logger: parse leading <LEVEL> in log messages

This commit adds support for systemd daemon log style, which is useful
when running in a 'daemon --foreground 2>&1 | logger' scenario, e.g.,
log redirection in Finit <https://github.com/troglobit/finit>

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

Author: troglobit

src/logger.c

@@ -193,6 +193,38 @@ static char *chomp(char *str)
         return str;
 }
 
+/*
+ * Parse possible systemd style log <level> prefix from message
+ * For details, see libsystemd/sd-daemon.h
+ *
+ * Returns the level, and updates buf to point past the prefix
+ */
+static int parse_level(char **buf, int severity)
+{
+	char *msg = *buf;
+
+	if (msg[0] == '<' && msg[2] == '>' && msg[1] >= '0' && msg[1] <= '7') {
+		int level;
+
+		switch (msg[1]) {
+		case '0': level = LOG_EMERG;    break;
+		case '1': level = LOG_ALERT;    break;
+		case '2': level = LOG_CRIT;     break;
+		case '3': level = LOG_ERR;      break;
+		case '4': level = LOG_WARNING;  break;
+		case '5': level = LOG_NOTICE;   break;
+		case '6': level = LOG_INFO;     break;
+		case '7': level = LOG_DEBUG;    break;
+		default:  level = severity;     break;
+		}
+
+		*buf = msg + 3;
+		return (severity & ~LOG_PRIMASK) | level;
+	}
+
+	return severity;
+}
+
 static int parse_prio(const char *arg, int *f, int *l)
 {
 	char buf[strlen(arg) + 1];
@@ -505,8 +537,11 @@ int main(int argc, char *argv[])
 	openlog_r(ident, log_opts, facility, &log);
 
 	if (!buf[0]) {
-		while ((fgets(buf, sizeof(buf), stdin)))
-			syslogp_r(severity, &log, msgid, sd, "%s", chomp(buf));
+		while ((fgets(buf, sizeof(buf), stdin))) {
+			char *msg = chomp(buf);
+
+			syslogp_r(parse_level(&msg, severity), &log, msgid, sd, "%s", msg);
+		}
 	} else
 		syslogp_r(severity, &log, msgid, sd, "%s", buf);