Dynamic security: Fix the plugin being able to be loaded twice.

Currently only a single plugin can interact with a unique $CONTROL
topic. Using multiple instances of the plugin would produce duplicate
entries in the config file.

Closes #2601.
Closes #2470.

ChangeLog.txt

@@ -27,6 +27,10 @@ Broker:
 - Dynamic security: Fix modifyClient and modifyGroup commands to not modify
   the client/group if a new group/client being added is not valid.
   Closes #2598.
+- Dynamic security: Fix the plugin being able to be loaded twice. Currently
+  only a single plugin can interact with a unique $CONTROL topic. Using
+  multiple instances of the plugin would produce duplicate entries in the
+  config file. Closes #2601. Closes #2470.
 
 Client library:
 - Fix threads library detection on Windows under cmake. Bumps the minimum

plugins/dynamic-security/plugin.c

@@ -482,6 +482,7 @@ void dynsec__config_save(void)
 int mosquitto_plugin_init(mosquitto_plugin_id_t *identifier, void **user_data, struct mosquitto_opt *options, int option_count)
 {
 	int i;
+	int rc;
 
 	UNUSED(user_data);
 
@@ -502,11 +503,46 @@ int mosquitto_plugin_init(mosquitto_plugin_id_t *identifier, void **user_data, s
 	plg_id = identifier;
 
 	dynsec__config_load();
-	mosquitto_callback_register(plg_id, MOSQ_EVT_CONTROL, dynsec_control_callback, "$CONTROL/dynamic-security/v1", NULL);
-	mosquitto_callback_register(plg_id, MOSQ_EVT_BASIC_AUTH, dynsec_auth__basic_auth_callback, NULL, NULL);
-	mosquitto_callback_register(plg_id, MOSQ_EVT_ACL_CHECK, dynsec__acl_check_callback, NULL, NULL);
+
+	rc = mosquitto_callback_register(plg_id, MOSQ_EVT_CONTROL, dynsec_control_callback, "$CONTROL/dynamic-security/v1", NULL);
+	if(rc == MOSQ_ERR_ALREADY_EXISTS){
+		mosquitto_log_printf(MOSQ_LOG_ERR, "Error: Dynamic security plugin can currently only be loaded once.");
+		mosquitto_log_printf(MOSQ_LOG_ERR, "Note that this was previously incorrectly allowed but could cause problems with duplicate entries in the config.");
+		goto error;
+	}else if(rc == MOSQ_ERR_NOMEM){
+		mosquitto_log_printf(MOSQ_LOG_ERR, "Error: Out of memory.");
+		goto error;
+	}else if(rc != MOSQ_ERR_SUCCESS){
+		goto error;
+	}
+
+	rc = mosquitto_callback_register(plg_id, MOSQ_EVT_BASIC_AUTH, dynsec_auth__basic_auth_callback, NULL, NULL);
+	if(rc == MOSQ_ERR_ALREADY_EXISTS){
+		mosquitto_log_printf(MOSQ_LOG_ERR, "Error: Dynamic security plugin can only be loaded once.");
+		goto error;
+	}else if(rc == MOSQ_ERR_NOMEM){
+		mosquitto_log_printf(MOSQ_LOG_ERR, "Error: Out of memory.");
+		goto error;
+	}else if(rc != MOSQ_ERR_SUCCESS){
+		goto error;
+	}
+
+	rc = mosquitto_callback_register(plg_id, MOSQ_EVT_ACL_CHECK, dynsec__acl_check_callback, NULL, NULL);
+	if(rc == MOSQ_ERR_ALREADY_EXISTS){
+		mosquitto_log_printf(MOSQ_LOG_ERR, "Error: Dynamic security plugin can only be loaded once.");
+		goto error;
+	}else if(rc == MOSQ_ERR_NOMEM){
+		mosquitto_log_printf(MOSQ_LOG_ERR, "Error: Out of memory.");
+		goto error;
+	}else if(rc != MOSQ_ERR_SUCCESS){
+		goto error;
+	}
 
 	return MOSQ_ERR_SUCCESS;
+error:
+	mosquitto_free(config_file);
+	config_file = NULL;
+	return rc;
 }
 
 int mosquitto_plugin_cleanup(void *user_data, struct mosquitto_opt *options, int option_count)