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

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/
> 
> ...
> test delayed_startup              ... ok
> test ondisk_startup               ... ok
> test concurrent_ddl_dml           ... ok
> test oldest_xmin                  ... ok
> test snapshot_transfer            ... ok
> ============== shutting down postmaster               ==============
> 
> ==========================
>  19 of 1005 tests failed. 
> ==========================

> The differences that caused some tests to fail can be viewed in the
> file ".../contrib/test_decoding/output_iso/regression.diffs".  A copy of the
> test summary that you see
> above is saved in the file
> ".../contrib/test_decoding/output_iso/regression.out".
> 
> The diff file contains:
> --- .../contrib/test_decoding/expected/delayed_startup.out  2018-02-08
> 16:57:42.816744192 +0300
> +++ .../contrib/test_decoding/output_iso/results/delayed_startup.out       
> 2019-01-16 17:46:33.138962021 +0300
> @@ -12,6 +12,8 @@
>  step s2start: SELECT data FROM
> pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids',
> 'false');
>  data
> 
> +BEGIN          
> +COMMIT         
>  step s1b: BEGIN ISOLATION LEVEL SERIALIZABLE;
>  step s1w: INSERT INTO do_write DEFAULT VALUES;
>  step s1c: COMMIT;
> 
> ======================================================================
> ...
> 
> 
> 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()?

Greetings,

Andres Freund