On Thu, Sep 24, 2026 at 3:49 PM Xuneng Zhou <[email protected]> wrote:
> Hi hackers,
>
> While working on eliminating potential deadlock risks of WAIT command, an
> agent noticed a similar risk for creating logical replication slots on hot
> standby. Here's how it goes:
>
> A statement such as:
>
> SELECT * FROM pg_create_logical_replication_slot('s', 'pgoutput');
>
> retains its executor-registered statement snapshot
> while DecodingContextFindStartpoint() searches for a consistent decoding
> point. Its WAL reader, read_local_xlog_page_guts(), limits reads to
> GetXLogReplayRecPtr().
>
> There seems to be a problematic ordering:
>
> 1) The slot-creation statement holds a snapshot and starts waiting for an
> additional WAL record to be replayed.
>
> 2) The operation in primary generates a cleanup record conflicting with
> that snapshot, for example through UPDATE followed by VACUUM while
> hot_standby_feedback
> is off
>
3) A running-transactions record needed to complete slot initialization,
> which follows the cleanup record
>
4) Startup process waits for the slot creator's snapshot before replaying
> cleanup, while the creator waits for the replay to advance
>
This seems serious if max_standby_streaming_delay = -1, since nothing left
> to break it automatically. I haven't found a report in the mailing lists
> since the debut of this feature. The reason for that could be an
> encounterance of it requires a fair coincidence of several factors and the
> symptom of being affected is manifested an interval of delay, which could
> be masked by the creation of the slot with finite delay setting
>
I don't see a clear solution to this potential issue, because the interface
> is a function, which means that the held snapshots cannot be popped cleanly
> since they belong to the surrounding executor.
>
Slot synchronization also has a replay-wait path that may participate in
> these cycles. I haven't looked into it yet.
>
Feedbacks are appreciated.
>
Sorry for the indentation issue..
--
Regards,
Xuneng Zhou
HighGo Software Co., Ltd.
