Failure of subscription tests with topminnow

Hi all,

topminnow has just failed in a weird fashion:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=topminnow&dt=2021-08-15%2013%3A24%3A58
# SELECT pid !=  FROM pg_stat_replication WHERE application_name = 'tap_sub';
# expecting this output:
# t
# last actual query output:
#
# with stderr:
# ERROR:  syntax error at or near "FROM"
# LINE 1: SELECT pid !=  FROM pg_stat_replication WHERE application_na...

Looking at the logs, it seems like the problem boils down to an active
slot when using ALTER SUBSCRIPTION tap_sub CONNECTION:
2021-08-15 18:44:38.027 CEST [16473:2] ERROR:  could not start WAL
streaming: ERROR:  replication slot "tap_sub" is active for PID 16336

There is only one place in 001_rep_changes.pl where this is used.

Thoughts?
--
Michael