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

3 files changed, 191 insertions, 0 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 891b4a6f..b2c9204f 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -71,5 +71,10 @@ if ENABLE_SERIAL
 libosmocore_la_SOURCES += serial.c
 endif
 
+if ENABLE_SYSTEMD_LOGGING
+libosmocore_la_SOURCES += logging_systemd.c
+libosmocore_la_LIBADD += $(SYSTEMD_LIBS)
+endif
+
 crc%gen.c: crcXXgen.c.tpl
 	$(AM_V_GEN)sed -e's/XX/$*/g' $< > $@
diff --git a/src/logging_systemd.c b/src/logging_systemd.c
new file mode 100644
index 00000000..7c966863
--- /dev/null
+++ b/src/logging_systemd.c
@@ -0,0 +1,121 @@
+/*
+ * (C) 2020 by Vadim Yanitskiy <axilirator@gmail.com>
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ */
+
+/*! \addtogroup logging
+ *  @{
+ * \file logging_systemd.c */
+
+#include <stdio.h>
+#include <syslog.h>
+
+/* Do not use this file as location in sd_journal_print() */
+#define SD_JOURNAL_SUPPRESS_LOCATION
+
+#include <systemd/sd-journal.h>
+
+#include <osmocom/core/talloc.h>
+#include <osmocom/core/utils.h>
+#include <osmocom/core/logging.h>
+
+/* FIXME: copy-pasted from logging_syslog.c */
+static int logp2syslog_level(unsigned int level)
+{
+	if (level >= LOGL_FATAL)
+		return LOG_CRIT;
+	else if (level >= LOGL_ERROR)
+		return LOG_ERR;
+	else if (level >= LOGL_NOTICE)
+		return LOG_NOTICE;
+	else if (level >= LOGL_INFO)
+		return LOG_INFO;
+	else
+		return LOG_DEBUG;
+}
+
+static void _systemd_output(struct log_target *target,
+			    unsigned int level, const char *log)
+{
+	/* systemd accepts the same level constants as syslog */
+	sd_journal_print(logp2syslog_level(level), "%s", log);
+}
+
+static void _systemd_raw_output(struct log_target *target, int subsys,
+				unsigned int level, const char *file,
+				int line, int cont, const char *format,
+				va_list ap)
+{
+	char buf[4096];
+	int rc;
+
+	rc = vsnprintf(buf, sizeof(buf), format, ap);
+	if (rc < 0) {
+		sd_journal_print(LOG_ERR, "vsnprintf() failed to render a message "
+					  "originated from %s:%d (rc=%d)\n",
+					  file, line, rc);
+		return;
+	}
+
+	sd_journal_send("CODE_FILE=%s, CODE_LINE=%d", file, line,
+			"PRIORITY=%d", logp2syslog_level(level),
+			"OSMO_SUBSYS=%s", log_category_name(subsys),
+			"OSMO_SUBSYS_HEX=%4.4x", subsys,
+			"MESSAGE=%s", buf,
+			NULL);
+}
+
+/*! Create a new logging target for systemd journal logging.
+ *  \param[in] raw whether to offload rendering of the meta information
+ *		   (location, category) to systemd-journal.
+ *  \returns Log target in case of success, NULL in case of error.
+ */
+struct log_target *log_target_create_systemd(bool raw)
+{
+	struct log_target *target;
+
+	target = log_target_create();
+	if (!target)
+		return NULL;
+
+	target->type = LOG_TGT_TYPE_SYSTEMD;
+	log_target_systemd_set_raw(target, raw);
+
+	return target;
+}
+
+/*! Change meta information handling of an existing logging target.
+ *  \param[in] target logging target to be modified.
+ *  \param[in] raw whether to offload rendering of the meta information
+ *		   (location, category) to systemd-journal.
+ */
+void log_target_systemd_set_raw(struct log_target *target, bool raw)
+{
+	target->sd_journal.raw = raw;
+	if (raw) {
+		target->raw_output = _systemd_raw_output;
+		target->output = NULL;
+	} else {
+		target->output = _systemd_output;
+		target->raw_output = NULL;
+	}
+}
+
+/* @} */
diff --git a/src/vty/logging_vty.c b/src/vty/logging_vty.c
index 200b45a1..02823504 100644
--- a/src/vty/logging_vty.c
+++ b/src/vty/logging_vty.c
@@ -732,6 +732,64 @@ DEFUN(cfg_no_log_syslog, cfg_no_log_syslog_cmd,
 }
 #endif /* HAVE_SYSLOG_H */
 
