Re: Notify downstream to discard the streamed transaction which was aborted due to crash.

On Fri, Jan 6, 2023 at 9:25 AM houzj(dot)fnst(at)fujitsu(dot)com
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> Hi,
>
> When developing another feature, I find an existing bug which was reported from Dilip[1].
>
> Currently, it's possible that we only send a streaming block without sending a
> end of stream message(stream abort) when decoding and streaming a transaction
> that was aborted due to crash because we might not WAL log a XLOG_XACT_ABORT
> for such a crashed transaction. This will cause the subscriber(with
> streaming=on) create a stream file but won't delete it until the apply
> worker restart.
>
> BUG repro(borrowed from Dilip):
> ---
> 1. start 2 servers(config: logical_decoding_work_mem=64kB)
> ./pg_ctl -D data/ -c -l pub_logs start
> ./pg_ctl -D data1/ -c -l sub_logs start
>
> 2. Publisher:
> create table t(a int PRIMARY KEY ,b text);
> create publication test_pub for table t
> with(PUBLISH='insert,delete,update,truncate');
> alter table t replica identity FULL ;
>
> 3. Subscription Server:
> create table t(a int,b text);
> create subscription test_sub CONNECTION 'host=localhost port=10000
> dbname=postgres' PUBLICATION test_pub WITH ( slot_name =
> test_slot_sub1,streaming=on);
>
> 4. Publication Server:
> begin ;
> insert into t values (generate_series(1,50000), 'zzzzzzzzz'); -- (while executing this restart publisher in 2-3 secs)
>
> Restart the publication server, while the transaction is still in an
> uncommitted state.
> ./pg_ctl -D data/ -c -l pub_logs restart -mi
> ---
>
> After restarting the publisher, we can see the subscriber receive a streaming
> block and create a stream file(/base/pgsql_tmp/xxx.fileset).
>
> To fix it, One idea is to send a stream abort message when we are cleaning up
> crashed transaction on publisher(e.g. in ReorderBufferAbortOld()). And here is
> a tiny patch which changes the same. I have confirmed that the bug is fixed and
> all regression tests pass. I didn't add a testcase because we need to make sure
> the crash happens before all the WAL logged transactions data are decoded which
> doesn't seem easy to write a stable test for this.
>
> Thoughts ?

Fix looks good to me. Thanks for working on this.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com