Add example plugin for topic hierarchy flattening.

Issue #3365

plugins/examples/CMakeLists.txt

@@ -17,6 +17,7 @@ add_subdirectory(payload-size-stats)
 add_subdirectory(plugin-event-stats)
 add_subdirectory(print-ip-on-publish)
 add_subdirectory(tick-interval)
+add_subdirectory(topic-hierarchy-flatten)
 add_subdirectory(topic-modification)
 add_subdirectory(topic-jail)
 add_subdirectory(wildcard-temp)

plugins/examples/topic-hierarchy-flatten/CMakeLists.txt

@@ -0,0 +1,3 @@
+set (PLUGIN_NAME mosquitto_topic_hierarchy_flatten)
+
+add_mosquitto_plugin_no_install("${PLUGIN_NAME}" "${PLUGIN_NAME}.c" "" "")

plugins/examples/topic-hierarchy-flatten/Makefile

@@ -0,0 +1,15 @@
+R=../../..
+include ${R}/config.mk
+
+PLUGIN_NAME=mosquitto_topic_hierarchy_flatten
+LOCAL_CFLAGS+=
+LOCAL_CPPFLAGS+=
+LOCAL_LDFLAGS+=
+LOCAL_LIBADD+=
+
+all : binary
+
+OBJS:=${PLUGIN_NAME}.o
+
+PLUGIN_NOINST:=1
+include ${R}/plugins/plugin.mk

