Horton

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

Peter Smith <smithpb2250@gmail.com>

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From: Peter Smith <smithpb2250@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Peter.B.Smith@fujitsu.com" <Peter.B.Smith@fujitsu.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Masahiko Sawada <sawada.mshk@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-10-09T00:14:24Z
Lists: pgsql-hackers 
On Thu, Oct 8, 2020 at 5:25 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > COMMENT
> > Line 177
> > +logicalrep_read_prepare(StringInfo in, LogicalRepPrepareData * prepare_data)
> >
> > prepare_data->prepare_type = flags;
> > This code may be OK but it does seem a bit of an abuse of the flags.
> >
> > e.g. Are they flags or are the really enum values?
> > e.g. And if they are effectively enums (it appears they are) then
> > seemed inconsistent that |= was used when they were previously
> > assigned.
> >
> > ;
>
> I don't understand this point. As far as I can see at the time of
> write (logicalrep_write_prepare()), the patch has used |=, and at the
> time of reading (logicalrep_read_prepare()) it has used assignment
> which seems correct from the code perspective. Do you have a better
> proposal?

OK. I will explain my thinking when I wrote that review comment.

I agree all is "correct" from a code perspective.

But IMO using bit arithmetic implies that different combinations are
also possible, whereas in current code they are not.
So code is kind of having a bet each-way - sometimes treating "flags"
as bit flags and sometimes as enums.

e.g. If these flags are not really bit flags at all then the
logicalrep_write_prepare() code might just as well be written as
below:

BEFORE
if (rbtxn_commit_prepared(txn))
flags |= LOGICALREP_IS_COMMIT_PREPARED;
else if (rbtxn_rollback_prepared(txn))
flags |= LOGICALREP_IS_ROLLBACK_PREPARED;
else
flags |= LOGICALREP_IS_PREPARE;

/* Make sure exactly one of the expected flags is set. */
if (!PrepareFlagsAreValid(flags))
elog(ERROR, "unrecognized flags %u in prepare message", flags);


AFTER
if (rbtxn_commit_prepared(txn))
flags = LOGICALREP_IS_COMMIT_PREPARED;
else if (rbtxn_rollback_prepared(txn))
flags = LOGICALREP_IS_ROLLBACK_PREPARED;
else
flags = LOGICALREP_IS_PREPARE;

~

OTOH, if you really do want to anticipate having future flag bit
combinations then maybe the PrepareFlagsAreValid() macro ought to to
be tweaked accordingly, and the logicalrep_read_prepare() code maybe
should look more like below:

BEFORE
/* set the action (reuse the constants used for the flags) */
prepare_data->prepare_type = flags;

AFTER
/* set the action (reuse the constants used for the flags) */
prepare_data->prepare_type =
flags & LOGICALREP_IS_COMMIT_PREPARED ? LOGICALREP_IS_COMMIT_PREPARED :
flags & LOGICALREP_IS_ROLLBACK_PREPARED ? LOGICALREP_IS_ROLLBACK_PREPARED :
LOGICALREP_IS_PREPARE;

Kind Regards.
Peter Smith
Fujitsu Australia

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