The situation is a service like
Type=notify
NotifyAccess=main
and the service uses some of the systemd helper utilities, e.g.
coredumpctl. The service process will pass NOTIFY_SOCKET to the helper
child (accidentally) and the result is a spurious notification and
the warning message:
> Jan 18 09:38:01 host systemd[1]: sdnotify.service: Got notification message from PID 13736, but reception only permitted for main PID 13549
Notification from helpers seem like an unintentional composition of the
commit c118b577fa ("coredumpctl: define main through macro") and commit
6b636c2d27 ("main-func: send main exit code to parent via sd_notify() on
exit"). The former used the handy macro for a main function, the latter
equipped any main function with the notification. (Further extended in
the commit 623a00020f ("notify: Add EXIT_STATUS field").)
Since notification from systemd utitilities are meant to extend
rudimentary exit()/wait() pair generally, they may happen to land into
service's NOTIFY_SOCKET. Tone down messages of notification that won't
match NotifyAccess=.
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