Re: BUG #15594: Unstable tests in contrib/test_decoding/output_iso/

17.01.2019 13:25, Petr Jelinek wrote:
> On 16/01/2019 20:09, Andres Freund wrote:
>> On 2019-01-16 14:56:34 +0000, PG Bug reporting form wrote:
>>> The following bug has been logged on the website:
>>>
>>> Bug reference:      15594
>>> Logged by:          Alexander Lakhin
>>> Email address:      exclusion@gmail.com
>>> PostgreSQL version: 11.1
>>> Operating system:   Ubuntu 18.04
>>> Description:        
>>>
>>> Some tests in contrib/test_decoding/output_iso/ occasionally fail.
>>> To reproduce it reliable you can do:
>>> # sed 's/ mxact / $(shell printf "mxact %.0s" `seq 1000` ) /' -i
>>> contrib/test_decoding/Makefile
>>> # make check -C contrib/test_decoding/
>>>
>>> ...
>>>
>>> I observe this issue with the tests: mxact, delayed_startup, and
>>> ondisk_startup.
>> Interesting - I've not seen that on the buildfarm before, or in local tests
>> with a lot of repetitions. I assume the problem vanishes if you add 'skip-empty-xacts', '1'
>> to the options of pg_logical_slot_get_changes()?
>>
> I bet those empty transactions are from autovacuum. The probability of
> hitting this could be exaggerated by the fact that the test is run
> within the same instance 1000x rather than running the make check 1000x
> so we produce plenty of catalog bloat for autovacuum to clean.
>
Yes, it's almost impossible to hit this with `make check`, but with
`make installcheck` the probability is much higher as we use an existing
instance.
I've reproduced it with `make check` just to make it simple.

Best regards,
Alexander