Make check_snmp backwards compatible in how it parses thresholds

Once upon a time, check_snmp used to accept inverse ranges in the
format of '2:1' to mean "alert if value is inside this range".

Since commit 7cb3ae09334796f3b54e4e6438e38c2cc679b360, ranges such
as those have instead triggered the error "Range format incorrect"
and resulted in an UNKNOWN warning state. This patch attempts to
fix the situation so that the old-style ranges continues to mean
exactly what the once did and people with lots of snmp checks can
avoid a bazillion false positives from their environments.

Signed-off-by: Andreas Ericsson <ae@op5.se>

plugins/check_snmp.c

@@ -144,6 +144,25 @@ double previous_value[MAX_OIDS];
 int perf_labels = 1;
 
 
+static char *fix_snmp_range(char *th)
+{
+	double left, right;
+	char *colon, *ret;
+	if (!(colon = strchr(th, ':')))
+		return th;
+	*colon = 0;
+
+	left = strtod(th, NULL);
+	right = strtod(colon + 1, NULL);
+	if (right >= left) {
+		return th;
+	}
+	ret = malloc(strlen(th) + strlen(colon + 1) + 2);
+	sprintf(ret, "@%s:%s", colon + 1, th);
+	free(th);
+	return ret;
+}
+
 int
 main (int argc, char **argv)
 {
@@ -228,6 +247,10 @@ main (int argc, char **argv)
 	for (i=0; i<numoids; i++) {
 		char *w = th_warn ? strndup(th_warn, strcspn(th_warn, ",")) : NULL;
 		char *c = th_crit ? strndup(th_crit, strcspn(th_crit, ",")) : NULL;
+		/* translate "2:1" to "@1:2" for backwards compatibility */
+		w = w ? fix_snmp_range(w) : NULL;
+		c = c ? fix_snmp_range(c) : NULL;
+
 		/* Skip empty thresholds, while avoiding segfault */
 		set_thresholds(&thlds[i],
 		               w ? strpbrk(w, NP_THRESHOLDS_CHARS) : NULL,