Fix zero length client ids being rejected for MQTT v5 clients.

This was happening when clean start was set to true.

ChangeLog.txt

@@ -23,6 +23,8 @@ Broker:
 - Disable TLS renegotiation. Client initiated renegotiation is considered to
   be a potential attack vector against servers. Closes #1257.
 - Fix incorrect shared subscription topic '$shared'.
+- Fix zero length client ids being rejected for MQTT v5 clients with clean
+  start set to true.
 
 Client library:
 - Fix typo causing build error on Windows when building without TLS support.

src/handle_connect.c

@@ -540,8 +540,12 @@ int handle__connect(struct mosquitto_db *db, struct mosquitto *context)
 			}else{
 				allow_zero_length_clientid = db->config->security_options.allow_zero_length_clientid;
 			}
-			if(clean_start == 0 || allow_zero_length_clientid == false){
+			if((context->protocol == mosq_p_mqtt311 && clean_start == 0) || allow_zero_length_clientid == false){
+				if(context->protocol == mosq_p_mqtt311){
 					send__connack(db, context, 0, CONNACK_REFUSED_IDENTIFIER_REJECTED, NULL);
+				}else{
+					send__connack(db, context, 0, MQTT_RC_UNSPECIFIED, NULL);
+				}
 				rc = MOSQ_ERR_PROTOCOL;
 				goto handle_connect_error;
 			}else{

test/broker/12-prop-assigned-client-identifier.py

@@ -5,12 +5,13 @@
 
 from mosq_test_helper import *
 
-rc = 1
 
-keepalive = 10
 
 
-connect_packet = mosq_test.gen_connect(None, proto_ver=5, keepalive=keepalive)
+def do_test(clean_start):
+    rc = 1
+    keepalive = 10
+    connect_packet = mosq_test.gen_connect(None, proto_ver=5, keepalive=keepalive, clean_session=clean_start)
 
     props = mqtt5_props.gen_string_prop(mqtt5_props.PROP_ASSIGNED_CLIENT_IDENTIFIER, "auto-00000000-0000-0000-0000-000000000000")
     connack_packet = mosq_test.gen_connack(rc=0, proto_ver=5, properties=props)
@@ -42,6 +43,10 @@ finally:
         (stdo, stde) = broker.communicate()
         if rc:
             print(stde.decode('utf-8'))
-
             exit(rc)
 
+
+do_test(True)
+do_test(False)
+exit(0)
+