[MM3-users] Cannot start lmtp runner due to error while attempting to bind on address ('127.0.0.1', 8024): address already in use

Ubuntu focal 20.04 mailman3 3.2.2-1

In /etc/mailman3/mailman.cfg:

# Where the LMTP server listens for connections.  Use 127.0.0.1 instead of
# localhost for Postfix integration, because Postfix only consults DNS
# (e.g. not /etc/hosts).
# Configuration is for a system where Mailman and the MTA are on the same host
lmtp_host: 127.0.0.1
lmtp_port: 8024

Stopping mailman3 and checking that no lmtp runner is active:

# systemctl stop mailman3
# ps -aux |grep mailman|grep -Pv "(postgres|grep)"
www-data  855395  0.0  0.4  82856 37904 ?        Ss   13:01   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini
www-data  855411  0.0  0.3 156588 25724 ?        Sl   13:01   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini
www-data  858518  0.0  0.2  82856 22532 ?        Ss   13:52   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini

Starting mailman3 and checking all active mailman3 runners:

# ps -aux |grep mailman|grep -Pv "(postgres|grep)"
www-data  855395  0.0  0.4  82856 37904 ?        Ss   13:01   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini
www-data  855411  0.0  0.3 156588 25724 ?        Sl   13:01   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini
www-data  858612  0.0  0.2  82856 22532 ?        Ss   13:54   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini
list      858626 17.8  0.9 103248 74680 ?        Ss   13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/master --force -C /etc/mailman3/mailman.cfg
list      858628 27.2  0.7  87176 62244 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=archive:0:1
list      858629 28.6  0.8  89116 64044 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=bounces:0:1
list      858630 29.2  0.8  90184 64972 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=command:0:1
list      858631 27.0  0.7  87560 62420 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=in:0:1
list      858632 28.8  0.8  89620 64508 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=lmtp:0:1
list      858633 26.6  0.7  84464 60800 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=nntp:0:1
list      858634 26.2  0.7  81964 58580 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=out:0:1
list      858635 28.4  0.7  88692 63648 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=pipeline:0:1
list      858636 26.4  0.7  83308 59820 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1
list      858637 26.8  0.7  84076 60828 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=retry:0:1
list      858638 29.2  0.8  90176 64964 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=virgin:0:1
list      858639 27.4  0.7  87564 62528 ?        R    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=digest:0:1

We clearly see that lmtp runner is active; then it exits after a few seconds:

# ps -aux |grep mailman|grep -Pv "(postgres|grep)"
www-data  855395  0.0  0.4  82856 37904 ?        Ss   13:01   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini
www-data  855411  0.0  0.3 156588 25724 ?        Sl   13:01   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini
www-data  858612  0.0  0.2  82856 22532 ?        Ss   13:54   0:00 /usr/bin/uwsgi --plugin python3 --ini /etc/mailman3/uwsgi.ini
list      858626  2.7  0.9 103248 74680 ?        Ss   13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/master --force -C /etc/mailman3/mailman.cfg
list      858628  4.3  0.9 103260 74704 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=archive:0:1
list      858629  4.3  0.9 103256 74964 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=bounces:0:1
list      858630  4.3  0.9 103256 74920 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=command:0:1
list      858631  4.3  0.9 103256 74804 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=in:0:1
list      858633  4.4  0.9 103924 75444 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=nntp:0:1
list      858634  4.3  0.9 103256 74392 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=out:0:1
list      858635  4.3  0.9 103260 74876 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=pipeline:0:1
list      858636  4.4  0.9 182412 79472 ?        Sl   13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1
list      858637  4.4  0.9 103256 74964 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=retry:0:1
list      858638  4.5  0.9 103256 74808 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=virgin:0:1
list      858639  4.4  0.9 103256 74776 ?        S    13:54   0:01 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=digest:0:1

The systemd service status gives a clue:

# systemctl status mailman3
● mailman3.service - Mailman3 server
   Loaded: loaded (/lib/systemd/system/mailman3.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2019-12-02 13:54:45 CET; 2min 51s ago
     Docs: man:mailman(1)
           https://mailman.readthedocs.io/
  Process: 858617 ExecStart=/usr/bin/mailman -C /etc/mailman3/mailman.cfg start --force (code=exited, status=0/SUCCESS)
 Main PID: 858626 (python3)
    Tasks: 14 (limit: 9284)
   Memory: 688.5M
   CGroup: /system.slice/mailman3.service
           ├─858626 /usr/bin/python3 /usr/lib/mailman3/bin/master --force -C /etc/mailman3/mailman.cfg
           ├─858628 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=archive:0:1
           ├─858629 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=bounces:0:1
           ├─858630 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=command:0:1
           ├─858631 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=in:0:1
           ├─858633 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=nntp:0:1
           ├─858634 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=out:0:1
           ├─858635 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=pipeline:0:1
           ├─858636 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1
           ├─858637 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=retry:0:1
           ├─858638 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=virgin:0:1
           └─858639 /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=digest:0:1

Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:     self.lmtp.start()
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:   File "/usr/lib/python3/dist-packages/aiosmtpd/controller.py", line 60, in start
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:     raise self._thread_exception
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:   File "/usr/lib/python3/dist-packages/aiosmtpd/controller.py", line 40, in _run
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:     ssl=self.ssl_context))
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:   File "/usr/lib/python3.7/asyncio/base_events.py", line 579, in run_until_complete
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:     return future.result()
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:   File "/usr/lib/python3.7/asyncio/base_events.py", line 1374, in create_server
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]:     % (sa, err.strerror.lower())) from None
Dec 02 13:54:52 samsung4-ubuntu mailman3[858617]: OSError: [Errno 98] error while attempting to bind on address ('127.0.0.1', 8024): address already in use

Of course the port is already in use: it is used by dovecot.

tcp6       0      0 ::1:8024                :::*                    LISTEN      0          1317360    129620/dovecot      
tcp        0      0 127.0.0.1:8024          0.0.0.0:*               LISTEN      0          1316324    129620/dovecot      

I must be missing something.