Re: Test of a partition with an incomplete detach has a timing issue

So I had a hard time reproducing the problem, until I realized that I
needed to limit the server to use only one CPU, and in addition run some
other stuff concurrently in the same server in order to keep it busy.
With that, I see about one failure every 10 runs.

So I start the server as "numactl -C0 postmaster", then another terminal
with an infinite loop doing "make -C src/test/regress installcheck-parallel";
and a third terminal doing this

while [ $? == 0 ]; do ../../../src/test/isolation/pg_isolation_regress --inputdir=/pgsql/source/master/src/test/isolation --outputdir=output_iso --bindir='/pgsql/install/master/bin' detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 detach-partition-concurrently-3 ; done

With the test unpatched, I get about one failure in the set.

On 2021-May-24, Noah Misch wrote:

> What if we had a standard that the step after the cancel shall send a query to
> the backend that just received the cancel?  Something like:

Hmm ... I don't understand why this fixes the problem, but it
drastically reduces the probability.  Here's a complete patch.  I got
about one failure in 1000 instead of 1 in 10.  The new failure looks
like this:

diff -U3 /pgsql/source/master/src/test/isolation/expected/detach-partition-concurrently-3.out /home/alvherre/Code/pgsql-build/master/src/test/isolation/output_iso/results/detach-partition-concurrently-3.out
--- /pgsql/source/master/src/test/isolation/expected/detach-partition-concurrently-3.out	2021-05-25 11:12:42.333987835 -0400
+++ /home/alvherre/Code/pgsql-build/master/src/test/isolation/output_iso/results/detach-partition-concurrently-3.out	2021-05-25 11:19:03.714947775 -0400
@@ -13,7 +13,7 @@
 
 t              
 step s2detach: <... completed>
-error in steps s1cancel s2detach: ERROR:  canceling statement due to user request
+ERROR:  canceling statement due to user request
 step s2noop: UNLISTEN noop;
 step s1c: COMMIT;
 step s1describe: SELECT 'd3_listp' AS root, * FROM pg_partition_tree('d3_listp')


I find this a bit weird and I'm wondering if it could be an
isolationtester bug -- why is it not attributing the error message to
any steps?

The problem disappears completely if I add a sleep to the cancel query:

step "s1cancel" 	{ SELECT pg_cancel_backend(pid), pg_sleep(0.01) FROM d3_pid; }

I suppose a 0.01 second sleep is not going to be sufficient to close the
problem in slower animals, but I hesitate to propose a much longer sleep
because this test has 18 permutations so even a one second sleep adds
quite a lot of (mostly useless) test runtime.

-- 
Álvaro Herrera                            39°49'30"S 73°17'W