+DEFUN(cfg_log_systemd_journal, cfg_log_systemd_journal_cmd,
+      "log systemd-journal [raw]",
+      LOG_STR "Logging to systemd-journal\n"
+      "Offload rendering of the meta information (location, category) to systemd\n")
+{
+#ifdef ENABLE_SYSTEMD_LOGGING
+	struct log_target *tgt;
+	bool raw = argc > 0;
+
+	log_tgt_mutex_lock();
+	tgt = log_target_find(LOG_TGT_TYPE_SYSTEMD, NULL);
+	if (tgt == NULL) {
+		tgt = log_target_create_systemd(raw);
+		if (tgt == NULL) {
+			vty_out(vty, "%% Unable to create systemd-journal "
+				"log target%s", VTY_NEWLINE);
+			RET_WITH_UNLOCK(CMD_WARNING);
+		}
+		log_add_target(tgt);
+	} else if (tgt->sd_journal.raw != raw) {
+		log_target_systemd_set_raw(tgt, raw);
+	}
+
+	vty->index = tgt;
+	vty->node = CFG_LOG_NODE;
+
+	RET_WITH_UNLOCK(CMD_SUCCESS);
+#else
+	vty_out(vty, "%% systemd-journal logging is not available "
+		"in this build of libosmocore%s", VTY_NEWLINE);
+	return CMD_WARNING;
+#endif /* ENABLE_SYSTEMD_LOGGING */
+}
+
+DEFUN(cfg_no_log_systemd_journal, cfg_no_log_systemd_journal_cmd,
+	"no log systemd-journal",
+	NO_STR LOG_STR "Logging to systemd-journal\n")
+{
+#ifdef ENABLE_SYSTEMD_LOGGING
+	struct log_target *tgt;
+
+	log_tgt_mutex_lock();
+	tgt = log_target_find(LOG_TGT_TYPE_SYSTEMD, NULL);
+	if (!tgt) {
+		vty_out(vty, "%% No systemd-journal logging active%s", VTY_NEWLINE);
+		RET_WITH_UNLOCK(CMD_WARNING);
+	}
+
+	log_target_destroy(tgt);
+
+	RET_WITH_UNLOCK(CMD_SUCCESS);
+#else
+	vty_out(vty, "%% systemd-journal logging is not available "
+		"in this build of libosmocore%s", VTY_NEWLINE);
+	return CMD_WARNING;
+#endif /* ENABLE_SYSTEMD_LOGGING */
+}
+
 DEFUN(cfg_log_gsmtap, cfg_log_gsmtap_cmd,
 	"log gsmtap [HOSTNAME]",
 	LOG_STR "Logging via GSMTAP\n"
@@ -926,6 +984,11 @@ static int config_write_log_single(struct vty *vty, struct log_target *tgt)
 		vty_out(vty, "log gsmtap %s%s",
 			tgt->tgt_gsmtap.hostname, VTY_NEWLINE);
 		break;
+	case LOG_TGT_TYPE_SYSTEMD:
+		vty_out(vty, "log systemd-journal%s%s",
+			tgt->sd_journal.raw ? " raw" : "",
+			VTY_NEWLINE);
+		break;
 	}
 
 	vty_out(vty, " logging filter all %u%s",
@@ -1127,5 +1190,7 @@ void logging_vty_add_cmds()
 	install_lib_element(CONFIG_NODE, &cfg_log_syslog_local_cmd);
 	install_lib_element(CONFIG_NODE, &cfg_no_log_syslog_cmd);
 #endif
+	install_lib_element(CONFIG_NODE, &cfg_log_systemd_journal_cmd);
+	install_lib_element(CONFIG_NODE, &cfg_no_log_systemd_journal_cmd);
 	install_lib_element(CONFIG_NODE, &cfg_log_gsmtap_cmd);
 }