Anti-critical-section assertion failure in mcxt.c reached by walsender

Hi,

While looking for something else, I noticed thorntail has failed twice
like this, on REL_12_STABLE:

TRAP: FailedAssertion("!(CritSectionCount == 0 ||
(context)->allowInCritSection)", File:
"/home/nm/farm/sparc64_deb10_gcc_64_ubsan/REL_12_STABLE/pgsql.build/../pgsql/src/backend/utils/mmgr/mcxt.c",
Line: 931)

In both cases, the last thing that process said was:

2021-05-05 20:25:51.650 MSK [3442631:6] 008_fsm_truncation.pl LOG:
received replication command: BASE_BACKUP LABEL 'pg_basebackup base
backup'   FAST NOWAIT

Unfortunately there is no libbacktrace in that release, and for some
reason we don't see a core being analysed... (gdb not installed,
looking for wrong core file pattern, ...?)

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=thorntail&dt=2021-05-05%2017%3A08%3A00
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=thorntail&dt=2021-04-22%2016%3A59%3A01