Dovecot error invalid service_fd

[Michel Verdier]

Hi all

On debian stable, dovecot 2.3.13+dfsg1-2+deb11u1 and postfix 3.5.17-0+deb11u1
I set postfix sending mails to dovecot for mailbox storage:

mailbox_transport = lmtp:unix:private/dovecot-lmtp

dovecot listen with lmtp:

service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    mode = 0666
    user = postfix
    group = postfix
  }
}
passdb {
  driver = pam
  args = session=yes cache_key=%u%r%l dovecot
}
userdb {
  driver = passwd
}

# ls -l /var/spool/postfix/private/dovecot-lmtp
srw-rw-rw- 1 postfix postfix 0 27 mars  11:24 /var/spool/postfix/private/dovecot-lmtp

I get logs like this:

postfix/lmtp[271076]: 6DB551008B3C1: to=..., relay=...[private/dovecot-lmtp], delay=0.7, delays=0.29/0.04/0.11/0.26, dsn=2.0.0, status=sent (250 2.0.0 ...
postfix/qmgr[197419]: 6DB551008B3C1: removed
dovecot: lmtp(271077): Disconnect from local: Client has quit the connection (state=READY)
dovecot: log(253615): Error: Received master input for invalid service_fd 27: 27 271079 BYE

I identify 271079 as the pid for dovecot auth worker process. If I add a
timeout in dovecot:

service auth-worker {
  idle_kill = 10s
}

Then I see error log coming 10 seconds after the end of every lmtp
connexion from postfix. There is no other log messages around even with
debug enable in dovecot. And mails are correctly delivered.

Am I missing something or is this to be ignored?

Cheers