logging: introduce 'systemd-journal' target

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

1 files changed, 7 insertions, 0 deletions

diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 36ce941c..6d0d5a3a 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -244,6 +244,7 @@ enum log_target_type {
 	LOG_TGT_TYPE_STDERR,	/*!< stderr logging */
 	LOG_TGT_TYPE_STRRB,	/*!< osmo_strrb-backed logging */
 	LOG_TGT_TYPE_GSMTAP,	/*!< GSMTAP network logging */
+	LOG_TGT_TYPE_SYSTEMD,	/*!< systemd journal logging */
 };
 
 /*! Whether/how to log the source filename (and line number). */
@@ -311,6 +312,10 @@ struct log_target {
 			const char *ident;
 			const char *hostname;
 		} tgt_gsmtap;
+
+		struct {
+			bool raw;
+		} sd_journal;
 	};
 
 	/*! call-back function to be called when the logging framework
@@ -392,6 +397,8 @@ struct log_target *log_target_create_gsmtap(const char *host, uint16_t port,
 					    const char *ident,
 					    bool ofd_wq_mode,
 					    bool add_sink);
+struct log_target *log_target_create_systemd(bool raw);
+void log_target_systemd_set_raw(struct log_target *target, bool raw);
 int log_target_file_reopen(struct log_target *tgt);
 int log_targets_reopen(void);