Horton

Re: Fix slot synchronization with two_phase decoding enabled

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-04-18T18:18:36Z
Lists: pgsql-hackers 
On Tue, Apr 8, 2025 at 10:14 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
>
> ----------
> Approach 2
> ----------
>
> Instead of disallowing the use of two-phase and failover together, a more
> flexible strategy could be only restrict failover for slots with two-phase
> enabled when there's a possibility of existing prepared transactions before the
> two_phase_at that are not yet replicated. During slot creation with two-phase
> and failover, we could check for any decoded prepared transactions when
> determining the decoding start point (DecodingContextFindStartpoint). For
> subsequent attempts to alter failover to true, we ensure that two_phase_at is
> less than restart_lsn, indicating that all prepared transactions have been
> committed and replicated, thus the bug would not happen.
>
> pros:
>
> This method minimizes restrictions for users. Especially during slot creation
> with (two_phase=on, failover=on), as it’s uncommon for transactions to prepare
> during consistent snapshot creation, the restriction becomes almost
> unnoticeable.

I think this approach can work for the transactions that are prepared
while the slot is created. But if I understand the problem correctly,
while the initial table sync is performing, the slot's two_phase is
still false, so we need to deal with the transactions that are
prepared during the initial table sync too. What do you think?

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

Commits

  1. Don't retreat slot's confirmed_flush LSN.

  2. Fix assertion failure during decoding from synced slots.

  3. Fix xmin advancement during fast_forward decoding.

  4. Fix slot synchronization for two_phase enabled slots.

  5. Again match pg_user_mappings to information_schema.user_mapping_options.