conf, log: On -h / --help, print usage to stdout, not stderr

Erik suggests that this makes it easier to grep for options, and with
--help we're anyway printing usage information as expected, not as
part of an error report.

While at it: on -h, we should exit with 0.

Reported-by: Erik Sjölund <erik.sjolund@gmail.com>
Link: https://bugs.passt.top/show_bug.cgi?id=52
Link: https://bugs.passt.top/show_bug.cgi?id=53
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

conf.c

@@ -717,10 +717,11 @@ static unsigned int conf_ip6(unsigned int ifi,
 }
 
 /**
- * usage() - Print usage and exit
+ * print_usage() - Print usage, exit with given status code
  * @name:	Executable name
+ * @status:	Status code for exit()
  */
-static void usage(const char *name)
+static void print_usage(const char *name, int status)
 {
 	if (strstr(name, "pasta")) {
 		info("Usage: %s [OPTION]... [COMMAND] [ARGS]...", name);
@@ -856,7 +857,7 @@ static void usage(const char *name)
 	info(   "    SPEC is as described for TCP above");
 	info(   "    default: none");
 
-	exit(EXIT_FAILURE);
+	exit(status);
 
 pasta_opts:
 
@@ -906,7 +907,16 @@ pasta_opts:
 	info(   "			Don't copy all addresses to namespace");
 	info(   "  --ns-mac-addr ADDR	Set MAC address on tap interface");
 
-	exit(EXIT_FAILURE);
+	exit(status);
+}
+
+/**
+ * usage() - Print usage and exit with failure
+ * @name:	Executable name
+ */
+static void usage(const char *name)
+{
+	print_usage(name, EXIT_FAILURE);
 }
 
 /**
@@ -1630,8 +1640,11 @@ void conf(struct ctx *c, int argc, char **argv)
 		case 'U':
 			/* Handle these later, once addresses are configured */
 			break;
-		case '?':
 		case 'h':
+			log_to_stdout = 1;
+			print_usage(argv[0], EXIT_SUCCESS);
+			break;
+		case '?':
 		default:
 			usage(argv[0]);
 			break;

log.c

@@ -43,17 +43,19 @@ static char	log_header[BUFSIZ];	/* File header, written back on cuts */
 
 static time_t	log_start;		/* Start timestamp */
 int		log_trace;		/* --trace mode enabled */
+int		log_to_stdout;		/* Print to stdout instead of stderr */
 
 #define BEFORE_DAEMON		(setlogmask(0) == LOG_MASK(LOG_EMERG))
 
 #define logfn(name, level)						\
 void name(const char *format, ...) {					\
+	FILE *out = log_to_stdout ? stdout : stderr;			\
 	struct timespec tp;						\
 	va_list args;							\
 									\
 	if (setlogmask(0) & LOG_MASK(LOG_DEBUG) && log_file == -1) {	\
 		clock_gettime(CLOCK_REALTIME, &tp);			\
-		fprintf(stderr, "%lli.%04lli: ",			\
+		fprintf(out, "%lli.%04lli: ",				\
 			(long long int)tp.tv_sec - log_start,		\
 			(long long int)tp.tv_nsec / (100L * 1000));	\
 	}								\
@@ -70,10 +72,10 @@ void name(const char *format, ...) {					\
 	if ((setlogmask(0) & LOG_MASK(LOG_DEBUG) && log_file == -1) ||	\
 	    (BEFORE_DAEMON && !(log_opt & LOG_PERROR))) {		\
 		va_start(args, format);					\
-		(void)vfprintf(stderr, format, args); 			\
+		(void)vfprintf(out, format, args); 			\
 		va_end(args);						\
 		if (format[strlen(format)] != '\n')			\
-			fprintf(stderr, "\n");				\
+			fprintf(out, "\n");				\
 	}								\
 }
 

log.h

@@ -22,6 +22,7 @@ void debug(const char *format, ...);
 	} while (0)
 
 extern int log_trace;
+extern int log_to_stdout;
 void trace_init(int enable);
 #define trace(...)							\
 	do {								\