Add `mosquitto_set_clientid` function for security plugins.

Signed-off-by: Tobias Assarsson <tobias.assarsson@gmail.com>

include/mosquitto_broker.h

@@ -462,6 +462,21 @@ mosq_EXPORT const char *mosquitto_client_username(const struct mosquitto *client
  */
 mosq_EXPORT int mosquitto_set_username(struct mosquitto *client, const char *username);
 
+/* Function: mosquitto_set_clientid
+ *
+ * Set the client id for a client.
+ *
+ * This effectively forces the client onto another message queue.
+ * Can be used to scope client ids by prefixing the client id with some user-specific data.
+ * eg. "client1" could become "user1-client1".
+ *
+ * Returns:
+ *   MOSQ_ERR_SUCCESS - on success
+ *   MOSQ_ERR_INVAL - if client is NULL, or if clientid is not a valid utf-8 string
+ *   MOSQ_ERR_NOMEM - on out of memory
+ */
+mosq_EXPORT int mosquitto_set_clientid(struct mosquitto *client, const char *clientid);
+
 
 /* =========================================================================
  *

src/linker-macosx.syms

@@ -29,6 +29,7 @@ _mosquitto_property_free_all
 _mosquitto_pub_topic_check
 _mosquitto_realloc
 _mosquitto_set_username
+_mosquitto_set_clientid
 _mosquitto_strdup
 _mosquitto_sub_matches_acl
 _mosquitto_sub_matches_acl_with_pattern

src/linker.syms

@@ -30,6 +30,7 @@
 	mosquitto_pub_topic_check;
 	mosquitto_realloc;
 	mosquitto_set_username;
+	mosquitto_set_clientid;
 	mosquitto_strdup;
 	mosquitto_sub_matches_acl;
 	mosquitto_sub_matches_acl_with_pattern;

src/plugin_public.c

@@ -268,6 +268,28 @@ int mosquitto_set_username(struct mosquitto *client, const char *username)
 	}
 }
 
+int mosquitto_set_clientid(struct mosquitto *client, const char *clientid)
+{
+    char *u_dup;
+    char *old;
+
+    if(!client) return MOSQ_ERR_INVAL;
+
+    int clientid_len = (int)strlen(clientid);
+    if(mosquitto_validate_utf8(clientid, clientid_len)){
+        return MOSQ_ERR_INVAL;
+    }
+
+    u_dup = mosquitto__strdup(clientid);
+    if(!u_dup) return MOSQ_ERR_NOMEM;
+
+
+    old = client->id;
+    client->id = u_dup;
+
+    mosquitto__free(old);
+    return MOSQ_ERR_SUCCESS;
+}
 
 static void disconnect_client(struct mosquitto *context, bool with_will)
 {