Re: pgsql: Fix skip-empty-xacts with sequences in test_decoding

On 2022-Feb-12, Tomas Vondra wrote:

> Fix skip-empty-xacts with sequences in test_decoding
>
> Regression tests need to use skip-empty-xacts = false, because there
> might be accidental concurrent activity (like autovacuum), particularly
> on slow machines. The tests added by 80901b3291 failed to do that in a
> couple places, triggering occasional failures on buildfarm.

I think you meant "... need to use skip-empty-xacts = true", since you
changed the value from 0 to 1, and the point is precisely to *skip*
those transactions. (The hidden double negative "skip=false" seems
quite confusing BTW.)

> Fixing the tests however uncovered a bug in the code, because sequence
> callbacks did not handle skip-empty-xacts properly. For trasactional
> increments we need to check/update the xact_wrote_changes flag, and emit
> the BEGIN if it's the first change in the transaction.

Hmm. Perhaps there should be a separate test script that runs various
things under skip-empty-xacts=0 and somehow protected against ancillary
activities (maybe a TAP test using autovacuum=0), just so that this new
code is covered ...

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