logit(): detect running in container to use stderr for logging

When running in a container we still want to use any syslog daemon
available for our logging needs.  However, the time between the first
logit() in Finit and any such daemon having started can be long.  In a
normal (non-containerized) setup we log to the kernel ring buffer, but
that's not available in a container scenario.  At least not for
unprivileged containers.  So we need to detect all these cases and be
prepared to fall back to log to the console, either using these LOG_CONS
flag to openlog(), or by simply calling vfprintf() to stderr.

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

src/log.c

@@ -59,18 +59,24 @@ void log_exit(void)
 	enable_progress(1);
 }
 
-static void log_open(void)
+static int log_open(void)
 {
-	int opts = LOG_PID;
+	int opts;
+
+	if (up)
+		return 1;
+	if (access("/dev/log", W_OK))
+		return 0;
+
+	opts = LOG_CONS | LOG_PID;
+	if (debug)
+		opts |= LOG_PERROR;
 
 	closelog();
-
-	if (debug)
-		opts |= LOG_PERROR; /* LOG_CONS | */
 	openlog("finit", opts, LOG_DAEMON);
 	setlogmask(LOG_UPTO(loglevel));
 
-	up = 1;
+	return up = 1;
 }
 
 /* Toggle debug mode */
@@ -95,15 +101,12 @@ void log_debug(void)
  */
 void logit(int prio, const char *fmt, ...)
 {
-	FILE *fp;
 	va_list ap;
+	FILE *fp;
 
 	va_start(ap, fmt);
 
-	if (up || fexist("/dev/log")) {
-		if (!up)
-			log_open();
-
+	if (up || log_open()) {
 		vsyslog(prio, fmt, ap);
 		goto done;
 	}
@@ -111,8 +114,7 @@ void logit(int prio, const char *fmt, ...)
 	if (LOG_PRI(prio) > loglevel)
 		goto done;
 
-	fp = fopen("/dev/kmsg", "w");
-	if (!fp) {
+	if (in_container() || !(fp = fopen("/dev/kmsg", "w"))) {
 		vfprintf(stderr, fmt, ap);
 		goto done;
 	}