
> I've reworked the locking part in the patch. The attached v4 patch
> should address all review comments including your previous
> comments[1].
>

Thank You for the patch. I have not reviewed fully, but please find
few comments:

1)
CreateReplicationSlot():

  Assert(cmd->kind == REPLICATION_KIND_LOGICAL);
 + EnsureLogicalDecodingEnabled();
  CheckLogicalDecodingRequirements();
  ReplicationSlotCreate(...);

We may have another race-condition here. We have
EnsureLogicalDecodingEnabled() before ReplicationSlotCreate(). It
means we are enabling logical-decoding before incrementing
LogicalDecodingCtl->n_logical_slots. So before we increment
LogicalDecodingCtl->n_logical_slots through ReplicationSlotCreate(),
another session may try to meanwhile drop the logical slot (another
one and last one), and thus it may end up disabling logical-decoding
as it will find n_logical_slots as 0.

Steps:
a) Create logical slot logical_slot1 on primary.
b) Create  publication pub1.
c) During Create-sub on subscriber, stop walsender after
EnsureLogicalDecodingEnabled() by attaching debugger.
d) Drop logical_slot1 on primary.
e) Release the walsender debugger.


2)
create_logical_replication_slot:

ReplicationSlotCreate(name, true
....
+ EnsureLogicalDecodingEnabled();
+ CheckLogicalDecodingRequirements();

Earlier we had CheckLogicalDecodingRequirements() before we actually
created the slot. Now we had it after slot-creation. It makes sense to
do Logical-Decoding related checks post EnsureLogicalDecodingEnabled,
but 'CheckSlotRequirements' should be done prior to slot-creation.
Otherwise we will end up creating the slot and later dropping it when
it should not have been created in the first place (for say wal_level
< replica).


3)
+ EnsureLogicalDecodingEnabled();
+

We can get rid of this from slotsync as this is no-op on standby


4)
pg_sync_replication_slots()
        if (!RecoveryInProgress())
                ereport(ERROR,

errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                errmsg("replication slots can only be
synchronized to a standby server"));

+ EnsureLogicalDecodingEnabled();

This API is called on standby alone, so EnsureLogicalDecodingEnabled
is not needed here either.

thanks
Shveta