Fix unused flags in CONNECT command being forced to be 0 in MQTT v3.1

This check is not required until v3.1.1.

Closes #2522. Thanks to garinocyr

ChangeLog.txt

@@ -7,6 +7,8 @@ Broker:
 - Fix bridge `restart_timeout` not being honoured.
 - Fix potential memory leaks if a plugin modifies the message in the
  MOSQ_EVT_MESSAGE event.
+- Fix unused flags in CONNECT command being forced to be 0, which is not
+ required for MQTT v3.1. Closes #2522.
 
 Client library:
 - Fix threads library detection on Windows under cmake. Bumps the minimum

src/handle_connect.c

@@ -458,9 +458,6 @@ int handle__connect(struct mosquitto *context)
  rc = MOSQ_ERR_PROTOCOL;
  goto handle_connect_error;
  }
- if(context->in_packet.command != CMD_CONNECT){
- return MOSQ_ERR_MALFORMED_PACKET;
- }
 
  /* Read protocol name as length then bytes rather than with read_string
  * because the length is fixed and we can check that. Removes the need
@@ -528,6 +525,9 @@ int handle__connect(struct mosquitto *context)
  rc = MOSQ_ERR_PROTOCOL;
  goto handle_connect_error;
  }
+ if((protocol_version&0x7F) != PROTOCOL_VERSION_v31 && context->in_packet.command != CMD_CONNECT){
+ return MOSQ_ERR_MALFORMED_PACKET;
+ }
 
  if(packet__read_byte(&context->in_packet, &connect_flags)){
  rc = MOSQ_ERR_PROTOCOL;

test/broker/data/CONNECT.json

@@ -3,6 +3,14 @@
  "comment": "CONNECT TESTS ARE INCOMPLETE",
  "group": "v3.1 CONNECT",
  "tests": [
+ { "name": "10 ok ", "connect":false, "expect_disconnect":false, "msgs":[
+ {"type":"send", "payload":"10 0F 0006 4D5149736470 03 01 000A 0001 70", "comment":"minimal valid CONNECT"},
+ {"type":"recv", "payload":"20 02 00 00", "comment": "CONNACK"}
+ ]},
+ { "name": "14 ok ", "connect":false, "expect_disconnect":false, "msgs":[
+ {"type":"send", "payload":"14 0F 0006 4D5149736470 03 01 000A 0001 70", "comment":"CONNECT with QoS=1"},
+ {"type":"recv", "payload":"20 02 00 00", "comment": "CONNACK"}
+ ]},
  { "name": "10 proto ver 2", "connect":false, "msgs":[
  {"type":"send", "payload":"10 0F 0006 4D5149736470 02 00 000A 0001 70", "comment":"CONNECT"},
  {"type":"recv", "payload":"20 02 00 01", "comment": "CONNACK identifier rejected"}