Fix reconnect delay backoff behaviour.

Closes #1027. Thanks to Harm Verhagen.

Bug: https://github.com/eclipse/mosquitto/issues/1027

ChangeLog.txt

@@ -10,6 +10,9 @@ Broker:
   because this can lead to confusing "Protocol not supported" errors if the
   network is down. Closes #1062.
 
+Library:
+- Fix reconnect delay backoff behaviour. Closes #1027.
+
 Client:
 - Always print leading zeros in mosquitto_sub when output format is hex.
   Closes #1066.

lib/loop.c

@@ -245,8 +245,12 @@ int mosquitto_loop_forever(struct mosquitto *mosq, int timeout, int max_packets)
 			}else{
 				pthread_mutex_unlock(&mosq->state_mutex);
 
-				if(mosq->reconnect_delay > 0 && mosq->reconnect_exponential_backoff){
-					reconnect_delay = mosq->reconnect_delay*reconnects*reconnects;
+				if(mosq->reconnect_delay_max > mosq->reconnect_delay){
+					if(mosq->reconnect_exponential_backoff){
+						reconnect_delay = mosq->reconnect_delay*(reconnects+1)*(reconnects+1);
+					}else{
+						reconnect_delay = mosq->reconnect_delay*(reconnects+1);
+					}
 				}else{
 					reconnect_delay = mosq->reconnect_delay;
 				}

lib/options.c

@@ -76,6 +76,8 @@ int mosquitto_reconnect_delay_set(struct mosquitto *mosq, unsigned int reconnect
 {
 	if(!mosq) return MOSQ_ERR_INVAL;
 	
+	if(reconnect_delay == 0) reconnect_delay = 1;
+
 	mosq->reconnect_delay = reconnect_delay;
 	mosq->reconnect_delay_max = reconnect_delay_max;
 	mosq->reconnect_exponential_backoff = reconnect_exponential_backoff;