Is a pg_stat_force_next_flush() call sufficient for regression tests?

Hi,

I noticed eelpout failed with this in stats.sql, in the pg_stat_io tests
added a couple months ago [1]:

@@ -1415,7 +1415,7 @@
:io_sum_vac_strategy_after_reuses >
:io_sum_vac_strategy_before_reuses;
?column? | ?column?
----------+----------
- t | t
+ t | f
(1 row)

The failure seems completely unrelated to the new commit, so this seems
like some randomness / timing issue. The failing bit does this:

----------------------------
VACUUM (PARALLEL 0, BUFFER_USAGE_LIMIT 128) test_io_vac_strategy;
SELECT pg_stat_force_next_flush();
SELECT sum(reuses) AS reuses, sum(reads) AS reads
FROM pg_stat_io WHERE context = 'vacuum' \gset io_sum_vac_strategy_after_
SELECT :io_sum_vac_strategy_after_reads > :io_sum_vac_strategy_before_reads,
:io_sum_vac_strategy_after_reuses >
:io_sum_vac_strategy_before_reuses;
----------------------------

So I'm wondering if pg_stat_force_next_flush() is enough - AFAICS this
only sets some flag for the *next* pgstat_report_stat() call, but how do
we know that happens before the query execution?

Shouldn't there be something like pg_stat_flush() that actually does the
flushing, instead of just setting the flag?

regards

[1]
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=eelpout&dt=2023-07-03%2011%3A09%3A13

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
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