[systemd-devel] Antw: [EXT] Re: Q: journal filtering and unit failures

[Ulrich Windl]

>>> Mantas Mikulenas <grawity at gmail.com> schrieb am 27.12.2021 um 10:52 in
Nachricht
<CAPWNY8VCjvefLQxkqH=7-XJQi2jTgvFOkbyTsyQM9G-cWLd4Ag at mail.gmail.com>:
> On Mon, Dec 27, 2021 at 11:26 AM Ulrich Windl <
> Ulrich.Windl at rz.uni-regensburg.de> wrote:
> 
>> Hi!
>>
>> debugging a problem I found out some surprising fact:
>> When filtering the journal with "journalctl -b
>> _SYSTEMD_UNIT=logrotate.service", I see an error only once, like this:
>> -- Logs begin at Mon 2020-11-30 11:35:08 CET, end at Mon 2021-12-27
>> 10:19:31 CET. --
>> Dec 18 00:00:20 h16 logrotate[41799]: Failed to kill unit
>> \x7b__SERVICE__\x7d.service: Unit \x7b__SERVICE__\x7d.s...
>> Dec 18 00:00:20 h16 logrotate[41799]: error: error running shared
>> postrotate script for '/var/log/iotwatch/MD10/...
>>
>> However when inspecting the full journal I also see:
>> Dec 18 00:00:20 h16 systemd[1]: logrotate.service: Main process exited,
>> code=exited, status=1/FAILURE
>> Dec 18 00:00:20 h16 systemd[1]: Failed to start Rotate log files.
>> Dec 18 00:00:20 h16 systemd[1]: logrotate.service: Unit entered failed
>> state.
>> Dec 18 00:00:20 h16 systemd[1]: logrotate.service: Failed with result
>> 'exit-code'.
>>
>> Actually I am surprised that those unit-related messages are not covered
>> by the filter.
>>
> 
> They're not generated **by** a process within logrotate.service, and
> journald does not allow a (fake) _SYSTEMD_UNIT to be set – not even if the
> sender is pid1. The same also applies to messages logged by
> systemd-coredump.
> 
> 
>> Is there a better way to get such related messages when filtering?
>>
> 
> Use `journalctl -u logrotate`, which combines several filters to get all
> messages related to the specified unit.
> 
> The actual filtering used by -u (as seen with SYSTEMD_LOG_LEVEL=debug) is a
> union of four filters – the equivalent command would be:
> 
> journalctl _SYSTEMD_UNIT=logrotate.service \
>   + MESSAGE_ID=fc2e22bc6ee647b6b90729ab34a250b1
> _UID=0 COREDUMP_UNIT=logrotate.service \
>   + _PID=1 UNIT=logrotate.service \
>   + _UID=0 OBJECT_SYSTEMD_UNIT=logrotate.service
> 
> The fourth is a generic filter (any process running as root can specify
> OBJECT_SYSTEMD_UNIT=) which I think came a bit later than the special-case
> pid1 and coredump ones.

OK, thanks.  Re-reading the manual page, I'd suggest to add references to
options like "-u" to the early section that deals with "MATCHES". A new user
(like I) will divert to the wrong path using "_SYSTEMD_UNIT=" when probably
"-u" was intended actually. Also the word "filter" occurs for option "-p", but
not for "-u". Maybe list all "filtering" options early in the DESCRIPTION...

Regards,
Ulrich


> 
> -- 
> Mantas Mikulėnas