Recent "could not register background process" buildfarm failures

In the past three weeks, two different buildfarm animals have
reported failures like this in test_shm_mq [1][2][3]:

# SELECT test_shm_mq(1024, 'a', 2001, 1);
# - test_shm_mq
# --------------
# -
# -(1 row)
# -
# +ERROR: could not register background process
# +HINT: You may need to increase "max_worker_processes".

test_shm_mq itself hasn't changed meaningfully in ages,
so it seems like some recent change somewhere else has increased
the probability of not being able to get a worker process.
Maybe autovacuum is eating too many workers? Any thoughts?

regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tayra&dt=2026-07-25%2005%3A43%3A54
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=akepa&dt=2026-08-17%2021%3A37%3A41
[3] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=akepa&dt=2026-08-18%2013%3A52%3A05