Re: Assertion failure on PG15 with modified test_shm_mq test

I don't think we have the infrastructure for a nice solution to this at the
moment - we need a fairly large overhaul of process initialization / shutdown
to handle these interdependencies nicely.

We can't move pgstat shutdown into on_dsm callback because that's too late to
allocate *new* dsm segments, which we might need to do while flushing
out pending stats.

See https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=fa91d4c91f28f4819dc54f93adbd413a685e366a
for a way to avoid the problem.