[pgpool] invalid read kind

Hi all,

we have a setup where we use PGPOOL instances between our application
and the main DB server. Lately there is an intermittent but annoying
error that sometimes pops up:

ERROR: unable to parse the query
DETAIL: invalid read kind
SSL SYSCALL error: EOF detected

Version numbers:
pgpool 3.7.20
postgresql 9.5.25

Relevant part of pgpool's log:

2022-03-10T09:44:50.000000+00:00 sanitized_hostname pgpool[8731]:
[2992409-1] 2022-03-10 09:44:50: pid 8731: LOG: statement: SELECT 'OK'
2022-03-10T09:44:50.000000+00:00 sanitized_hostname pgpool[8731]:
[2992410-1] 2022-03-10 09:44:50: pid 8731: LOG: DB node id: 0 backend
pid: 29433 statement: SELECT 'OK'
2022-03-10T09:44:50.000000+00:00 sanitized_hostname pgpool[8731]:
[2992411-1] 2022-03-10 09:44:50: pid 8731: ERROR: unable to parse the
query
2022-03-10T09:44:50.000000+00:00 sanitized_hostname pgpool[8731]:
[2992411-2] 2022-03-10 09:44:50: pid 8731: DETAIL: invalid read kind
2022-03-10T09:44:50.000000+00:00 sanitized_hostname pgpool[8731]:
[2992412-1] 2022-03-10 09:44:50: pid 8731: LOG: statement: DISCARD
ALL
2022-03-10T09:44:50.000000+00:00 sanitized_hostname pgpool[8731]:
[2992413-1] 2022-03-10 09:44:50: pid 8731: LOG: DB node id: 0 backend
pid: 29433 statement: DISCARD ALL
2022-03-10T09:44:50.000000+00:00 sanitized_hostname pgpool[8731]:
[2992414-1] 2022-03-10 09:44:50: pid 8731: FATAL: failed to read kind
from backend 0
2022-03-10T09:44:50.000000+00:00 sanitized_hostname pgpool[8731]:
[2992414-2] 2022-03-10 09:44:50: pid 8731: DETAIL: kind == 0

The query does not appear to be relevant, it's well-formed and not the
same every time.

Now, what does this mean, and how to deal with it?

Best regards,
Peter Juhasz

Comnica Kft.