Re: pgsql: Skip empty transaction stream in test_decoding.

On Mon, Nov 9, 2020 at 8:40 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Mon, Nov 9, 2020 at 7:11 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> >
> > /home/pgbfarm/buildroot/HEAD/pgsql.build/contrib/test_decoding/output_iso/results/concurrent_stream.out
> > 2020-11-08 12:31:10 -0700
> > @@ -13,7 +13,6 @@
> > opening a streamed block for transaction
> > streaming change for transaction
> > closing a streamed block for transaction
> > -committing streamed transaction
> > ?column?
> >
> > This smells like a race condition.
> >
>
> It is possible. I see that autovacuum has triggered (as per below
> logs) during this test which might have some impact on the output but
> I am not sure at this stage. I'll look into this.
>

I am able to reproduce this issue locally. This is an issue in
test_decoding. The problem is that in a streaming transaction just
before the last stream (which has no changes but a commit message)
another concurrent empty transaction occurred (which in this case is
caused by autovacuum). Now, in test_dcoding plugin, we track whether
there is any change occured at the LogicalDecodingContext level
(xact_wrote_changes) due to which the empty transaction in above
context can toggle the flag and commit of streaming transaction will
get confused. The first idea that occured to me to fix this issue is
to keep track of whether we have decoded any changes at transaction
level (ReorderBufferTxn) instead of keeping at LogicalDecodingContext
level as streaming transactions can be interleaved with other
transactions which was not the case previously. I'll think more on
this and start a thread on pgsql-hackers.

--
With Regards,
Amit Kapila.