Add `mosquitto_client_protocol_version()` function.

This can be used by plugins to determine which version of MQTT a client
has connected with.

ChangeLog.txt

@@ -14,6 +14,8 @@ Broker:
   Closes #1735.
 - Add `mosquitto_plugin_publish()` function, which can be used by plugins to
   publish messages.
+- Add `mosquitto_client_protocol_version()` function which can be used by
+  plugins to determine which version of MQTT a client has connected with.
 
 Client library:
 - Client no longer generates random client ids for v3.1.1 clients, these are

src/linker-macosx.syms

@@ -5,6 +5,7 @@ _mosquitto_client_id
 _mosquitto_client_keepalive
 _mosquitto_client_certificate
 _mosquitto_client_protocol
+_mosquitto_client_protocol_version
 _mosquitto_client_sub_count
 _mosquitto_client_username
 _mosquitto_set_username

src/linker.syms

@@ -5,6 +5,7 @@
 	mosquitto_client_id;
 	mosquitto_client_keepalive;
 	mosquitto_client_protocol;
+	mosquitto_client_protocol_version;
 	mosquitto_client_sub_count;
 	mosquitto_client_username;
 	mosquitto_log_printf;

src/mosquitto_broker.h

@@ -131,6 +131,18 @@ void *mosquitto_client_certificate(const struct mosquitto *client);
 int mosquitto_client_protocol(const struct mosquitto *client);
 
 
+/*
+ * Function: mosquitto_client_protocol_version
+ *
+ * Retrieve the MQTT protocol version with which the client has connected. Can be one of:
+ *
+ * 3 - for MQTT v3 / v3.1
+ * 4 - for MQTT v3.1.1
+ * 5 - for MQTT v5
+ */
+int mosquitto_client_protocol_version(const struct mosquitto *client);
+
+
 /*
  * Function: mosquitto_client_sub_count
  *

src/plugin.c

@@ -77,6 +77,21 @@ int mosquitto_client_protocol(const struct mosquitto *client)
 }
 
 
+int mosquitto_client_protocol_version(const struct mosquitto *client)
+{
+	switch(client->protocol){
+		case mosq_p_mqtt31:
+			return 3;
+		case mosq_p_mqtt311:
+			return 4;
+		case mosq_p_mqtt5:
+			return 5;
+		default:
+			return 0;
+	}
+}
+
+
 int mosquitto_client_sub_count(const struct mosquitto *client)
 {
 	return client->sub_count;