This change implements 'systemd-journal' logging target, that is
similar to the existing 'syslog' target. The key difference is
that 'systemd-journal' allows us to offload rendering of the meta
information, such as location (file name, line number), subsystem,
and logging level, to systemd. Moreover, we can attach arbitrary,
user-specific fields [1] to the logging messages, so they can be
used for advanced log filtering (e.g. by IMSI/TMSI/TLLI):
$ journalctl OSMO_SUBSYS=DMSC -f
Since we don't want to make libsystemd a required dependency, this
feature is optional, and needs to be enabled at build-time:
$ ./configure --enable-systemd-logging
The new logging target can be configured in the same way as any
other one - via the VTY interface, or using the configuration file:
log systemd-journal [raw]
logging level set-all notice
logging filter all 1
Two logging handlers are available: generic and raw. The first one
behaves similarly to both 'syslog' and 'stderr', i.e. all the meta
information is rendered by libosmocore itself, and then passed to
systemd together with the logging message. The later is more like
the 'gsmtap' target, so all available meta information is handed
over to systemd in form of fields [1]:
- CODE_FILE / CODE_LINE - location info,
- PRIORITY - syslog-compatible logging level,
- OSMO_SUBSYS - Osmocom-specific sub-system (e.g. DMSC),
- OSMO_SUBSYS_HEX - same as OSMO_SUBSYS, but encoded in hex,
- MESSAGE - the logging message itself,
and then can be rendered in any supported format (e.g. JSON).
More details about the API can be found in [2].
[1] https://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html
[2] https://www.freedesktop.org/software/systemd/man/sd-journal.html
Change-Id: I609f5cf438e6ad9038d8fc95f00add6aac29fb23
