v126: Mailman ModuleNotFoundError: No module named 'Defaults'

Symptoms

Mailman fails to start on version 126.

restartsrv_mailman[1386141]: File "/usr/local/cpanel/3rdparty/mailman/bin/mailmanctl", line 106, in <module>
restartsrv_mailman[1386141]: from Mailman import mm_cfg
restartsrv_mailman[1386141]: File "/usr/local/cpanel/3rdparty/mailman/Mailman/mm_cfg.py", line 49, in <module>
restartsrv_mailman[1386141]: from Defaults import *
restartsrv_mailman[1386141]: ModuleNotFoundError: No module named 'Defaults'
systemd[1]: mailman.service: Control process exited, code=exited, status=1/FAILURE

Description

In version 126, some Mailman functions have been updated. Primarily, the Python was updated from version 2 to 3, and how modules are loaded from their system paths in Python 3. This requires that a few variables be present in the configuration file. However, if the configuration contains any custom changes, it prevents the RPM install from replacing the configuration file.

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-46518. Follow this article to receive an email notification when a solution is published in the product. 

Workaround

1. Make a backup:

cp -p /usr/local/cpanel/3rdparty/mailman/Mailman/mm_cfg.py /usr/local/cpanel/3rdparty/mailman/Mailman/mm_cfg.py.backup

2. Copy the rpm version:

cp -p /usr/local/cpanel/3rdparty/mailman/Mailman/mm_cfg.py.rpmnew /usr/local/cpanel/3rdparty/mailman/Mailman/mm_cfg.py

3. Restart:

/scripts/restartsrv_mailman