plugins/examples/topic-hierarchy-flatten/mosquitto_topic_hierarchy_flatten.c

@@ -0,0 +1,161 @@
+/*
+Copyright (c) 2020-2025 Roger Light <roger@atchoo.org>
+
+All rights reserved. This program and the accompanying materials
+are made available under the terms of the Eclipse Public License 2.0
+and Eclipse Distribution License v1.0 which accompany this distribution.
+
+The Eclipse Public License is available at
+   https://www.eclipse.org/legal/epl-2.0/
+and the Eclipse Distribution License is available at
+  http://www.eclipse.org/org/documents/edl-v10.php.
+
+SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
+
+Contributors:
+   Roger Light - initial implementation and documentation.
+*/
+
+/*
+ * This is an *example* plugin which looks at messages coming in and if their
+ * topic matches a topic filter, makes the payload available on another topic,
+ * flattening the hierarchy if the topic filter includes a wildcard.
+ *
+ * The input topic filter and output topic can be configured in the plugin
+ * config. The "plugin_opt_republish" option can be set to true/false. If set
+ * to true, then the original incoming messages are unaffected and a new
+ * message is published for each matching message. If set to false, the
+ * original message has its topic replaced with the output topic.
+ *
+ * Compile with:
+ *   gcc -I<path to mosquitto-repo/include> -fPIC -shared mosquitto_topic_hierarchy_flatten.c -o mosquitto_topic_hierarchy_flatten.so
+ *
+ * Use in config with:
+ *
+ *   plugin /path/to/mosquitto_topic_hierarchy_flatten.so
+ *   plugin_opt_input_topic_filter my/+/topics
+ *   plugin_opt_output_topic the/single/output/topic
+ *   plugin_opt_republish true
+ *
+ * Note that this only works on Mosquitto 2.1 or later.
+ */
+#include <stdio.h>
+#include <string.h>
+
+#include "mosquitto.h"
+
+#define PLUGIN_NAME "topic-hierarchy-flatten"
+
+#define UNUSED(A) (void)(A)
+
+MOSQUITTO_PLUGIN_DECLARE_VERSION(5);
+
+struct plugin_data{
+	mosquitto_plugin_id_t *pid;
+	char *input_topic_filter;
+	char *output_topic;
+	bool republish;
+};
+
+
+static int callback_message_in(int event, void *event_data, void *userdata)
+{
+	struct mosquitto_evt_message *ed = event_data;
+	struct plugin_data *data = userdata;
+	bool result;
+	int rc;
+
+	UNUSED(event);
+
+	mosquitto_topic_matches_sub(data->input_topic_filter, ed->topic, &result);
+
+	if(result){
+		if(data->republish){
+			mosquitto_property *props = NULL;
+			if(ed->properties){
+				rc = mosquitto_property_copy_all(&props, ed->properties);
+				if(rc){
+					return rc;
+				}
+			}
+			rc = mosquitto_broker_publish_copy(NULL, data->output_topic, (int)ed->payloadlen,
+					ed->payload, ed->qos, ed->retain, props);
+			if(rc){
+				mosquitto_property_free_all(&props);
+				return rc;
+			}
+		}else{
+			ed->topic = mosquitto_strdup(data->output_topic);
+			if(!ed->topic){
+				return MOSQ_ERR_NOMEM;
+			}
+		}
+	}
+
+	return MOSQ_ERR_SUCCESS;
+}
+
+int mosquitto_plugin_init(mosquitto_plugin_id_t *identifier, void **userdata, struct mosquitto_opt *opts, int opt_count)
+{
+	struct plugin_data *data = mosquitto_calloc(1, sizeof(struct plugin_data));
+	if(!data){
+		return MOSQ_ERR_NOMEM;
+	}
+	*userdata = data;
+	data->republish = true;
+
+	for(int i=0; i<opt_count; i++){
+		if(!strcmp(opts[i].key, "input_topic_filter")){
+			data->input_topic_filter = mosquitto_strdup(opts[i].value);
+			if(!data->input_topic_filter){
+				return MOSQ_ERR_NOMEM;
+			}
+		}else if(!strcmp(opts[i].key, "output_topic")){
+			data->output_topic = mosquitto_strdup(opts[i].value);
+			if(!data->output_topic){
+				return MOSQ_ERR_NOMEM;
+			}
+		}else if(!strcmp(opts[i].key, "republish")){
+			data->republish = !strcmp(opts[i].value, "true");
+		}
+	}
+
+	if(!data->input_topic_filter){
+		data->input_topic_filter = mosquitto_strdup("input/#");
+		if(!data->input_topic_filter){
+			return MOSQ_ERR_NOMEM;
+		}
+	}
+
+	if(!data->output_topic){
+		data->output_topic = mosquitto_strdup("output");
+		if(!data->output_topic){
+			return MOSQ_ERR_NOMEM;
+		}
+	}
+
+	mosquitto_log_printf(MOSQ_LOG_INFO, PLUGIN_NAME ": Input topic filter is '%s'", data->input_topic_filter);
+	mosquitto_log_printf(MOSQ_LOG_INFO, PLUGIN_NAME ": Output topic is '%s'", data->output_topic);
+	mosquitto_log_printf(MOSQ_LOG_INFO, PLUGIN_NAME ": Republish is %s", data->republish?"true":"false");
+
+	data->pid = identifier;
+	return mosquitto_callback_register(data->pid, MOSQ_EVT_MESSAGE_IN, callback_message_in, NULL, data);
+}
+
+/* mosquitto_plugin_cleanup() is optional in 2.1 and later. Use it only if you have your own cleanup to do */
+int mosquitto_plugin_cleanup(void *userdata, struct mosquitto_opt *opts, int opt_count)
+{
+	struct plugin_data *data = userdata;
+	UNUSED(opts);
+	UNUSED(opt_count);
+
+	if(data){
+		mosquitto_callback_unregister(data->pid, MOSQ_EVT_MESSAGE_IN, callback_message_in, NULL);
+
+		mosquitto_FREE(data->input_topic_filter);
+		mosquitto_FREE(data->output_topic);
+		mosquitto_FREE(data);
+	}
+
+	return MOSQ_ERR_SUCCESS;
+}

plugins/examples/topic-hierarchy-flatten/test.conf

@@ -0,0 +1,2 @@
+plugin ./mosquitto_topic_hierarchy_flatten.so
+plugin_opt_republish true

plugins/examples/topic-hierarchy-flatten/test.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+../../../src/mosquitto -c test.conf -v