Horton

Re: [HACKERS] logical decoding of two-phase transactions

vignesh C <vignesh21@gmail.com>

View thread View on postgr.es
From: vignesh C <vignesh21@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2021-03-08T14:39:10Z
Lists: pgsql-hackers 
On Mon, Mar 8, 2021 at 6:25 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Mar 8, 2021 at 4:20 PM vignesh C <vignesh21@gmail.com> wrote:
> >
> > On Mon, Mar 8, 2021 at 11:30 AM Ajin Cherian <itsajin@gmail.com> wrote:
> > >
> > > On Fri, Mar 5, 2021 at 9:25 PM vignesh C <vignesh21@gmail.com> wrote:
> > >
> > >
> > > Created new patch v53:
> >
> > Thanks for the updated patch.
> > I had noticed one issue, publisher does not get stopped normally in
> > the following case:
> > # Publisher steps
> > psql -d postgres -c "CREATE TABLE do_write(id serial primary key);"
> > psql -d postgres -c "INSERT INTO do_write VALUES(generate_series(1,10));"
> > psql -d postgres -c "CREATE PUBLICATION mypub FOR TABLE do_write;"
> >
> > # Subscriber steps
> > psql -d postgres -p 9999 -c "CREATE TABLE do_write(id serial primary key);"
> > psql -d postgres -p 9999 -c "INSERT INTO do_write VALUES(1);" # to
> > cause a PK violation
> > psql -d postgres -p 9999 -c "CREATE SUBSCRIPTION mysub CONNECTION
> > 'host=localhost port=5432 dbname=postgres' PUBLICATION mypub WITH
> > (two_phase = true);"
> >
> > # prepare & commit prepared at publisher
> > psql -d postgres -c \
> > "begin; insert into do_write values (100); prepare transaction 'test1';"
> > psql -d postgres -c "commit prepared 'test1';"
> >
> > Stop publisher:
> > ./pg_ctl -D publisher stop
> > waiting for server to shut
> > down...............................................................
> > failed
> > pg_ctl: server does not shut down
> >
> > This is because the following process does not exit:
> > postgres: walsender vignesh 127.0.0.1(41550) START_REPLICATION
> >
> > It continuously loops at the below:
> >
>
> What happens if you don't set the two_phase option? If that also leads
> to the same error then can you please also check this case on the
> HEAD?

It succeeds without the two_phase option.
I had further analyzed this issue, see the details of it below:
We have the below code in WalSndDone function which will handle the
walsender exit:
if (WalSndCaughtUp && sentPtr == replicatedPtr &&
!pq_is_send_pending())
{
QueryCompletion qc;

/* Inform the standby that XLOG streaming is done */
SetQueryCompletion(&qc, CMDTAG_COPY, 0);
EndCommand(&qc, DestRemote, false);
pq_flush();

proc_exit(0);
}

But in case of with two_phase option, replicatedPtr and sentPtr never
becomes same:
(gdb) p /x replicatedPtr
$8 = 0x15faa70
(gdb) p /x sentPtr
$10 = 0x15fac50

Whereas in case of without two_phase option, replicatedPtr and sentPtr
becomes same and exits:
(gdb) p /x sentPtr
$7 = 0x15fae10
(gdb) p /x replicatedPtr
$8 = 0x15fae10

I think in case of two_phase option, replicatedPtr and sentPtr never
becomes the same which causes this process to hang.

Regards,
Vignesh

Commits

  1. Add prepare API support for streaming transactions in logical replication.

  2. Doc: minor improvements for logical replication protocol documentation.

  3. Fix test failure in 021_twophase.pl.

  4. Refactor to make common functions in proto.c and worker.c.

  5. Unify PostgresNode's new() and get_new_node() methods

  6. Fix potential buffer overruns in proto.c.

  7. Add support for prepared transactions to built-in logical replication.

  8. Refactor function parse_subscription_options.

  9. Allow enabling two-phase option via replication protocol.

  10. Don't use Asserts to check for violations of replication protocol.

  11. Improve psql tab completion for options of subcriptions and publications

  12. Rearrange logrep worker's snapshot handling some more.

  13. doc: Update information of new messages for logical replication.

  14. ALTER SUBSCRIPTION ... ADD/DROP PUBLICATION

  15. Allow pgoutput to send logical decoding messages.

  16. Refactor function parse_output_parameters.

  17. Avoid repeated decoding of prepared transactions after a restart.

  18. Fix an oversight in ReorderBufferFinishPrepared.

  19. Allow multiple xacts during table sync in logical replication.

  20. Fix replication of in-progress transactions in tablesync worker.

  21. Fix 'skip-empty-xacts' option in test_decoding for streaming mode.

  22. Use Enum for top level logical replication message types.

  23. Add support for streaming to built-in logical replication.

  24. Fix the logical streaming test.

  25. Implement streaming mode in ReorderBuffer.

  26. Extend the logical decoding output plugin API with stream methods.

  27. Store 2PC GID in commit/abort WAL recs for logical decoding