Allow MQTT v5.0 outgoing bridges to fall back to MQTT v3.1.1

This applies if connecting to a v3.x only broker.

ChangeLog.txt

@@ -8,6 +8,8 @@ Broker:
   bridging to e.g. Amazon or Google.
 - Add support for MQTT v5 bridges to handle the "retain-available" property
   being false.
+- Allow MQTT v5.0 outgoing bridges to fall back to MQTT v3.1.1 if connecting
+  to a v3.x only broker.
 
 Client library:
 - Client no longer generates random client ids for v3.1.1 clients, these are

src/handle_connack.c

@@ -41,6 +41,18 @@ int handle__connack(struct mosquitto_db *db, struct mosquitto *context)
 	if(packet__read_byte(&context->in_packet, &reason_code)) return 1;
 
 	if(context->protocol == mosq_p_mqtt5){
+		if(context->in_packet.remaining_length == 2 && reason_code == CONNACK_REFUSED_PROTOCOL_VERSION){
+			/* We have connected to a MQTT v3.x broker that doesn't support MQTT v5.0
+			 * It has correctly replied with a CONNACK code of a bad protocol version.
+			 */
+			log__printf(NULL, MOSQ_LOG_NOTICE,
+					"Warning: Remote bridge %s does not support MQTT v5.0, reconnecting using MQTT v3.1.1.",
+					context->bridge->name);
+
+			context->protocol = mosq_p_mqtt311;
+			context->bridge->protocol_version = mosq_p_mqtt311;
+			return MOSQ_ERR_PROTOCOL;
+		}
 		rc = property__read_all(CMD_CONNACK, &context->in_packet, &properties);
 		if(rc) return rc;
 		mosquitto_property_free_all(&properties);